It’s all in the timing. Delaying herbicide application until later stages of weed growth can result in lower crop yields. A field survey found that when glyphosate is applied to glyphosate-resistant soybeans with weeds taller than 6 inches, crop yields can be reduced by as much as 27 percent.

The journal Weed Science examines weed competition across 64 site-years in 10 southern Wisconsin counties. Glyphosate-resistant soybean crops were sprayed with glyphosate to reduce competition from common lambsquarters, velvetleaf, dandelion, and amaranthus. Researchers compared the timing of herbicide application at each site against the crop yield.

In economic terms, the average yield loss of 5 percent predicted in this study translates into a loss of $26.72 per acre at 2011 crop prices. A loss greater than 5 percent—up to 27 percent—was predicted for one quarter of the sites in this study. The cost of glyphosate applied postemergence at the labeled rate would be about $10.93 per acre or 2 percent of the value of the soybean yield.

Full text of the article, “Soybean Yield Loss Potential Associated with Early-Season Weed Competition across 64 Site-Years,” Weed Science, Vol. 61, No. 3, July-September 2013, is now available.