In a new USDA Economic Research Service report, researchers examine the use of glyphosate on corn and soybean crops, and management of glyphosate-resistant weeds in both crops.
The report, The Economics of Glyphosate Resistance Management in Corn and Soybean Production, found that glyphosate resistance is more prevalent in soy crops than corn crops, and soybean production relies more heavily on glyphosate use.
The report also noted that managing glyphosate resistance is more cost-effective than ignoring glyphosate resistance, and there are economic incentives that stress cooperative use of resistance management practices.
According to the report, reliance on glyphosate, by many growers, as the sole herbicide to control weeds is "the primary factor underlying the evolution of [glyphosate-resistant] weeds" and ultimately, using herbicides with different modes of action, can result in fewer GR weeds.
Growers reporting data to USDA said that GR weeds are more prevalent in soybeans; and GR-weed infestations were found on 5.6% of the corn acres in 2010 and declines in glyphosate effectiveness in about 40% of soybean acres in 2012.
More tillage also is used on corn than soybeans, ERS said, which could contribute to the GR weed disparity between corn and soy fields.
Glyphosate resistance management
Managing resistance is more cost-effective than ignoring it, the USDA report said, based on a 20-year simulation of herbicide choices that are short-term fixes and more long-term herbicide choices that are resistance management tactics.
According to the report, 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
Simulations also show that weed seed dispersal from a field where resistance was ignored 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 neighbors to use [resistance management practices] and may also be aware of the incentive," the report said.
Additional findings of the corn and soybean glyphosate study include:
• Glyphosate-tolerant varieties were planted to more soybean than corn acres
• Much more glyphosate (pounds of active ingredient) was applied to soybean than to corn fields;
• Herbicide use practices were consistent with glyphosate-resistance management on fewer soybean acres (60%) than on corn acres (82%); and
• Glyphosate-resistance management was more likely to be done proactively on corn acres and more likely to be done reactively, in response to GR weeds, on soybean acres
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