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Controlling glyphosate-resistant grass in irrigated cotton

When glyphosate-resistant summer grass starts to cause yield losses in cotton, growers obviously need to add some non-glyphosate options to their system to protect yields and prevent further weed blowouts. The questions then become where to add non-glyphosate tactics to get most benefit, and how many are needed?
Developer David Thornby used the Barnyard Grass Understanding and Management tool, BYGUM to investigate three key questions relevant to irrigated cotton systems.

Scenario #1 – Ask BYGUM: the value of glyphosate
What’s the remaining value of glyphosate in rotations with glyphosate-resistant awnless barnyard grass?
BYGUM developer David Thornby has developed a series of scenarios to demonstrate this new decision support tool.
Since the first confirmation of glyphosate resistance in awnless barnyard grass, many other resistant populations have been found, and these populations don’t all display the same level of resistance.
While glyphosate is no longer effective as a stand-alone control measure against any of them, some populations are less strongly resistant than others. In the case of the first-confirmed population, field rates still had around a 40 percent efficacy on small seedlings. For other more recently confirmed populations, efficacy even on small seedlings is much lower.
Given that glyphosate is going to be applied to these populations anyway, it’s important not to overstate the usefulness of glyphosate by hoping to be able to rely on it for some level of control. BYGUM can test the difference for us, between populations with strong resistance and those with moderately strong resistance.
David said he used a simple irrigated rotation, with modest use of non-glyphosate options in an otherwise glyphosate dominated system. He varied the effectiveness of glyphosate from around 40 percent (‘moderate’ resistance) to around five percent (‘strong’ resistance).
Key outcomes
The results show two key things. First, both systems are still making money after five years. High levels of crop competition keep seed production per escaping weed low, and the addition of a few effective tactics reduces the number of surviving plants to moderate/know levels of between six and 14 plants per square metre at the end of the fifth season.
Secondly, however, both systems are heading towards failure. Weed and seedbank numbers are increasing, however slowly. And while gross margins are the same at the end of season one, there is a predicted difference of around $500/ha between the gross returns in season five.
David says there are three lessons here. First, strong crop competitive effects might mask the seriousness of resistance issues in irrigated cotton, should they be present. Second, there are good reasons to determine just how strongly resistant your resistant awnless barnyard grass population is, if you’re going to be sticking with a system that is predominately about the use of glyphosate. And third, allowing a slow decline with somewhat-inadequate weed management looks likely to have a substantial cost as the years pass by. We’ve made many assumptions in this example – in particular, that irrigated cotton is planted and provided with resources to allow for strong competitiveness against the weed. We’ve also made assumptions about crop and herbicide costs, average yields and prices. “You could run BYGUM with a different set of assumptions that fit your experiences, and see if the results change.
Scenario #2 – Testing the value of a cover crop
Can summer cover crops be used to get on top of weed populations?
In a one-in-one-out rotation of dryland cotton, summer fallows offer a chance to get on top of weed populations through vigorous use of non-crop herbicides. However, with no crop competition present, they can also offer weed escapes an opportunity to set a lot of seed, especially when the key herbicide in both crop and fallow, glyphosate, is no longer effective.
“Cover crops allow growers to maintain some competition even in non-crop seasons,” David Thornby says. “A good stand of millet (as simulated in BYGUM), sprayed out before seed set, allows for a combination of late season herbicide use to clean up survivors and mid-season competition with glyphosate- resistant barnyard grass, reducing seed set per plant.”
The first scenario is a basic one-in- one-out rotation. This contains the assumption that the barnyard grass population is resistant to glyphosate, and that an early season residual and mid-season inter-row cultivation are used to provide some control in crop: summer fallows use two cultivations and a double knock. In scenario 2 David replaced the second summer fallow with a cover crop.
The cover crop includes cultivation, a double knock, spray out (assuming this is with a non-glyphosate product effective on glyphosate, such as paraquat), and a late application of paraquat over the now-dead millet. “The cover crop is more expensive than the summer fallow, and actual plant numbers per square metre are not reduced all that much (1.1 to 0.8 per square metre),” David says. “But there is a substantial difference in seeds returned to the seed bank.”
The comparison scenario shows a substantial increase in the yield from the final cotton crop, due to the strong reduction in seed bank numbers at the end of the cover crop season. “The benefits of the cover crop come due, as expected, in the following crop, where the seed bank has been driven down and emerging weed numbers are low,” David said. “Over the course of the whole rotation, incorporating one cover crop every second summer fallow is predicted to be worth almost $200/ha in increased yield.
“There is more than one way to protect future yields in a dryland rotation, but using cover crop competition certainly seems to bear looking at. “We’ve made many assumptions in this example – in particular, that planting time and summer rainfall are conducive to good cover crop growth, resulting in high competition, that the barnyard grass population is strongly resistant to glyphosate, and that the cover crop doesn’t reduce moisture availability to the following cotton crop. “We’ve also made assumptions about crop and herbicide costs, average yields and prices. “Once again, users could run BYGUM with a different set of assumptions that fit their experiences, and see if the results change.”
Scenario #3 – Residual answers to resistance
Can you control a glyphosate-resistant grass by adding a residual in irrigated cotton?
In situations where a glyphosate-resistant summer grass starts to cause yield losses in cotton, growers obviously need to add some non-glyphosate options to their system to protect yields and prevent further weed blowouts. The questions then become where to add non-glyphosate tactics, to get most benefit, and how many are needed?
David Thornby used BYGUM to investigate this question. First he compared a system with glyphosate-resistant barnyard grass where only glyphosate is used with the same system with an early-season (pre- or at-planting) residual added.

Residuals tripled gross margins
The yield results of adding a single residual are striking.
The glyphosate-only system is still producing some yields (Figure 2 – primarily due to the competitiveness of irrigated cotton), but end-of-season weed numbers are very high and the potential of the system is being seriously under-utilised. “Adding a single residual can reduce early-season weed numbers dramatically, and because this is when most of the competition effects occur, this has a huge benefit for the bottom line,” David said.
Figure 1: Irrigated cotton rotation with an early-season residual. While we’ve used ‘a residual’ in the notation here, a rotation of suitable products from different modes of action should be used in the field. The glyphosate-alone system is the same as this one, without the early season residual.
Figure 2: A comparison of gross margin and barnyard grass numbers at end-of-season between glyphosate alone (top) and glyphosate plus a single, early-season residual.
“Gross margins are almost tripled compared to glyphosate alone when the weed population is strongly glyphosate resistant.
“However, end-of-season weed numbers (and seedbank density) are still unacceptably high, so a single residual doesn’t appear to be enough of an addition, despite the dramatic effect.
“A single year of poor control from the residual (rather than the average of around 85 percent efficacy) would certainly result in a blowout.”
Adding a mid- or late-season tactic provides some insurance against weed blowouts and seed production. Because late weed germinants in vigorous cotton stands don’t produce a lot of seed per plant, the effects on yield aren’t so dramatic. However reducing surviving plants and especially reducing the seed bank size are critically important insurance against future blowouts and selection of resistance for other modes of action.
David tried two different tactics in BYGUM, adding either a layby residual to each crop or a mid-season knockdown.
Adding a layby reduces the seed bank somewhat, and cuts surviving plants at end-season down to around 25 per square metre. This still appears to be too many survivors for comfort, but it does represent a substantial improvement over an early- season residual alone, and offers insurance against future blowouts.
BYGUM predicts that it can be sustained at least for the five-year rotation. This comes at a cost, however: the reductions in late-season weed numbers are offset by the price of the extra residual and BYGUM includes a penalty due to phytotoxicity.
In comparison with the layby system, a system with a mid-season knockdown, rotating between options including Group A herbicides, shielded paraquat and inter-row tillage improves the gross margin (due to a combination of taking out some weed competition and having some options with lower phytotoxicity-related yield penalties), but leaves more end-of-season survivors.
“So this is not an ideal system either – but is certainly an improvement in all ways over a single-residual system,” David said.
“These analyses show that while a single early-season residual can do a lot of heavy lifting in terms of reducing weed competitiveness, it’s not enough on its own for long-term sustainability. Late or mid-season tactics provide some insurance.”
“BYGUM predicts that while good returns can be sustained at least for five years with this ‘plus two’ strategy, more non-glyphosate tactics would be needed to drive the seed bank to very low levels. We’ve made many assumptions in this example—in particular, that irrigated cotton is planted and provided with resources to allow for strong competitiveness against the weed, that resistance to glyphosate is quite strong, and that good efficacy is generally the case for residual applications. Pre-simulation weed numbers are assumed to be moderate and we’ve also made assumptions about crop and herbicide costs, average yields and prices.”
Case study reproduced courtesy of CRDC, following publication in CRDC’s Spotlight magazine, Winter 2016. To access BYGUM, visit: www.cottoninfo.com.au/barnyard-grass-understanding-and-management-bygum.

