Early-season weed competition resulted in as much as 40-bu./acre yield loss according to data found in some South Dakota State University Research Plots. "Everyone knows that weeds in fields create competition for water and nutrients and can cause significant yield loss, but many neglect the effects of just early-season weeds on crop yield," says Mark Rosenberg South Dakota State University agronomy and weeds field specialist.

Rosenberg encourages growers to begin controlling weeds within three to five weeks after crop emergence to prevent lasting effects on crop growth and yield. However, he says in some cases this is not early enough because at high weed densities, some SDSU research plots measured 20-40-bu./acre in corn yield loss even though glyphosate was first applied two to three weeks after corn and weed emergence.

 

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"Corn is much more sensitive to early-season weed competition than soybeans, which is why we often recommend using pre-emergence herbicides in corn to prevent yield loss and minimize selection for glyphosate-resistant weeds," Rosenberg says. "Pre-emergence herbicides are also beneficial in soybeans, but mostly for the benefit of increasing the consistency of weed control, managing glyphosate-resistant weeds and increasing flexibility in the timing of post-emergence applications."