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Using fodder crops to manage resistant weeds?

with Simon Falkiner, Research Consultant, Falkiner Ag

No till, continuous cropping has seen some huge productivity gains in Australian agriculture however herbicide resistance in weeds is a real Achilles Heel.

Simon Falkiner, research consultant with Falkiner Ag says the rapid increase in herbicide resistance in the high rainfall zones is alarming and he recommends that growers seriously consider the value of pasture and fodder crops to regain control.

Simon Falkiner says herbicide resistance has occurred faster in the high rainfall zone of south-western Victoria than many growers realise. He suggests growers consider including a fodder crop in the rotation to drive down weed numbers in just a few years.

“There is no doubt that some growers are losing money on weedy crops,” he says. “Herbicide resistant weeds can and do cause negative gross margins even in harvestable crops.”

“Fodder crops are a seriously good option to clean up weedy paddocks and still generate income. For most growers, reintroducing livestock of their own or on agistment will take best advantage of the buoyant livestock market. Where the property is located close enough to dairies, feedlots or an export market, making hay or silage is another option.”

A two-year fodder or pasture phase with grazing and harvesting biomass prior to weed seed set will remove over 90 per cent of annual ryegrass seeds from the seed bank. Gaining control of wild radish takes longer due to the longevity of the seed in the soil however the same principles apply — do everything possible to stop seed set.

“There are several ways to make fodder and pasture production a profitable part of a cropping enterprise,” says Mr Falkiner. “Grazing offsets the cost of taking a paddock out of grain production but has little impact on weed numbers. The whole benefit for managing herbicide resistant weeds lies in controlling the late germinating in-crop weeds that survive grazing and chemical applications, and preventing them from setting seed.”

What is the best method to stop weed seed set in a fodder crop?

Short answer: The method doesn’t matter; just the timing and a double knock.

Longer answer: Hay, silage, spray-topping and green and brown manuring are equally effective at controlling annual ryegrass. It is essential that the crop is ended before the annual ryegrass sets any viable seed. If some viable seed is present, then silage is a safer option than hay to avoid transporting weed seeds to other locations. The correct timing of the operation generally does not allow for a grain or oilseed harvest. Using the feed on farm is the most profitable option at the moment and, as many growers say, the hay market can be very fickle.

Grazing animals do not play a direct role in the management of weeds. The real benefits lie in using the crop to generate income through livestock production and or harvesting the biomass before the weeds have set viable seed.

Which fodder crops and pastures are the best for weed management?

Short answer: The choice of crop, and the management of the crop itself, is less important than the timing of the operation to prevent seed set in weeds.

Longer answer: The choice of fodder species may take many things into account such as improved soil fertility, the range of in-crop herbicide options, livestock preference, marketing opportunities, biomass production and so on. Arrowleaf clover and perennial ryegrass converted to silage and brown manured Balansa clover and field peas all provided an 89 to 91 per cent reduction in annual ryegrass numbers. Forage oats (silage) and serradella (brown manure) were less useful for weed management but still achieved weed reduction over 80 per cent.

Using fodder crops to manage weeds.

How does fodder cropping compare to chemical fallowing or burning stubble for weed control?

Short answer: Stubble burning does not kill enough weed seeds. Chemical fallowing continues the heavy reliance on herbicides.

Longer answer: Fodder cropping provides more opportunities to include non-herbicide tactics into the cropping rotation. In the high rainfall zone, moisture conservation is less of a priority than in lower rainfall zones, allowing both summer and winter fodder cropping options, which can help drive down the seed bank. Even in high stubble load conditions, stubble burning does not generate the high temperatures for long enough to kill enough weed seeds on the soil surface. There is no doubt that weedy crops cost growers money, and even generate negative gross margins – a two-year break from cropping in paddocks infested with herbicide resistant weeds could be exactly what’s needed to solve a very costly problem.

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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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Testing for herbicide resistance

“Testing takes the guesswork out of the equation and gives farmers baseline information that they can use to monitor changes in the weeds on their farms,” he said. “If low level resistance is identified early there are many more management options available compared to situations where full blown resistance has taken hold.” Dr Boutsalis said the over use and over reliance on particular herbicides will unavoidably lead to herbicide resistance developing. “We often hear of farmers applying herbicide even though they are not sure if it will work,” he said. The $300 to $400 cost of testing is insignificant compared to the cost of wasted herbicide, lost production and the costs of driving down a large seed bank of resistant weeds. What herbicide resistance tests are available to farmers in Australia? Short answer: The ‘quick’ test using the whole plant and the ‘seed’ test. Longer answer: The ‘quick’ test uses plant samples collected on farm and sent to the laboratory. The plants are revived and planted into pots then tested against the required herbicides. The ‘seed’ test requires the collection of ripe seed, which is planted out at the laboratory. After dormancy has been broken and the seedlings have started to grow they are tested for their response to herbicides. Both tests are equally accurate. The ‘quick’ test can not test for resistance to some pre-emergent herbicides, such as trifluralin. Which is the most common test that farmers use? Short answer: The seed test. Longer answer: Collecting seed before or at harvest is the most common method used. The collected seed must be mature, from green to when the seed changes colour. Before harvest collect 30 to 40 ryegrass seedheads or several handfuls of wild oats seed. After harvest it is common to find seedheads still in the paddock or samples of contaminated grain can be sent for analysis. Where is the best place to collect samples? Short answer: From suspicious or high risk areas. Longer answer: Herbicide resistance can develop in high risk areas like fencelines or at random through a paddock. Visual observations and changes on the yield monitor in the header can indicate good places to collect seed. If collecting plant samples, look for weeds at the early tillering stage that appear to have ‘escaped’ previous herbicide treatment. Collect 50 to 100 small plants or fewer larger plants. Shake off the soil from the roots, place in a plastic bag and send to the laboratory. What’s involved in sending samples? Short answer: Pick, pack, register and ship. Longer answer: Each sample needs to arrive at the laboratory with suitable identification and instructions. Register the samples online to get a unique sample number and to provide the information required, such as which herbicides you want to test against. Plant Science Consulting and Charles Sturt University both offer commercial herbicide resistance seed testing. Find the details under Point 4 of the 10 Point Plan on the WeedSmart website.   How to ask a WeedSmart question Ask your questions about the spread of herbicide resistance, or any herbicide resistance management strategy, using this blog or using Twitter @WeedSmartAU.

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