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Paired rows take off on Twitter

Recently we started a hashtag on Twitter to share information about paired row sowing. We used the hashtag #Pairedrow. You can still use it now to engage in the discussion and tag WeedSmart. Thank you to everyone who has already contributed to the discussion about paired row sowing on Twitter. Here is a summary of the pros and cons.

You’ve got to love the names of some of the brands out there! Below are the names of some of the paired row sowing options that growers had tried.
Rootboot, Stiletto boot, Agmaster twin seeker, Bourgault, Ausplow, Conservapac, Morris C2 boot, Seed master, Burando Hill.

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Testing for weed susceptibility to herbicide pays off

Sam and Emily Eagle run 2500 merino ewes on their 3000 ha mixed farm near Horsham, Victoria. They say the livestock and cropping activities complement each other, keeping their pastures and crops performing at their best.
Herbicide resistant annual ryegrass is their main weed challenge with one test revealing resistance to glyphosate (65 per cent) and clethodim (80 per cent), and full susceptibility to chlorsulfuron (Group B, Glean).
Sam and Emily Eagle run a mixed farming enterprise near Horsham, Victoria where grazing and cropping are mutually beneficial for weed management.
“We test annual ryegrass from two or three paddocks each year to monitor any changes in susceptibility to the herbicides we use,” said Sam. “Knowing which herbicides are effective makes it easier to plan our herbicide use without relying solely on the products that still work. Every year we have at least one tactic in place specifically to reduce the weed seed bank.”
Knowing that the tested weeds were susceptible to Glean gave Sam an opportunity to regain control of a potential blow-out situation, using a herbicide that is much cheaper than alternatives that he might have chosen if he had to make the decision without the herbicide resistance test results.
The Eagle’s agronomist usually collects the seed for testing and the results are considered to be representative of the whole paddock, each one being around 35 to 70 ha.
“We can fairly safely assume that all our weeds have some level of resistance so we concentrate on managing survivors, mostly treating with a double-knock whenever possible,” he said. “Annual ryegrass is our main problem weed however we are keeping a close eye on brome grass that is present on one of our lease blocks.”

Triazine resistance on one block precludes the use of TT canola so the Eagles grow conventional canola on this block, aiming for the most competitive, highest yielding crop possible.
Along fence lines Sam uses a 2-year program where he slashes in spring in one year and then sprays a knockdown + residual herbicide mix the next year. “When we slash, we know that the weeds will still set seed. We keep the slasher low to the ground to ensure any seed heads present at harvest are below header height so they won’t get spread,” said Sam.
The pasture paddocks are de-stocked over summer with the sheep grazing on the stubble. If the stubble runs out early the sheep are returned to the containment area where they are fed screenings, hay and grain until the pastures are ready. The ewes return to the pastures to lamb in autumn.
Sam and Emily use narrow windrow burning in the canola as their harvest weed seed control tool. They have had trouble using this tactic in cereals, where the fires often don’t burn right to the ground, leaving weed seed concentrated in bands. On the other hand, the canola burns well, destroying the weed seed, and Sam is able to safely burn several paddocks on the one day. Grazing the canola narrow windrows has not caused any problems with burning or with weed seed being spread.
Narrow windrow burning in canola has worked very well for Sam, driving down herbicide resistant ryegrass numbers.
“Canola actually gives us a few opportunities to control late germinating weeds,” said Sam. “Firstly with an over the top spray to desiccate the crop, secondly windrowing the crop early and third, using the narrow windrow chute at harvest in preparation for narrow windrow burning in autumn. We also spray top wheat and barley, with the sheep providing the second knock for any survivor weeds.”
Growing faba beans, canola, wheat and malt barley enables them to use a different pre-emergent herbicide each year of the rotation. At the end of this 4-year program any paddocks that are carrying a weed burden are thoroughly cleaned using a pasture and a 3-year hay program. Moby forage barley sown with clover gives a nutrient boost to the perennial ryegrass pasture phase, which may last up to ten years.
“We supply hay for export and generally grow two oat crops and one vetch,” said Sam. “Any failed crops or additional production is stored as silage in underground pits to drought-proof our breeding flock. Silage is a particularly good way to clean up weeds because we spray out when the crop is actively growing and not under any moisture or heat stress, then cut in early September.”
At the end of a 4-year cropping program any paddocks that are carrying a weed burden are thoroughly cleaned using a pasture and a 3-year hay program. The Eagles supply hay for export and generally grow two oat crops and one vetch.
The 2017 seeding represents the beginning of the Eagles’ fully aligned controlled traffic farming (CTF) system. The transition to 12 m wide CTF has taken several years but Sam and Emily are convinced that the efficiencies gained will be well worth the investment.
They sow all crops on 300 mm row spacing and aim to achieve the most competitive crops possible. Although Sam knows 380 mm row spacing would make some management operations easier, they pick up extra yield and suppress weeds with the narrower spacing.
In the seven years that Sam and Emily have been managing the farms they have seen the benefits of the rotation in keeping weed numbers low. “All of our worst paddocks have now had the ‘rotation treatment’ and we have avoided weed blow-outs,” said Sam. “Two wet years in a row could potentially challenge our weed management but having the sheep in the system gives us more options while still earning income from each paddock.”
Related links

10 Point Plan – Test for resistance to establish a clear picture of paddock-by-paddock farm status
Plant Science Consulting herbicide resistance testing
CSU Herbicide resistance testing

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Delay spraying stressed weeds after rain

WEEDSMART DRY SEASON SPECIAL
Dry season agronomy is difficult. In affected areas there is a reasonable chance that pre-emergent herbicides applied at or before seeding will not work as well as they usually would, even when it does rain. This fact, combined with the significantly reduced crop competition in most paddocks, will mean weeds will have the opportunity to grow in greater numbers.
Knowing that spraying moisture stressed weeds in dry conditions is less effective, many growers will be looking for any sign of rainfall as an opportunity to quickly apply post-emergent sprays in an attempt to reduce weed seed set.

