Postemergence Soybean Herbicide Injury: Is There a Yield Penalty?

June 23, 2000
Postemergence herbicide applications for weed control in soybeans are nearly completed in some areas of Illinois. Depending on the herbicide applied, some of these recently treated soybean fields are easily identified from a "windshield survey" due to the large amount of soybean injury. Often when this injury is observed by soybean producers, the first question asked is, "Will this soybean injury mean a reduction in soybean yield at the end of the season?" Knowing that this question weighs heavily on the minds of many soybean producers, the Soybean Research and Development Council, comprising the Illinois and Iowa Soybean Program Operating Boards, worked in conjunction with the University of Illinois, Southern Illinois University, and Iowa State University to conduct research trials on evaluating the effect of post- emergence herbicides on soybean yield.

This research was conducted over a variety of environments in Illinois and Iowa from 1997 to 1999. The implications of early and late planting, and early and late application timings of Pursuit, Blazer, and Roundup Ultra, were investigated.

What type of soybean injury was experienced?

Early-season injury from postemer-gence herbicides was fairly consistent. The level of soybean injury that was experienced from these herbicides often depended on the type of herbicide (diphenyl ether, ALS inhibitor, or EPSP-synthase inhibitor), the soybean growth stage at application, planting times (early or late), and environmental conditions before and after herbicide application. At all locations, soybean recovered from early-season injury in a relatively short time. The recovery period frequently depended on the local environmental conditions. Recovery was often slower in environments with low soil moisture and high air temperatures. However, at all locations early-season herbicide injury was not apparent by 21 days after application.

Did injury reduce soybean yield?

For the three-year, multi-location study, more than 95% of the (288) postemergence herbicide applications did not reduce soybean yield regardless of the herbicide or level of soybean injury observed. Significant yield loss occurred in only 3.5% of the treatments. These few instances often did not correlate to severe early-season injury and were usually associated with late herbicide applications to late-planted soybeans. These factors (late-planted soybean and late-season herbicide applications) were more conducive to environmental conditions that may have not been ideal for soybean recovery.

What is the bottom line?

Injury from postemergence herbicides is not a good predictor of soybean yield loss. There are a number of factors that likely influence the potential for postemergence herbicide injury to cause soybean yield reductions. These high risk factors include late planting dates, late herbicide applications, and poor environmental conditions for soybean recovery, such as low soil moisture and high temperatures. If no visual injury develops from the postemergence herbicide, there is a minimal risk of soybean yield reductions. However, just because herbicide injury is observed, soybean yields are not necessarily reduced.

The results of this extensive research indicated that the risk of soybean yield loss from labeled applications of postemergence herbicides is low. Soybean yield loss is more likely to occur from allowing weeds to compete too long with the soybeans, or failing to adequately control weeds.

This project was supported by checkoff funds from the Soybean Research and Development Council, comprising the Illinois and Iowa Soybean Program Operating Boards.--Christy Sprague and Aaron Hager

Author: Aaron Hager Christy Sprague