In last week's ICM newsletter I discussed the causes of malformed soybean leaves. The first concern when this problem develops is to identify the source of the problem, then the issue is to determine whether yields will be affected. This article summarizes results of studies evaluating the effect of dicamba on soybean yields.

One of the best studies on this topic was conducted by Behrens and Leushen at the University of Minnesota. They reported that significant dicamba injury to soybean from vapor drift could occur up to 3 days after application to corn. In one out of five experiments they observed minor injury due to volatilization on the 4th day after application. Rainfall after application greatly reduced vapor movement of dicamba.

The researchers reported that low levels of foliar injury (leaf cupping) did not influence yield potential (Table 1). Soybean injury was evaluated 3 weeks after dicamba drift by using a scale of 0 (no injury) to 100 (complete kill). Slight leaf malformations (injury rating of 10) were observed up to 200 ft downwind of treated corn. More severe injury was observed closer to the corn (injury ratings of 60-70), with terminal bud kill and axillary bud release resulting in short, bushy beans and delayed maturity. Significant yield losses were not observed unless severe early-season injury was observed.

Weidenhamer and coworkers concluded that there was no yield reduction without height reduction of soybeans, regardless of foliar symptoms (Dicamba injury to soybean. 1989. Agronomy Journal 81: 637-643). Yield reductions greater than 10 percent were indicated by severe morphological symptoms of injury, such as terminal bud kill, splitting of the stem, swollen petioles, and curled, malformed pods. Symptoms such as crinkling and cupping of terminal leaves occurred at rates much lower than those required to cause yield reductions.

A third study was conducted in South Dakota during the mid-70s by Auch and Arnold. Similar experiments were conducted during 3 years, although dicamba was applied at different stages of soybean development in each year (Table 2). Dicamba was applied at rates of 0.001, 0.01, and 0.056 kg/ha (equivalent to 0.03, 0.3, and 1.6 oz Banvel per acre). The researchers did not provide information on early-season injury other than to say that all rates caused leaf cupping. Significant findings in this study were that the yield response varied from year to year and that exposure of soybeans to dicamba during the bloom stage was more likely to affect yields than exposure during the vegetative stage (Table 2).

In summary, dicamba injury on soybeans is a common problem throughout Iowa in many years. Research has shown that minor distortion of soybean leaves that occurs prior to bloom usually does not affect soybean yields. However, each situation is different and it is impossible to predict the final impact on yield from symptoms that occur early in the season. Remember that other factors may induce symptoms typical of dicamba, complicating diagnosis of this problem. I suspect that the cupping that frequently shows up following early-season postemergence applications on soybeans is merely a cosmetic response as reported in the above-cited research with dicamba, although I am unaware of any research on this response.

Table 1. Relationship between early-season dicamba injury and yields of two soybean varieties. Yields expressed as percentage of nondamaged plants.

Source: Behrens and Lueschen. 1997. Weed Science 27: 486-493.

Table 2. Influence of soybean growth stage and dicamba rate on soybean growth and yield.

Source: Auch and Arnold. 1979. Weed Science 26: 471-475. Data in bold significantly different than untreated control.