Australian Herbicide Resistance Initiative communication lead, Peter Newman says waiting for new leaves to grow after a rainfall event is likely to give better results than spraying just a few days after rain.
“When plants are stressed, one of their survival mechanisms is to thicken the cuticle of their leaves,” he says. “This reduces moisture loss from the leaf during hot, dry periods and also reduces the uptake of post-emergent herbicides.”
“These leaves remain thicker and waxier, even when it rains and the plant freshens up,” he says. “Waiting for new leaves to emerge after rain will result in a much better level of control and help minimise weed seed set. The use of the correct adjuvants and spray quality to counteract the increased waxiness of the leaves will also improve herbicide efficacy.”
However, waiting for new growth also has its problems if dry conditions return. “These decisions are not easy, but if rain has fallen and more rain is forecast, perhaps waiting for new growth of the weeds will give the best results,” he says.
Department of Agriculture and Food WA research conducted by Dr David Minkey and John Moore in the 1990s demonstrated the impact of moisture stress and low humidity on herbicide efficacy. Their research showed a 20-fold difference in efficacy of glyphosate sprayed on weeds of the same age under favourable and stressful environmental conditions.
“Unfortunately, spray events are going to be difficult to time and the results are probably going to be less than optimal. This is out of the grower’s control in most instances,” says Peter. “Given that there is a good chance of more weeds surviving in-crop weed control efforts, implementing some form of harvest weed seed control will be an even higher priority this year.”
“High numbers of annual ryegrass is a concern but we know that we can get back on top of a ryegrass seed bank in just a few years,” he says. “Wild radish however produces seed that remains viable in the soil for 5 to 10 years so it takes much longer to drive weed seed numbers down if this weed blows out.”
If faced with a crop failure, spraying out early could be a good option and for crops that are harvested consider a low cost harvest weed seed control option, such as chaff lining, to minimise the potential impact of a weed blow-out.
Chaff lining involves placing a chute on the rear of the harvester that concentrates the chaff-only fraction into a narrow band between the wheeltracks of the header. The straw is chopped and spread as usual. The chute can generally be built on farm at a very low or even nil cost.
Relevant links

AHRI Insight – Why thirst weeds are hard to kill
Spray application manual – product requirements module
WeedSmart search – chaff lining

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How chaff lining works

WEEDSMART DRY SEASON SPECIAL
Weeds are a constant factor in farming, regardless of the seasonal conditions. The lack of autumn rains in areas like the Eyre Peninsula in SA and no follow-up rain for dry sown crops in some Western Australian districts means there will be many crops with variable establishment and variable growth rates, making weed control more challenging.

Social media shows impact of 2017’s dry conditions
Twitter has provided stark visual examples of just how dry it is this season in some parts of the country. @ZimStead is an agronomist in Western Australia and recently shared the below two pictures taken a year apart from the same farm in North Tammin. The more lush picture was taken on June 1, 2016, and the second, showing the effects of dry conditions was taken on June 1, 2017. 
North Tammin June 2016 (photo @ZimStead)
North Tammin June 2017 (photo @ZimStead)

Chaff lining – an effective tool
It’s understandable that in a dry year, grain growers staple the chequebook shut and stop spending money on their crops to keep costs down and minimise their loss.  However, this can result in weeds setting seed in crop, which can quickly undo many years of hard work to erode weed seed banks.
Perhaps this may be the year for growers to consider adopting the simple, low-cost option chaff lining, to cost-effectively deal with weeds setting seed in these low yielding crops.

Chaff lining involves placing a chute on the rear of the harvester that concentrates the chaff only into a single line in the centre of the header.  The straw is chopped and spread as usual.  The idea is to drive the harvester on the same tracks for years to come, repeatedly placing chaff lines in the same place each year.
Growers using this practice find that these chaff lines are often a lot less weedy than they would expect and they believe that a lot of the weed seeds rot, however, unfortunately, we don’t have any data to confirm or deny this observation as yet.
There’s also the option to place a narrow windrow of all residue in the same place in a future crop which can be burnt to remove the chaff line and destroy surviving weed seeds. Our aim in a dry year should be to set ourselves up for success next year, and chaff lining could be a low cost, easy tool to put growers on the right track.
Join the conversation on Twitter by following @WeedSmartAU and using the hashtag #chaffline

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Ranking paddocks for weed expenditure in a dry year

WEEDSMART DRY SEASON SPECIAL
Weeds are a constant factor in farming, regardless of the seasonal conditions. The lack of autumn rains in areas like the Eyre Peninsula in SA and no follow-up rain for dry sown crops in some Western Australian districts means there will be many crops with variable establishment and variable growth rates, making weed control more challenging.
Making decisions early about a paddock’s weed control program is one way to make sure that money spent on weed control is cost-effective. The The Big 6 is a useful tool for growers and advisors to use when planning an integrated weed management strategy and also for situations like this where adjustments are required due to an unexpected set of environmental circumstances.
Australian Herbicide Resistance Initiative (AHRI) southern extension agronomist, Greg Condon says ranking paddocks according to their potential returns and future spending is a good way to channel resources to the most critical areas.
“The key is to look at how to minimise losses rather than to try to optimise production. From a weed management perspective this usually means looking at each paddock and considering the likely effect of different weed control options,” he says.
“For example, the future financial cost of a paddock with low ryegrass numbers will be far less than a paddock on the verge of a wild radish blowout if no herbicide is applied. If the future cost of ‘doing nothing’ will be high, then higher level expenditure is justified.”
AHRI southern extension agronomist, Greg Condon suggests growers talk to their advisors about ways to minimise the future cost of weed control as well as implementing low cost options for the current year in areas experiencing a dry start to the season.
The next step is to rank the crops in order of their potential value. For lower value crops, consider spray fallowing, grazing or cutting for hay. Wherever possible, avoid using expensive herbicides and look at low cost cultural weed control measures that will reduce weed seed set. Canola, pulses and feed barley all have crop top registrations that can be used to prevent seed set and lessen the weed burden for future years. This will be a valuable option for crops that do not warrant a full post-emergent spray program if the season stays dry.
“Cereals are generally lower risk crops that tend to handle drier conditions better than broadleaf crops. One exception to this is short season Roundup Ready canola, which can be quite resilient, and is supported with low cost post-emergent herbicide options,” says Greg.
The other thing to consider when ranking paddocks is the crop’s yield potential. For example, a canola crop on a deep ripped fallow would get priority over a low fertility weedy block. Looking at the establishment of crops is a good way to rank the paddocks ‘in the middle’. Putting available resources into crops that have the best establishment means that there will be some competition for weeds and if the paddock has had a history of harvest weed seed control then the weed burden should also be lower.
Ranking paddocks and crops early in the season will help target expenditure and minimise costs for this season and future seasons in terms of weed burden. As the season unfolds, harvest weed seed control will be the next major decision to impact on the weed seed bank.

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Pulses to attack weeds on many fronts

