Late herbicide application can cut soybean yields

It’s all in the timing: Delaying herbicide application until later stages of weed growth can result in lower crop yields.

A field survey has found that when glyphosate is applied to glyphosate-resistant soybeans with weeds taller than six inches, crop yields can be reduced by as much as 27 per cent.

The July-September edition of 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, estimating yield loss by combining field survey information with predictive models using WeedSOFT, a computer-based agricultural weed management system.

The WeedSOFT model takes into consideration weed species, crop growth stage, row spacing, weed height and weed density. With this information, the total competitive load — and yield loss — can be estimated for a large geographic area.

Postplant herbicide application is recommended when weeds are six inches or less in height. However, factors such as weather, time constraints or equipment availability might delay the herbicide application past the ideal time.

This survey found that most herbicide applications were actually taking place later than the optimum time: weeds were sprayed on average at seven inches tall, and 17 per cent of the fields had weeds taller than nine inches before treatment.

In economic terms, the average yield loss of five per cent predicted in this study translates into a loss of US$26.72 per acre at 2011 crop prices. A loss greater than five per cent — up to 27 per cent — 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 two per cent of the value of the soybean yield.

The results of this study suggest an earlier application of glyphosate could be economically beneficial. The authors also suggest a pre-emergence herbicide, at a cost of about $12.55 per acre, could also improve yields. The use of a pre-emergence herbicide can allow successful use of a single treatment of glyphosate at a later stage of weed growth.

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