The widespread use of dicamba in corn, combined with the high sensitivity of soybeans to this herbicide, results in numerous cases of soybean injury each year. When dicamba injury occurs, whether from spray drift, volatilization, or sprayer contamination, the common question is, How much will yields be affected? As with any source of crop stress, it is impossible to accurately predict yield loss potential from dicamba injury that happens early in the growing season. This article summarizes results of controlled studies to help evaluate situations that occur in the field.

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Soybean leaf cupping can be triggered by growth regulator herbicides.

Behrens and Leushen (University of Minnesota) studied the volatilization of dicamba from cornfields into soybean fields and the resultant injury. They reported that significant injury to soybean due to volatilization from cornfields could occur up to 3 days after application (Dicamba volatility. 1979. Weed Science 27:486-493). In one of five experiments they observed minor injury due to volatilization on the fourth day after application. Rainfall events 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 application (WAA) by using a scale of 0 (no injury) to 100 (complete kill). Slight leaf malformations (injury rating of 10) were observed up to 200 feet 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 (Dicamba injury to soybean. 1989. Agronomy Journal 81:637-643) concluded that there was no yield reduction without height reduction, regardless of foliar symptoms. "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-1970s by Auch and Arnold (Dicamba use and injury on soybeans in South Dakota. 1978. Weed Science 26:471-475). Similar experiments were conducted during 3 years, although soybean stage at dicamba application varied among the experiments. Dicamba was applied at rates of 0.001, 0.01, and 0.056 kilograms/hectare (equivalent to 0.03, 0.3, and 1.6 ounces Banvel/acre) (Table 2). The researchers did not provide information on early-season injury other than to say that all rates caused leaf cupping. The important points in this study are that the yield response varied widely from year to year, and that exposure of soybean to dicamba during the bloom stage is more likely to affect yields than exposure during the vegetative stage of growth.

The most recent study was conducted in Kansas (Al-Khatib and Peterson. 1999. Soybean response to simulated drift from selected sulfonylurea herbicides, dicamba, glyphosate and glufosinate. Weed Technology 13:264-270). Dicamba was applied to soybeans at the V2-V3 growth stage at 1/100, 1/33, 1/10, and 1/3 of the label rate (16 ounces/acre). Experiments were conducted in 1997 and 1998, data presented in Table 3 are averaged over the 2 years because results were similar. Visual injury ratings were higher 30 days after application (DAA) than at 7 DAA. As would be expected, the level of injury increased with increasing herbicide rates. The lowest dicamba rate resulted in 35 percent visual injury 30 DAA, but yields were reduced only by 2 percent. The 1/33 rate (0.5 ounces Banvel) resulted in a 10 percent yield loss. Several other herbicides (Beacon, Basis, Exceed, Roundup, and Liberty) were evaluated at equivalent fractions of their label rates (data not shown). Dicamba was the most injurious of the herbicides evaluated. Roundup and Liberty did not affect yields at 1/3 of the label rate, whereas Beacon and Accent caused less than a 20 percent yield loss at this rate. Exceed was the second most damaging herbicide, but the yield loss differed significantly between the 2 years. In 1997 the 1/3 rate of Exceed reduced soybean yields approximately 35 percent, whereas in 1998 an 85 percent loss occurred.

In summary, dicamba injury on soybean 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 develop shortly after application. Remember that other factors can induce symptoms typical of dicamba, complicating diagnosis of this problem. There are no controlled studies of the effects of this phenomenon on soybean yields; however, it is likely that yields will not be affected if the symptoms are limited to a few trifoliolate leaves.

Table 1. Relationship between early-season dicamba injury and yields of two soybean varieties.

^a Parentheses indicate increased yield compared with untreated control.

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

^a Data in red significantly different from untreated control.

Table 3. Response of soybean to simulated dicamba drift.

^a Label rate: 16 ounces Banvel/acre; 0.5 pound dicamba/acre.