Evidence is mounting that shows pulses can play an important part in weed control, particularly in the war against herbicide resistant annual ryegrass. In the past, pulses have often been named as ‘the weak link’ in crop rotations when it comes to weed control however recent research indicates that pulses can make a valuable contribution to an integrated weed management program.
“Field pea is a particularly useful crop for growers to launch a strategic multi-pronged attack on grass weeds,” says former NSW DPI researcher, Dr Eric Armstrong. “The idea is to apply a double knockdown prior to sowing, use pre- and post-emergent herbicides, crop top in spring and harvest early to stop weed seed set.”
Dr Eric Armstrong says that when crop topping, timing is critical to prevent seed set of the target weeds. For crop topping to be effective, spray the paddock before the weed seeds are viable.
“Hitting weeds before, during and at the end of the pulse crop phase with different herbicide and cultural tools has a big impact on weed numbers and largely eliminates the addition of weed seed to the soil bank. Not many other crops allow this many tactics to be applied to weeds in one season,” he says.
Leader of the NSW DPI Pulse team, Mr Mark Richards says the early flowering and maturing field pea varieties PBA Pearl and PBA Oura are the best candidates for crop topping, even though they were not released with this management strategy in mind. These earlier maturing varieties of field pea provide better weed control options than Kaspa field pea, chickpea and lupin, all of which generally mature later, sometimes after the target weeds have set seed.
Early flowering and maturing field pea varieties PBA Pearl and PBA Oura are the best candidates for crop topping.
“As early maturing varieties of faba bean, lentil and lupin become available, their adoption in crop rotations for weed control alone will be justified, especially given the broad spectrum of herbicides registered for use in pulse crops,” he says.
Dr Armstrong recommends growers give greater consideration to the value of pulse crops in the rotation to achieve agronomic stability and improved profitability. “If weeds are a priority, growers can include a pulse crop with the intention of crop topping to prevent weed seed set—accepting a small yield penalty in return for bringing the weeds under control,” he says. “Brown manuring the pulse crop is a last option and would only be chosen in very high weed burden situations.”
“When crop topping, timing is critical to prevent seed set of the target weeds,” says Dr Armstrong. “The weed seeds must be at or before the milky dough stage for crop topping to be an effective weed control measure.”
If brown manuring is chosen, end the crop when the weeds are flowering and before the weed seeds reach the milky dough stage to be sure of 100 per cent seed set control.
If herbicide resistant weeds are the main priority, spray out the paddock before the weed seeds reach milky dough stage.
Some of the decisions surrounding the use of manuring and crop topping can occur during the growing season as seasonal conditions dictate. However, Dr Armstrong recommends growers give careful consideration to the use of pulses when they are planning their crop sequence several years in advance.
“When choosing a pulse crop, consider the species’ adaptation to the farm and whether it is suited to crop topping and or desiccation as part of an integrated weed control program,” he says.
From an economic point of view, it is important to calculate returns over the whole crop sequence rather than for each crop on its own as this will take into account any benefits or savings in soil moisture, soil nitrogen or pest and disease management.
“Although there are significant soil health benefits in green or brown manuring a pulse crop it is rarely economical across the crop rotation,” Mr Richards says. “This is because growing two cash crops and a manure crop, or fallow, rarely earns as much income as three consecutive cash crops, even though the crop grown following a manure crop is likely to produce higher yield.”
A 5-year study led by Tony Swan, Senior Experimental Scientist, CSIRO Agriculture clearly identified that a two-year break from cereal production and the implementation of all available weed seed control options is the best way to run down the ryegrass seedbank.
Where herbicide resistant annual ryegrass is a major problem, an alternate three-year sequence of wheat-hay (sprayed afterwards) in year one, pulse-grain (spray topped) in year two, and Roundup Ready (RR) canola in year three can be profitable and also reduce the seed bank to extremely low levels.
Mr Mark Richards leads the New South Wales Department of Primary Industries (NSW DPI) Pulse team at Wagga Wagga that was previously led by senior research agronomist, Dr Eric Armstrong. Both researchers have worked with pulses for many years and are passionate about the options pulse crops can provide the farming system.
The on-going research investment by NSW DPI and the Grains Research and Development Corporation (GRDC) continues to examine weed control options and management in pulse crops.

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Strip grazing cattle to manage weeds

As the 2013–2016 drought bit harder in NSW’s Riverina, Ardlethan farmers Lou and Charlie Clemson were thinking about ways to better utilise more of their property, particularly the 200 ha of non-arable country.
Using a NSW State Government drought assistance grant to supplement their own funds they have installed a laneway through the middle of their property, Wongajong, to link the hilly paddocks with the stockyards and provide easy access to all the cropping paddocks between.
Lou Clemson says the laneway allows easy movement of stock through the cropping area and also provides an excellent confinement area with troughed water and creep feeders. The timbered hill paddock is key to the system’s success, providing native pasture for the breeding herd.
Lou says the changes to their business and their weed management have been amazing. “We now have another income stream, and cattle have been very profitable since the drought,” she said. “And we are using less herbicide to manage weeds.”
The re-introduction of livestock to Wongajong started in 2010 when the Clemsons bought 300 steers to make use of a frosted wheat crop. “We fed the steers for 5 months and made good money out of what would have otherwise been a failed crop,” said Lou. “After that we fenced off the hill and added the central laneway and watering points. We are really happy with how well this system is working and see benefits across our whole cropping operation.”
“Growing early varieties is the key to maximising the feed benefit of fodder crops such as Moby barley and Brenan and Naparoo feed wheats, which we plant in February or March,” she said. “In paddocks where we have some annual ryegrass pressure we have sown feed barley in February–March, weaned calves onto it in May and grazed until September. This 73 ha paddock alone turned off 200 prime yearlings.”
With some welcome rain in December, the Clemsons sprayed out the barley and took the opportunity to sow cowpeas over summer. “This year we weaned part of the herd in January and the calves went straight onto the cowpeas where they remained until late April. The cowpeas even podded up and we could have stripped them but decided to just continue grazing. The added nitrogen will also boost the next crop and build soil fertility.”
Cowpeas planted opportunistically on some December rain provided perfect feed for newly weaned calves from January until late April. In addition to the feed value the cowpeas have also provided a boost to soil fertility.
With two back to back seasons of crop competition and sustained grazing pressure, the ryegrass is well under control and the Clemsons have sown Condo grain wheat this winter. After removing the young stock from the cowpeas the Clemsons introduced them to creep feeders offering hay, feed barley and a magnesium calcium supplement in the central laneway.
Lou said their system relies on the 200 ha of hilly country where they run their breeding herd. The cows calve in the hill paddock in July then have access to crop stubbles straight after harvest until January when the cows return to the hill paddocks and the calves are weaned onto feed.
“In this system, Wongajong can carry 200 breeding cows and 50 replacement heifers but no more,” she said. “This is working so well that we are keen to build our herd and will replicate the laneway and fodder cropping system on a second property nearby. We have been expanding our area for both cropping and livestock with additional land purchased and leased.”
Looking across Wongajong, the top paddock grew cowpeas that were grazed over summer, the next paddock down the hill has feed barley ready for grazing and then the laneway where the young cattle are introduced to hay and grain once their rumens are mature. The Lemken speed tiller will be used to prepare the cowpea paddock for sowing the high yielding, short season Condo wheat.
“We identify paddocks that need a spell from cropping and also look at the weed burden,” said Lou. “Grazing barley is our go-to crop for weedy paddocks. It does a good job competing with annual ryegrass and gives us several options such as making hay or grazing and then spraying out.”
Canola has been a mainstay crop for the Clemsons along with barley for grazing, hay and feed grain, and wheat for grazing and feed grain.
The cattle strip graze the fodder crops at a density of 2 head/ha to maximise the feed value and weed control benefits. Electric fencing is used where necessary to manage the grazing intensity and provide fresh feed. Most of the fodder crops will be grazed out and sprayed in spring to clean-up for the following winter. The Clemsons are now planning to extend their fodder cropping program to hay and silage production.
Forage barley has proven to be the most useful crop for livestock production and weed control.
Charlie and Lou use a Lemken Helidor speed tiller to lightly cultivate paddocks that have been sprayed out after grazing. Paddocks are then rested for seven months, from late spring through summer, before they are re-sown. The combination of several non-herbicide weed control tactics such as strip grazing, fodder conservation and the speed tiller have resulted in an overall reduction in herbicide use.
“Sowing early gives us the greatest number of choices,” said Lou. “Some years we would expect about 10 per cent of our crops to be frost affected, but by sowing early we can achieve higher yields and this can make up for losses to frost.”
Lou said there has been a noticeable change in their attitude towards weeds with Charlie being much less stressed. Having the canola and wheat sprayed on time is important while there is more flexibility with spraying times for the grazing crops.
“If there are weeds visible in the paddock we can stay relaxed knowing that once the cattle have finished grazing and been sold, the paddock will be sprayed out before the ryegrass sets seed, and any survivors will be killed with the Lemken Helidor,” she said.
The Clemsons use a Lemken speed tiller straight after harvest to manage stubble and weeds. Provided there is some soil moisture present the cultivation to a depth of up to 10 cm or so causes about 80 per cent of the weed seeds to germinate, allowing a very effective spray opportunity.
Cultivation with the Lemken Helidor machine stimulates weed germination and breaks up the stubble.
Operating at 15–16 km/hr, the 12 m wide cultivator also breaks up and spreads the stubble and throws soil over some of the stubble to aid decomposition. The cattle still have access to valuable feed in paddocks where the Lemken has been used. This light cultivation means there is no need to burn stubble and seeding the next crop is easy.
“Summer cropping opportunities seem to be more common in recent years and we have had success with both cowpeas and sorghum,” said Lou. “The Supa Sudan sorghum we grew this past summer came back three times and provided excellent feed value but it also used too much soil moisture and might limit our immediate cropping choices for that paddock.”
“In winter, forage barley is followed by TT canola then a grazing wheat with the option to graze, make hay or harvest the grain.”
In addition to annual ryegrass incursions the Clemsons have also seen black oats, wild radish and brome grass populations cleaned up through this combination of grazing and cropping.
 

