Timely postemergence herbicide applications in soybean can increase profits

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

Early season weed competition may be one of the biggest contributors to unseen yield losses in soybean. Appropriately timing postemergence herbicide applications is critical to preserving crop yields. Several studies have shown that delaying postemergence weed control strategies can substantially reduce crop yield. This has become more of a concern with the large number of glyphosate-resistant crop acres that rely on postemergence applications of glyphosate for weed control. We are not saying that postemergence herbicide application timings are not important in non-glyphosate-resistant crops, but herbicides generally used in these crops are most effective on small weeds. Therefore, growers that are growing non-glyphosate-resistant crops are probably already making postemergence herbicide applications early. If they are not, it is important to also follow the outlined guidelines for postemergence herbicide applications.

In glyphosate-resistant soybeans, there is the temptation to delay postemergence glyphosate applications until all of the weeds have emerged. Delaying these applications can rob soybeans of this maximum yield potential. As mentioned before, research throughout the Midwest and Canada has shown the importance of early season weed control. For example, research conducted in Ontario has shown that delaying herbicide applications more than four weeks after soybean emergence can reduce yield about 0.75 bushel/acre/day.

Over a four-year period, MSU conducted research to study the effect of glyphosate application timing and row spacing on corn and soybean yield loss from weed competition. This research showed that in at least one of the years that this research was conducted, if weeds were allowed to reach 6-inches in height before control, yield was reduced in narrow row soybeans and 30-inch row corn. Depending on when this yield loss occurred, soybean yield was reduced from 2.5 to 5.5 bushel/acre/day. This starts to add up when you consider the economics. Considering these crop loss estimates, delaying herbicide applications three days would cost you any where from $56.25 to $123.75/acre in soybeans ($7.50 new crop soybean.) Waiting to make postemergence herbicide applications can cost you money. Therefore, it is important to follow the recommendations to avoid yield losses due to early season weed competition.


Weeds should be controlled in:

  • 7.5 and 15-inch row soybean before weeds exceed 4-inches.
  • 30-inch row soybean before weeds exceed 6-inches.

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