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How to manage annual ryegrass in chickpea crops

with Bhagirath Chauhan, weeds researcher, QAAFI

Annual ryegrass is becoming increasingly prevalent in the northern cropping region, and many populations already have a high level of resistance to the major Group B and Group A herbicides registered for use in chickpea crops.

To keep this important crop as a viable option, growers are looking for ways to add non-chemical in-crop options to an integrated weed control program to prevent a yield-limiting blow-out in ryegrass populations.

QAAFI weeds researcher Baghirath Chauhan has completed several studies to investigate ways to make pulse crops, including chickpeas and mungbeans, more competitive against weeds.

The principles of crop competition are fairly well known but making the necessary changes to planting gear can be daunting, so it is important to know that any changes will achieve the desired effect.

To assist growers to better implement crop competition in chickpeas, A/Professor Bhagirath Chauhan, principal research fellow and weed team leader, QAAFI, UQ looked at the effect of narrow rows, variety and early weed control to assess which was the most powerful suppressant of annual ryegrass.

“In a weed-free environment, it has been shown that narrow-sown chickpeas will produce higher yield, so we wanted to see if narrow sowing also suppresses weed growth and seed set,” says Bhagirath. “We also wanted to understand whether a more-prostrate variety like PBA Seamer would suppress more weeds than the more-erect PBA HatTrick. The third aspect we considered was the effect of weed infestation at different growth stages of the crop.”

The results were pretty conclusive: PBA Seamer sown at 25 cm and kept weed-free for at least the first three weeks after planting is a winning combination for ryegrass control.

Crop competition is one of the WeedSmart ‘Big 6’ tactics that will be the centre of discussion at the two WeedSmart Week events this year, one in Emerald from 13 to 15 August and the other in Horsham from 27 to 29 August. To register go to www.weedsmart.org.au/events

What is the best way to reduce ryegrass growth and seed set?

Short answer: Narrowing the row spacing and ensuring good early weed control are the most effective tactics in chickpeas.

Longer answer: Plant architecture made some difference, but only in very weedy conditions. Narrowing row spacing from 75 cm to 25 cm reduced weed biomass by 16 per cent and reduced seed set by 26 per cent.

Keeping the crop weed-free for at least three weeks had the biggest effect, driving down weed biomass at the end of the season by 52 per cent and weed seed set by 48 per cent. This shows that, once established, chickpea can hold its own against weeds that emerge later in the season.

Can more competitive crops also produce higher grain yield?

Short answer: Yes. If you can’t do narrower rows then put an emphasis on early weed control.

Longer answer: Averaged across both cultivars and all weed infestation periods, sowing chickpeas on 25 cm row spacing (same seeding rate) produced 20 per cent more grain than sowing on 75 cm row spacing. This is most likely due to the crop plants being more evenly spaced and able to better exploit the available soil and light resources.

This research was conducted across two growing seasons and demonstrated that controlling annual ryegrass for the first three weeks after planting increased crop yield by a whopping 200 per cent compared to the season-long weedy scenario. Annual ryegrass that emerges 6 weeks or more after planting does not impact on chickpea yield, but if allowed to set seed, can contribute to the weed seed bank present at seeding the following year.

Is annual ryegrass a serious weed in chickpea?

Short answer: Yes, annual ryegrass is a yield-limiting weed and is adapting to farming systems further north than its traditional range.

Longer answer: Averaged over row spacing and cultivar, the penalty attributable to annual ryegrass was 1.2 t/ha less grain yield between the weed-free plots (1.8 t/ha) and the season-long weedy plots (0.6 t/ha). Without any competition, season-long weedy plots produced more than 129 annual ryegrass seed spikes per m2.

By planting PBA Seamer at 25 cm row spacing and keeping the crop weed-free for three weeks, the number of annual ryegrass spikes is reduced to just 8 per m2.  

How can I achieve this early weed control?

Short answer: Start the year ahead in the paddocks you plan to grow chickpeas and do everything possible to reduce the ryegrass seed bank using effective herbicides, weed seed burial, competitive cereals and harvest weed seed control tactics or hay-making. Back this up with registered pre-emergents for chickpea and as many non-herbicide tactics in-crop as possible.

Longer answer: Annual ryegrass is a master at evolving herbicide resistance. In southern regions it has evolved resistance to the registered in-crop herbicides for chickpeas. This will also occur in the northern region if steps are not taken to preserve the efficacy of Group A post-emergent chemistry across the crop sequence.