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Making pre-em herbicides work in high stubble

Whether stubble is standing or laying flat on the ground it represents a challenge for farmers using pre-emergent herbicides to control weeds early in the cropping season.
Most growers and advisors are aware that products such as trifluralin are quite tightly bound if they contact stubble during application, however the behaviour of newer products has been largely unknown.
For pre-emergent herbicides to be effective the product must be placed in contact with the soil and must provide an even layer of chemical to intercept germinating weeds. Decisions at harvest will have a direct bearing on pre-emergent choices and efficacy for the next season.
Yaseen Khalil is a PhD candidate at the UWA School of Agriculture and Environment, studying the availability of pre-emergent herbicides applied to crop residue and then exposed to different rainfall simulations.
Decisions at harvest will have a direct bearing on pre-emergent choices and efficacy for the next season. Yaseen Khalil, PhD candidate at the UWA School of Agriculture and Environment, is studying the availability of pre-emergent herbicides applied to crop residue and then exposed to different rainfall simulations.
The three pre-emergent herbicides investigated were trifluralin, Sakura (pyroxasulfone) and Arcade (prosulfocarb). As expected, trifluralin was very resistant to leaching from crop residue with minimal amounts of the product washing off stubble in rainfall events simulated just one day after herbicide application.
On the other hand, a sufficient quantity of Arcade was leached by rainfall applied after 7 days to provide some control of annual ryegrass. Sakura was the clear ‘stand-out’ when it comes to efficacy of leachate. Product applied to 4 t/ha crop residue plots provided 100 per cent control of annual ryegrass when just 5 mm of simulated rainfall was applied 14 days after the herbicide application.
“The implications for growers are that they can rely on significant amounts of Sakura and Arcade being washed off crop residues and into the soil if there is a rainfall event of at least 5 mm within about one week of applying these pre-emergent herbicides,” says Mr Khalil. “If the crop residue is already wet when the pre-emergent herbicides are applied, the products are more tightly held and less product leaches off the residue in subsequent rainfall events, compared with spraying onto dry stubble.”
Mr Khalil’s supervisor University of Western Australia senior lecturer, Dr Ken Flower says an important factor to consider is the unevenness of residue spread at harvest. “While the residue level across the paddock may average out to an acceptable 3 t/ha it is common for the residue to be as high as 10 to 15 t/ha directly behind the header,” he says. “This in itself has implications when it comes to growers’ decisions about their weed management tactics.”
Australian Herbicide Resistance Initiative communication lead Peter Newman says getting the pre-emergent product through the crop residue and onto the soil at application time is still the priority.
“It is good to know that some products remain effective and can be leached off the crop residue with rainfall following the application,” he says. “Yaseen’s research has added to the growing knowledge bank about the most effective use of pre-emergent herbicides in no-till, stubble retention systems.”
Whilst quite extensive in itself, this study only compared three pre-emergent herbicides. The chemical properties of the over 50 active ingredients that possess pre-emergent, or residual, activity vary enormously, and they are found in 10 mode of action groups. The efficacy of these products relies on different environmental, soil and crop residue conditions.

“Understanding how these products can best fit into an integrated weed management program on a farm requires considerable thought, taking many, many factors into consideration,” says Mr Newman. “There are great opportunities to use these products to add diversity to weed management but it is essential that they are not over-relied on as 100 per cent weed control is uncommon, even when the conditions at application are as good as possible.”
“We used to talk a lot about rotating herbicides from one year to the next, now we are more focused on herbicide mixes as the best way to go — mixing two pre-emergent herbicides together where possible, and then perhaps rotate to another mix next year,” he says. “They say that rotating herbicides buys you time, but mixing herbicides buys you shots. Mixing and rotating buys you time and shots, which is the best that we can hope for.”
Mr Newman recommends growers take the time to discuss a variety of options with agronomists and take on board research like Yaseen’s, along with their own observations, to build a robust and ever-changing weed management program. “The aim is to keep overall weed numbers low as this is the best way to minimise the risk of herbicide resistance,” he says.
Other relevant resources

Podcast: Herbicides and stubble – some wash off, some don’t
AHRI Insight – Herbicide and stubble
GRDC Pre-emergent herbicides manual
What’s the safest way to manage pre-emergent herbicides at seeding

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Satellite imagery to detect and monitor weeds

