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Early intervention: patch management of herbicide resistance

Local knowledge and early intervention can slow the development of herbicide resistance in weeds. This is the view of DAFF Queensland senior research scientist, Dr David Thornby, who has been using computer modelling to investigate the way herbicide resistant populations become established on farms.

“Several management strategies have been developed that are effective in reducing the weed seed bank even after herbicide resistant weeds have become widespread on a farm, though they are most effective if they are implemented before resistance is widespread,” he said. “Our modelling shows that, under the right conditions, local eradication of resistant biotypes is possible, provided action is taken early and the weed species is not very mobile.”

The Spatial Herbicide Resistance Analyser (SHeRA, developed with funding assistance from the Cotton Research and Development Corporation) is used to predict the effect of management tactics on the fates of individual weeds and the movement of seeds and pollen. “Our main interest is in glyphosate resistance but the principles are the same for other herbicides,” he said.

“When patches of resistant weeds are small, up to 100 m2, growers can readily consider intensive management options that would be considered too expensive or risky to be done over the whole paddock or the whole farm.”

Dr Thornby is suggesting that growers become more conscious of the signs of herbicide resistance. “Glyphosate is such a widely used product that a large proportion of growers are likely to have been selecting for glyphosate resistance over many years,” he said. “Other than in very high value and high input systems, glyphosate is relied on as a relatively inexpensive method of controlling a wide range of weeds, particularly in no-till systems.”

“In dryland cropping systems glyphosate-resistant awnless barnyard grass is a significant problem and sow thistle and sweet summer grass are emerging as being at risk of developing resistance.”

Weeds with high rates of reproduction are the most likely to develop resistance. In addition to using strategies to reduce the risk of herbicide resistance, such as rotating chemical modes of action and including non-chemical weed control methods, Dr Thornby is urging growers to assess the efficacy of each herbicide application.

“Herbicide resistance can start with one resistant plant that survives the chemical application,” he said. “If that single plant is noticed and removed before setting seed then a large problem can theoretically be averted. The difficulty is in finding the surviving plants and having strategies in place to remove them immediately.”

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SHeRA is used to predict the likely expansion of a herbicide resistant patch through seed dispersal and pollen flow to surrounding susceptible weed populations. The management practices of the grower can restrict or assist the expansion of the patch. For instance, continuing to use glyphosate alone on resistant weed populations promotes rapid expansion while intense intervention within and around the patch, such as double knocking, chipping or cultivating can contain, and potentially eradicate, the resistant population.

“Because of gene flow in pollen it is important to actively manage the patch and the area surrounding it,” said Dr Thornby. “The patch itself can be considered the ‘eradication zone’, the area surrounding it might be the ‘containment zone’ and the rest of the paddock might be managed using best practice spray techniques and agronomy. This way the risk of widespread herbicide resistance is contained while not adding significantly to the production costs.”

“This approach relies on finding patches while they are a manageable size and taking action immediately.”

Herbicide resistant patches frequently occur in non-cropped areas such as along fencelines, irrigation channels and around farm infrastructure so these ‘hot spots’ need close monitoring so they do not become a source of resistant seed that could be carried into cropping areas.

If patches of resistant weeds are evident inside a paddock, chances are that it is the in-crop or fallow weed management practices that are the main cause. Dr Thornby suggested that cultural practices need to be given more consideration as weed control methods to avoid over-reliance on herbicides.

“Minimum tillage has many benefits but there is a place for cultivation to control weeds,” he said. “But cultivation alone is not the answer either because some weeds prosper with soil disturbance. The routine use of harvest weed seed control methods, growing competitive crops and using crop rotations to manage weeds are also very important and require an understanding of weed ecology and the use of local experience.”

“When choosing which crop type and variety to grow in rotation, weed management and herbicide resistance need to be part of the decision,” he said. “Non-competitive crops provide resistant weeds with an opportunity to spread, which then limits your crop choices and increases production costs in the future.”

Strategic management of patches is unlikely to be effective against highly mobile weeds. Some weeds, such as fleabane are mobile because their seeds are widely transported on the wind. Other weeds, such as ryegrass have high rates of gene flow in pollen and are likewise considered highly mobile.

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