An over-reliance on pre-emergent herbicide use will also select for herbicide resistance, just as it has for post-emergent herbicides. To minimise this risk, it is important to use a diverse range of weed management tactics in-crop, such as crop competition, inter-row cultivation or chipping, to remove survivor weeds before they set seed. Where possible, rotate registered pre-emergent herbicide modes of action groups J, D and K between years and consider mixing pre-emergent modes of action groups where permitted, always at full label rates for all active components of the mix. ALWAYS READ AND FOLLOW LABEL INSTRUCTIONS.

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How do you manage summer weeds without spraying at night?

Concerns are being raised about the practical implications of this for summer weed control programs. Mary O’Brien, a private consultant with extensive experience in managing spray drift, is keen to see growers fully adopt spray application practices that maximise herbicide efficacy and minimise off-target drift.   Mary O’Brien says the ‘community drift’ that can occur when a number of applicators are each putting a small amount of product in the air at the same time can have very damaging effects on off-target sites. “The bottom line is that allowing spray to drift is like burning money,” she says. “Any product that doesn’t hit the target is wasted and the efficacy of the spray job is reduced, mildly resistant biotypes may survive as a result of low dose application and there is potential damage to sensitive crops and the environment.” “The difficulty is that many growers want to spray at night to cover more ground when conditions are cooler and potentially weeds are less stressed. Having a restriction on night spraying does restrict the time available to cover the areas required.” Having heard these concerns from growers across the country Mary keeps coming back to the fact that if there was a limitation to capacity at planting or at harvest, growers would scale up to get the job done in a timely manner. “Buying another spray rig or employing a contractor is an additional cost, especially after a couple of tough seasons, but I really think this is insignificant against the cost of losing key products and the resultant escalation in herbicide resistance to the remaining herbicides,” says Mary. “This problem is not confined to 2,4-D or even to herbicides. I recently spoke to a stone fruit grower who was forced to dump his whole crop after a positive MRL return for a fungicide he had never even heard of, let alone used.” What about just slowing down and lowering the boom during night spraying? Short answer: This, coupled with a good nozzle, will reduce drift but it will never eliminate it. Longer answer: The correct ground speed and boom height will have a large effect on the amount of product that remains in the air. The problem is that it only takes 1 per cent of the product remaining in the air to cause off-target damage. Once there are a few operators putting just 1 per cent of their product in the air at the same time, the amount of product quickly accumulates and can potentially be very damaging. Mary calls this ‘community drift’. Isn’t it better to spray weeds at night when it’s cooler? Short answer: Not really. Longer answer: Research by Bill Gordon showed that even if you keep everything else the same, night spraying can put at least three times more product in the air than daytime application, even if weather conditions are similar and there is no temperature inversion in place. The main difference between day and night is how the wind is moving across the landscape, rather than the wind speed. Under inversion conditions, the air moves parallel to the ground surface and this means that the product can move significant distances away from the target before coming to the ground. To achieve the best results through daytime spraying, applicators should focus on treating small, actively growing weeds. When there is good soil moisture, weeds are unlikely to be stressed even when the temperature is quite high. Temperature inversion conditions are more common at night and in the early morning. These conditions generate a laminar flow of air across the landscape allowing small droplets to travel many kilometres away from the target site before coming to ground. Can I use other products at night and just avoid using 2,4-D? Short answer: The current changes to 2,4-D labels has drawn a lot of attention but the problem is the same for all crop protection sprays – herbicides, fungicides and insecticides. Longer answer: Different products have different properties and some may work better at night but the problem is the sensitivity of some crops to certain products, such as 2,4-D. All products are tested for their efficacy and the label provides detailed information about the required spray quality and spray application conditions. Many products have explicit label instructions regarding wind speed, temperature inversions (or laminar flow) and night spraying. Given the high risk of drift at night, applicators need to be very confident that there is no inversion present, and weather conditions should be measured at least every 15 minutes to ensure wind speed remains above 11 kilometres per hour. An on-board weather station is the best way to monitor conditions. A visual demonstration using smoke to simulate the the lateral movement of small spray droplets when a temperature inversion is in place. What can I do to improve spray efficacy and avoid spray drift? Short answer: If you do just one thing – change your nozzle. Longer answer: All the factors that increase drift also reduce efficacy. To improve efficacy and reduce drift, use a better nozzle (larger spray quality) and appropriate water rates (matched to spray quality and stubble load), slow down and keep the boom low. Wind is required to push product downward and onto the target, and remember that the 3–15 km/h wind speed is for day time conditions only, this does not apply at night.
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Does ambient temperature affect herbicide performance?

