By Christy Sprague
Early season weed competition may be one of the biggest contributors to unseen yield losses in soybeans. Appropriately timing post-emergence herbicide applications is critical to preserving crop yields.
Several studies have shown that delaying post-emergence 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 post-emergence applications of glyphosate for weed control. We are not saying that post-emergence 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 post-emergence herbicide applications early. If they are not, it is important to also follow the below outlined guidelines for post-emergence herbicide applications.
In glyphosate-resistant soybeans, there is the temptation to delay post-emergence glyphosate applications until all of the weeds have emerged. Delaying these applications can rob soybeans of its 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 bushels per acre per day.
Over a four-year period, Michigan State University 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 bushels per acre per day. This starts to add up when you consider the economics. Considering these crop loss estimates, delaying herbicide applications three days would cost you anywhere from $114.40 to $251.60 per acre in soybeans ($15.25 June soybean price).
Waiting to make post-emergence herbicide applications can cost you money. Therefore, it is important to follow the recommendations below to avoid yield losses due to early-season weed competition.
Michigan State University Extension recommendations: Weeds should be controlled in:
•7.5- and 15-inch row soybeans before weeds exceed 4 inches.
•30-inch row soybeans before weeds exceed 6 inches.
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Sprague write for Michigan State University Extension, Department of Plant, Soil and Microbial Sciences