Having ‘eyes in the sky’ checking crops every ten days is a useful tool for growers and agronomists monitoring crops and managing weeds. Satellite images provide a very objective means of identifying weedy areas and measuring the response to management tactics.
From his family farm in northern NSW, remote sensing tech enthusiast Ben Boughton runs a satellite imagery business, Satamap, utilised by growers and agronomists from across Australia.
Satamap founder and grain grower Ben Boughton uses satellite imagery to assist with the management of herbicide resistant weeds.
“Using the satellite images agronomists can look for anomalies in biomass production on their clients’ farms and can then go directly to the area on the farm to identify the problem,” says Ben. “In the case of weeds, it is easy to calculate the size of weedy patches and to monitor the impact of herbicide or cultural practices.”
“An example on our own farm was to clearly see the impact when we sprayed a paddock where awnless barnyard grass was a problem and were not able to get back with the second knock due to rain,” he says. “The recovery and spread of what we now know for sure was glyphosate resistant barnyard grass showed very clearly in the satellite images taken before and after the event.”
Glyphosate-resistant awnless barnyard grass is one of the Boughton’s main challenges, with a patch visible on satellite imagery following a fallow double knock where the second knock application was prevented.
Agronomists use the Satamap service to observe trends across their district and to make their farm visits more time efficient. In fallow paddocks the satellite images show the extent of vegetation (weed) growth that can be hard to see amongst tall stubble. The images show an average reflectance over 10 m by 10 m area so a similar sized patch on the map may represent a number of large individual plants spread out across the area, or a carpet of grass. That is the reason why ‘boots on the ground’ are still required to investigate exactly what species are present and to plan appropriate management.
“The satellite images are not a diagnostic tool on their own but they do provide valuable information for the grower and their advisor,” he says. “Some agronomists are even using imagery from the previous year to plan variable rate application of pre-emergent herbicide to treat areas where the weed seed bank could be expected to be high in paddocks with known problems.”
The use of satellite imagery links well with UAV (unmanned aerial vehicles, aka drones) to assist with the ground-truthing process across a farm.
The Boughtons property, north-east of Moree, NSW is predominantly winter cropping with up to 25 per cent summer cropping if the season permits. Ben says summer cropping is a great way to help clean up black oats coming out of winter cereals.
The farm is 100 per cent dryland cropping with no internal fencing and all paddocks set up in a 3 m controlled traffic configuration. Ben has found the tramlines can present a weed control challenge with black oats and phalaris taking advantage of the lack of crop competition.
The crop rotation is generally wheat, barley, chickpea to give a two year break for the chickpeas and avoid planting wheat on wheat.
“We aim for good, even crop establishment to generate strong crop competition early in the season,” he says. “This is even more important in paddocks where we dry sow and can’t do a pre-sowing knockdown on black oats.”
With Group A-resistant black oats on the increase Ben uses crop competition in barley to support a one-off use of Group A (Axial) and switches to Group B (Atlantis) in wheat. Ben is conscious of the risk of losing Group A efficacy altogether and is looking for alternatives to use in the barley.
“Sorghum is a good crop to combat Group A-resistant black oats using a glyphosate spray at the end of the winter fallow and planting in September,” he says. “Chickpea is also providing a useful crop for black oats control using Verdict and Select, both Group A, with high rates of oil and ammonium sulphate.”
Ben is using pre-emergent products such as Flame in the summer fallow but is very conscious of the impact these applications can have on crop rotation choices, particularly in years where the summer rainfall is limited.
“It is important to have a rotation plan worked out so you can take advantage of summer cropping opportunities and Flame can even cause crop damage in barley the following winter if there has not been enough summer rain,” he says.
Fleabane is another challenging weed to manage due to its natural tolerance of glyphosate and the need to be very careful with applications of Group I products, such as picloram in fallows, adjacent to neighboring cotton crops. Ben says Tordon 242 applied early in wheat crops to treat late germinating fleabane is providing very effective control with some residual activity.
 

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BYGUM: Barnyard grass management for cotton growers

Glyphosate resistant awnless barnyard grass is now a common problem in Australian cotton farming systems. Growers and researchers have identified management tools and practices that can reduce the impact of this weed, however the economic fall-out has been more difficult to ascertain.
Dr David Thornby, former weeds researcher with the Department of Agriculture and Fisheries and now consulting researcher with Innokas, has developed a computer modelling program that allows growers and agronomists to test barnyard grass management scenarios and assess the economic impact prior to implementation, across a 5-year rotation.

“The ecology and seed bank behaviour of awnless barnyard grass is quite well understood,” he says. “Getting a handle on the economic value of different management strategies is a useful feature of the Barnyard Grass Understanding and Management [BYGUM] decision making tool. The analysis also shows the yield penalty incurred in each crop as a direct result of herbicide resistant barnyard grass incursions and the effect different management strategies would have on the weed seedbank.”
The BYGUM program was adapted from the Australian Herbicide Resistance Initiative’s Ryegrass Integrated Management [RIM] tool, which fulfils a similar purpose for assessing management scenarios for resistant annual ryegrass in southern cropping zones, but does not include parameters for cotton production or fallow management.
“We had 15 years of weed ecology and seed bank research to draw on to adapt the RIM model to suit a barnyard grass in a sub-tropical environment growing both summer and winter crops,” says Dr Thornby. “We also had large data sets for herbicide efficacy and the effect of other weed management practices on barnyard grass populations.”
BYGUM provides growers with a robust means to evaluate five-year rotations including testing the economic value of fallows and fallow weed management, winter and summer crop sequences, cover crops, tillage, harvest weed seed control, different herbicide options and more.
The computer program has a simple step-through wizard design for users to:

Define the basics (prices, costs, herbicide options, base yields)
Build a rotation (five years) and specify weed controls
Check the results and compare with other scenarios

When using BYGUM to assess a current or proposed strategy, users can enter parameters about the size of the seed bank and the cost, type and efficacy level of herbicides. They can easily test what will happen if the chosen herbicide doesn’t work or show the effect of using a different herbicide or another weed management tool.
“The cost of managing resistant barnyard grass can be assessed across different strategies and under different seasonal conditions,” he says. “The user can define many different parameters or use the default settings, including commodity prices and yields.”
The non-herbicide management tools included in the model are harvest weed seed management (for the grain phase of the system), cover crops and tillage. Dr Thornby says brown manuring millet and leaving it standing for improved moisture conservation is an effective tool that growers could investigate. He says tillage operations used in cotton systems for the purposes of pupae busting and bed formation generally have little effect on barnyard grass populations or the seedbank.
“Tillage that is timed to control barnyard grass soon after emergence can be very effective,” he says. “A portion of the seed that is present in the soil however will remain viable for 12–18 months if buried to a depth of 10 cm and seed can germinate from a depth of 5 cm given the right conditions.”
Over winter, there is significant mortality of seed in the seed bank, which means tillage following early germinations in spring can really drive down the seed bank as the majority of the seed present has germinated and very few viable seeds are buried. In most years it is safe to assume that five barnyard grass cohorts will germinate between September and March, largely in response to rainfall and temperature conditions.
“In-crop, barnyard grass plants that germinate in the first cohort will have the greatest opportunity to establish using abundant moisture and nutrient resources and produce large quantities of seed before the crop is able to compete,” he says. “Later cohorts tend to be shaded out by the crop and produce far less seed.”
Although BYGUM has been developed specifically for analysing control options for BYG in cotton it can also be used for other summer crops, particularly sorghum, and for summer fallows. Dr Thornby says it also has application for other summer grasses, such as feathertop Rhodes grass, but suggests that users carefully check the herbicide efficacy parameters. It is also not recommended for use on other grasses in Central Queensland farming systems.
BYGUM has been developed with the financial and intellectual support of CRDC, UWA, GRDC, QAAFI, DAF and UQ. It has been tested by leading cotton growers and agronomists.
Relevant links

Try BYGUM for yourself
Sample scenarios tested in BYGUM

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Don’t start mixing until the water quality is right