with Chris Preston, Associate Professor, Weed Management
 at 
The University of Adelaide Temperature affects the absorption, translocation and metabolic degradation of herbicides applied to plants. Herbicides applied under the wrong conditions can appear to fail, however the reason may not be herbicide resistance. Dr Chris Preston, Associate Professor, Weed Management
 at The University of Adelaide says most herbicides have a temperature range at which they are most effective in controlling target weeds. “Applying herbicides outside the optimal temperature range is likely to contribute to a spray failure, even in susceptible populations,” he says. “Alternatively, applying herbicides within the correct temperature range can improve the control in populations known to have a level of resistance to that herbicide.” Dr Chris Preston suggests testing whole plants rather than seed for responses to a range of post-emergent herbicides. The Quick-Test is conducted in the same growing season as herbicide will be applied so the testing will occur under similar conditions to field conditions. Dr Preston says the effect of frost on the efficacy of clethodim is a striking example. Spraying clethodim in non-frosty conditions achieves vastly better results than spraying after three days of frost, even on populations that are resistant to this chemical mode of action. “Combining the optimal temperature with optimal weed size will give the best results possible,” he says. “The current common practice of applying clethodim to tillered ryegrass in the coldest months is not making the best use of this herbicide.” As a general rule of thumb, Group A (fops), paraquat (Group L) and glyphosate (Group M) are more effective at lower temperatures while Group A (dims), atrazine (Group C) and glufosinate (Group N) are more effective at higher temperatures. However, weeds that are resistant to paraquat become less resistant in warmer temperatures. “The other implication of this research is the effect of ambient temperature on herbicide test results,” says Dr Preston. “Seed collected in winter and grown out in the glasshouse in summer will be tested for resistance in conditions that are not representative of field conditions when growers are next treating that weed species. The Quick-Test using whole plants overcomes this problem and improves the reliability of herbicide susceptibility testing.” How can I get the best performance out of clethodim? Short answer: Avoid applying clethodim during frosty periods. Longer answer: Twice as much clethodim is required to kill susceptible annual ryegrass if the product is applied after three days of frost. Even higher rates are required if the plants have resistance to clethodim. Planning to apply clethodim for grass control outside the coldest months of June and July, and avoiding night spraying in winter, will see better results in both resistant and susceptible populations, particularly in tillered plants. Clethodim is most active when temperatures are over 20 degrees C. Weed seed that is tested during summer may return false negative results, which could translate into spray failure in the field the next season. Twice as much clethodim is required to kill susceptible annual ryegrass if the product is applied after three days of frost. Even higher rates are required if the plants have resistance to clethodim. When it is it too hot for glyphosate? Short answer: Efficacy is much better at 20 degrees C than at 30 degrees C. Longer answer: Spraying glyphosate resistant barnyard grass at lower temperatures is more effective than under hotter conditions. If barnyard grass is tested for herbicide resistance during the cooler parts of the year it may appear susceptible to the field rate of glyphosate but then when this rate is applied to the population in summer there may be many survivors. When glyphosate is taken up rapidly it tends to limit its own translocation, which can mean that although symptoms may appear more rapidly in warmer temperatures, plant kill is less reliable. Which herbicide resistance test should I use? Short answer: The weed resistance Quick-Test for post-emergent herbicides. Longer answer: The Quick-Test involves testing whole plants rather than seed for responses to a range of herbicides and rates. The Quick-Test is conducted in the same growing season as herbicide will be applied so the testing will occur under similar conditions to field conditions. The results of the Quick-Test are available within the same season, potentially giving growers an opportunity to apply an effective weed control tactic before the end of the season. The Quick-Test is not available for many pre-emergent herbicides. The Quick-Test is available through Plant Science Consulting and results are normally available after four weeks. Relevant links Maximising clethodim performance and the impact of frost fact sheet Keeping clethodim working in broafleaf crops Plant Science Consulting herbicide resistance testing – Quick-Test GRDC Update Paper – New developments and understanding in resistance mechanisms and management

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