US - Farmers are reporting the presence of glyphosate herbicide-resistant weeds in the US, especially in soybeans, but simulations have shown that farmers can benefit economically from persuading neighbours to manage resistance.
Glyphosate, usually known by many trade names, including Roundup, has been the most widely used herbicide in the United States since 2001, and it effectively controls many weed species.
However, the use of genetically-modified glyphosate-tolerant crop varieties as well as over-reliance on the herbicide has led to glyphosate resistance in weeds.
14 glyphosate-resistant weed species currently affect US crop-production areas, and they can reduce crop yields and increase weed-control costs. Recent surveys suggest that the amount of affected cropland is increasing.
Growers have reported glyphosate weed infestations on 5.6 per cent of corn acres in 2010 and declines in glyphosate effectiveness in about 40 per cent of soybean acres in 2012, with the majority of those acres in the Corn Belt and Northern Plains of US.
Glyphosate resistance management led to higher yields
Simulation results over a 20-year period show that using herbicide resistance management practices can improve farm returns.
Herbicide choices that help manage glyphosate resistance differ from short-term herbicide choices that ignore glyphosate resistance in three important ways.
Choices that manage resistance use glyphosate during fewer years, often combine glyphosate with one or more alternative herbicides and avoid applying glyphosate in consecutive growing seasons.
As a result, glyphosate resistance is managed more cost effectively, and after about two consecutive years of managing resistance, the cumulative impact of the returns received exceeds that received when ignoring resistance.
Surveys show corn and soybean growers responded similarly to the presence of glyphosate-resistant weeds on their farms.
The most common survey response was to use other herbicides in addition to glyphosate. Growers used this practice on over 84 per cent of corn acres with GR weeds and on 71 per cent of soybean acres with reduced glyphosate effectiveness.
The next most common response was to increase the amount of glyphosate used. Growers used this practice on 25 percent of corn acres with GR weeds and 39 percent of soybean acres with reduced glyphosate effectiveness.
Corn and soybean growers who reported reduced glyphosate effectiveness realised lower returns than those who did not.
In addition, corn and soybean growers who used glyphosate alone received lower yields and returns than similar corn and soybean growers who used at least one other herbicide in combination with glyphosate.
Although the crop growers using more than one herbicide had higher production costs, the additional costs were more than offset by higher yields.
Economic incentives to persuade neighbours to manage resistance
Simulations showed that weed-seed dispersal from a field where crop growers ignore resistance when managing weeds could reduce the returns on nearby fields, and that the reduction could be larger for a nearby field where growers manage resistance than where they ignore it.
This result suggests that corn and soybean growers have an economic incentive to encourage neighbours to use resistance management practices and may also be aware of the incentive.
Some soybean growers in Arkansas and North Carolina are responding to such incentives and collaborating in the management of herbicide-resistant weeds.
TheCropSite News Desk
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