Water quality is often overlooked as a possible contributor to herbicide failure and can lead to confusion over the herbicide resistance status of weeds on a property. The careful management of spray events is highlighted in the The Big 6 for managing herbicide resistance.
Spray application specialist, Craig Day of Spray Safe and Save at Cowra, NSW says water should be considered as one of the chemicals in any mix, given that water quality varies markedly depending on its source.
Spray application specialist, Craig Day of Spray Safe and Save at Cowra, NSW says water should be considered as one of the chemicals in any mix, given that water quality varies markedly depending on its source.
“It would be great if all herbicide sprays were applied using rainwater but that is often not practical,” he says. “The pH, hardness, electrical conductivity and dissolved solids in water all interact with the herbicides and adjuvant products in a mix. It is essential that these parameters are all addressed before any crop protection products are added to the water.”
Mr Day says a water test is a cheap way to ensure that the herbicide applied will be as effective as possible in a weed control program. “Generally, water is considered hard when the calcium carbonate levels exceed 300–400 ppm,” he says. “If glyphosate is added to hard water, the calcium and glyphosate ions react, effectively reducing the uptake of the glyphosate into the plant. By adding ammonium sulfate to the water, the positively charged calcium ions bind with the negatively charged sulfate ions. When glyphosate is added to properly agitated and dissolved ammonium sulfate solution the glyphosate does not bind to calcium ions.”
It is critical that no undissolved ammonium sulfate, via a handling system or within the spray tank, comes into contact with 2,4-D amine formulations. The resulting precipitate is irreversible.
“Hard water will affect formulations of 2,4-DB, glyphosate, 2,4-D amine, Lontrel Advance and Tigrex. If hard water is used with these formulations, there is a greater potential for a reduction in the effectiveness of the chemical,” he says.
Water with a pH over 8 supports alkaline hydrolysis, which can affect the efficacy of many herbicide and insecticide products, and is often associated with hard water. Like the example of 2,4-D in hard water, chemicals tend to separate out over time in a high pH environment and the mix become less effective over time.
“The trend toward larger spray tanks means that product is in the tank longer and will be adversely affected when there is a high pH,” says Mr Day. “At a pH of 8, which is common for tap water in many areas, many products will only remain fully potent for a matter of 1.5 or 2 hours at the most. The use of an acidifying surfactant helps lower the pH to an optimal 4.5 or 6.”
“Aside from herbicides, organophosphates and carbamates are particularly susceptible to alkaline hydrolysis so pH needs careful consideration when spraying aphids with a carbamate formulation.”
When obtaining a water quality test, Mr Day suggests that growers ask specifically for the presence of bicarbonates in water to be included in the report. “A bicarbonate level of 75 ppm and above will lead to reductions in the efficacy of 2,4-D amine and Group A formulations,” he says. “Ammonium sulfate can be used to reduce the effect of bicarbonates on Group A herbicides, but will produce little improvement on 2,4-D amine formulations.”
Both total dissolved salts and saline water cause stability issues that can result in separation and blocking of filters, especially if there is insufficient agitation during the mixing process. Also, high salt levels in water will resist manipulations of pH.
Mr Day recommends growers get their water source tested regularly. “If you take two samples and retain the second jar of water you can use it to calibrate your eye using a pool and spa testing kit and the test results. This can become an ongoing monitoring tool,” he says.

Another problem Mr Day commonly sees is the incorrect use of batching tanks, where products are concentrated in small tanks without sufficient water. He suggests that growers avoid having the entire tank load amount of acidifying adjuvant in direct contact with other chemistry and that 2,4-D amine is never allowed to sit on undissolved ammonium sulfate.
“Similarly, high concentrations of glyphosate and 2,4-D amine in a batching tank can cause the glyphosate to lose its effectiveness,” he says. “It is very important that water is conditioned first, and that sufficient water and ample agitation is used when preparing a spray load. Time is always against growers but there is no point hurrying just to apply a load that won’t work because the chemistry is wrong in the tank.”
As new products come to market, Mr Day suggests growers ask their advisors where the product fits in the mixing order and to highlight any interactions with water quality parameters. “Ask your advisor to provide the correct mixing order when writing the advice sheet,” he says. “You will then have this advice to follow and can record the mixing order on file. This information, and a water quality test, will help unravel any issues with a spray’s efficacy and, in combination with a herbicide resistance test, assist with developing a robust herbicide program.”
The GRDC GrowNote Herbicide Use technical manual provides detailed information about the water quality and adjuvant requirements for each herbicide MOA group.

More resources:

GRDC-GrowNotes-Herbicides – provides water quality and adjuvant advice for use with each MOA group.
NSW DPI water quality and herbicide interaction table (see below) in ‘Weed control in winter cropping’ publication
Spray Application GrowNote video playlist

From the NSW DPI ‘Weed control in winter cropping 2016’ publication. http://www.dpi.nsw.gov.au/agriculture/broadacre-crops/guides/publications/weed-control-winter-crops

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Spray small multi-resistant wild radish twice

Western Australian growers are regaining control over herbicide resistant weeds as a result of widespread adoption of integrated weed management systems.
Australian Herbicide Resistance Initiative communications manager, Peter Newman, says many crops are cleaner than ever and growers should be acknowledged for their consistent efforts over many years to implement a wide range of herbicide and non-herbicide tactics to drive down the weed seed bank on their farms.
“There was a serious problem here over the last two decades and growers have now made managing herbicide resistance in weeds a component in every farming system,” he says. “We were caught once, and had to learn the hard way. Hopefully growers in other areas will be able to take advantage of the experience and research that has helped WA growers get back in the driver’s seat with weed control.”
One piece of research that the RSCN in Geraldton, WA requested that DAFWA and then Crop Circle Consulting and Landmark conduct in 2012 and 2013 has given growers additional confidence when managing herbicide resistant wild radish.
“The objective of the 2-year study was to identify paddocks that had high populations of wild radish that was resistant to several herbicide groups,” says Mr Newman. “Although Velocity® (pyrasulfatole + bromoxynil, Group H/C) was working well in most situations, growers were aware that if there was over-reliance on the one product that soon enough that option would be gone.”
“Some growers were using two or three shots of Velocity® in a single growing season and they had little confidence in other herbicides due to their radish being multiple resistant.”
The first year trial achieved a 22 to 36 per cent yield improvement when wild radish was controlled and the researchers established that the key strategy was to spray the wild radish as early as possible while avoiding any crop damage.
“Applying Velocity at the 2-leaf stage of the wheat crop is safe for the crop and kills the majority of the small – 1 to 2 leaf stage – wild radish plants present,” he says. “When this is followed with a second spray of a herbicide from another mode of action at the crop’s 5-leaf stage, when the radish is no bigger than 4-leaf stage, the level of control is very high even in multi-resistant wild radish populations.”
Spraying small radish twice is a proven tactic to combat multi-resistant populations.
This strategy was tested on three properties in the Geraldton area in 2013 and proved very effective. Grant Thompson from Crop Circle Consulting and Landmark led the trial work in 2013 and says the result that was the most surprising was that there were many options available for use as the second spray.
The paddocks chosen for the trials all had known resistance to a wide range of herbicides from mode of action groups B, C, F and I.
“Provided the paddock was sprayed early with a product that did not damage the crop, it didn’t make much difference what herbicide was used for the second spray,” he says. “This was a great result for growers as if gave them more opportunities to rotate modes of action across the whole rotation.”
Even multi-resistant wild radish plants succumb to a second herbicide spray provided the first spray is applied when the radish plants are small.
“For example, growers may choose to use Jaguar (Group F/C) as the first spray in cereals and avoid using Velocity twice in the one season,” he says. “Jaguar applied at 1 L/ha when the wheat is 2-leaf and the radish is 1 to 2-leaf is giving excellent control of the first cohort of weeds in most situations.”
The trials also highlighted the importance of achieving excellent coverage with each spray. Mr Thompson stresses the importance of using the correct adjuvant, appropriate nozzles and not skimping on the water rate.
Control improves yield and minimises costs
Treating the wild radish early was very important for preserving yield. Even with one early spray, all registered treatments doubled the grain yield over the nil treatment. The second spray helps achieve optimal weed control while remaining profitable.
In the two-spray system the herbicide applications cost $19 to $29 per ha and achieved a yield benefit worth an average of $90–130/ha, and as high as $260/ha in very weedy situations.
Knowing the herbicide resistance profile of the weeds present helps growers to choose the mode of action herbicide most likely to give the best results. Herbicide resistance testing and an array of integrated weed management strategies are detailed on the Weedsmart website.
When followed with a harvest weed seed control tactic such as narrow windrow burning or chaff cart, the contribution of new seed to the weed seed bank can realistically be zero. Consistently applied over several seasons can see the weed population driven right down and, if there are no weeds, there are no herbicide resistant weeds.
Wild radish flourishing in each control patch in the trial – illustrating the multi-resistance traits in the population at this site.
Relevant links

Webinar with Peter Newman and Grant Thompson
GRDC Update paper ‘Spray resistant radish early for best efficacy and yield’
Wild Radish factsheet

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Best seed, best establishment, fewer weeds

Crop competition is an important strategy in the Weedsmart Big 6 to manage herbicide resistance – but it’s not all about row width and seeding rate – the size of the seed sown makes a difference too.
Rohan Brill, Research and Development agronomist, NSW DPI based in Wagga Wagga, along with colleagues at Trangie and Tamworth, has been teasing apart whether the improved early performance of canola hybrids over open-pollinated (OP) cultivars comes from the generally-larger seed size of hybrids or from hybrid vigour.
Rohan Brill, Research and Development agronomist, NSW DPI based in Wagga Wagga, says most of the improved early growth in canola hybrids lies in the larger seed size, with heterosis providing an additional benefit. Sowing large canola seed, regardless of the cultivar, is key to strong early crop growth and the crop’s ability to compete with weeds. (Photo: Gregor Heard)
“It seems that most of the improved early growth in hybrids lies in the larger seed size, with heterosis providing an additional benefit,” he says. “Our study showed that sowing large canola seed, regardless of the cultivar, is key to strong early crop growth and the crop’s ability to compete with weeds.”
As a reliable rule of thumb, Mr Brill recommends cleaning and grading all farmer-retained OP canola seed to collect planting seed that is 2 mm in diameter or larger. In both glasshouse and field experiments following this rule led to improved open-pollinated TT canola emergence and early biomass production. Seed size had a greater effect on early biomass production than did cultivar type (hybrid vs OP).
“A 10 per cent difference in seed diameter produces a 33 per cent difference in seed volume, so it is very important not to skimp on the 2 mm diameter rule,” he says. “There was a stark contrast between plots sown with 2 mm seed compared to 1.8 mm seed. It is very hard to tell the difference visually between these two seed sizes but the visual difference in early vigour and biomass production was very clear.”
A 10 per cent difference in seed diameter produces a 33 per cent difference in seed volume, so it is very important not to skimp on the 2 mm diameter rule. There was a stark contrast between plots sown with 2 mm seed (left) compared to 1.8 mm seed (right).
“This trial has shown that grading planting seed to extract the largest seeds is a far better way to increase crop competition and yield than increasing the seeding rate of smaller seed,” he says. “For example, a grower might take 30 tonne of grain from a clean paddock and screen it hard at harvest to take out 2 tonne of the best seed and sell the rest. Then store the 2 tonne of seed in a cool place before grading it (including using a gravity table) to 2 mm and above prior to planting. With the larger but still relatively cheap seed, a sowing rate of at least 3 kg/ha is recommended, resulting in a plant stand of 30 plants/m2 even at an establishment rate of 50 per cent. There is little penalty from sowing at even higher sowing rates, as DAFWA research found that too few plants has a greater economic penalty than too many plants.”
The DAFWA canola seed size ready-reckoner is an excellent resource for growers to use and is available on the Department’s website. Simply line up retained OP canola seed along a 10 cm section of a ruler and count the number of seeds. To achieve best results there should be 50 seeds or less to take up the 10 cm length. Use this information to calculate seed rate.
“Sowing larger seed is essential if planting early, in early-mid April rather than mid-May, to maximise grain yield potential in canola,” he says. “The larger seed can be safely sown slightly deeper than optimal for each soil type and still achieve reliable crop establishment.”
Sowing in early-mid April into residual soil moisture gives the crop a head-start on weed germinations and the strong crop growth provides valuable support for the pre-emergent herbicides. The only catch is to make sure that the cultivar selected for sowing early will flower in the optimal window for your district.
Larger seeds possess more energy resources than smaller seeds, enabling the seedling to push through more soil and stubble and to grow past insect damage and outcompete weeds. While planting larger OP cultivar seed will help close the gap in early vigour between OP and hybrid cultivars, the herbicide tolerant traits of hybrids continue to provide other benefits.
Mr Brill says the extra effort to grade seed to 2 mm will see an improvement in yield provided attention is also given to good soil nutrition, timing and harvester speed. “Driving the header too fast has been shown to cause large yield losses,” he says. “This represents a direct loss of income and the volunteer canola plants suck moisture from the soil, potentially compromising the early sowing opportunity for the following season.”
Crop competition is the cheapest form of weed control and it usually comes hand in hand with improved yield; a genuine win all-round.
Sown side-by-side, one hour apart, these Gem canola plots were sown at 2.6 kg/ha with 80 kg/ha MAP. The only difference is that the farmer retained seed on the left was graded to 2 mm while the plot on the right was sown with purchased seed, straight from the bag. (Photo: Warwick and Di Holding, Pontara Grain, Yerong Creek)
Related resources

Watch a recording of Rohan’s full webinar presentation

Estimating the size of retained canola seed

DAFWA canola seeding rate information

Competitive canola cultivars

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Multi-resistance is game changing for herbicide tolerant crops

American farmers seem to have regained the upper hand on glyphosate resistant Palmer amaranth and waterhemp, which had overwhelmed many crops from 2008 to 2012, but there are still some longer term challenges.
Jason Norsworthy, professor of weed science at the University of Arkansas says that although many paddocks are cleaner than they have been for years, he doesn’t believe that the current weed management practices are sustainable.
Jason Norsworthy, professor of weed science at the University of Arkansas says many growers in the States are now only one or two herbicides away from having no herbicide control options for Palmer amaranth (pigweed) in either conventional or herbicide tolerant soybean crops.
In the USA, herbicide tolerant varieties and hybrids have come to dominate in the sown areas of cotton (89%), soybean (94%) and corn (89%) across all regions. The soybean and corn growing regions overlap significantly and there are some regions where all three crops are grown.
Dr Norsworthy says many growers in the States are now only one or two herbicides away from having no herbicide control options for Palmer amaranth in either conventional or herbicide tolerant soybean crops. “Four modes of action are already lost on many populations of Palmer amaranth,” he says. “The multiple-resistance to these four modes of action is a real game changer for growers. It is simply not sustainable to use a herbicide or a technology until it breaks and expect to be able to switch to another option.”
The practice of continuing to use a herbicide after resistance had established is one of the key reasons why many growers are in a serious predicament, according to Dr Norsworthy. He says that ALS (Group B) resistance has inevitably led to glyphosate (Group M) resistance, which has in turn led to PPO (Group G) resistance. It is only a matter of time before PPO (Group G) resistance is also widespread in weeds like Palmer amaranth.
“PPOs are currently being applied both at planting and in-crop in soybean crops where there are few other herbicide options,” he says. “This high selection pressure will inevitably lead to failure of the herbicide leaving growers with dicamba [Group I] products as their last resort.”
The use of low rates of dicamba in corn crops has already exposed many weed populations to low dose selection to this mode of action and researchers have demonstrated full dose dicamba survivors can evolve in just three generations in Amaranthus spp.
In Australia, there are almost as many species resistant to glyphosate (13 species) as are present in the United States (15 species); however, there are not as many herbicide tolerant crop options.
Dr Norsworthy suggests that Australian growers continue to adopt diverse weed management practices, including harvest weed seed control, competitive crops, strategic tillage and pasture phases rather than over-relying on herbicide tolerant hybrids as they come available. “We are encouraging American growers to look more toward non-herbicide practices to try to break the cycle of over-reliance on single herbicide modes of action,” he says. “Once multiple resistance takes hold there really are less options available and most of them will have a production penalty, but if these practices are not adopted then production systems will fail.”
Australian Herbicide Resistance Initiative (AHRI) director, Stephen Powles in a soybean trial plot in America overwhelmed with herbicide resistant Palmer amaranth (pigweed), a prime candidate for harvest weed seed control.
Related information

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