Effect of dicamba on soybean yields
by Bob Hartzler

Updated July 5, 2001 - 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.   The tendency of soybeans to develop symptoms characteristic of dicamba in the apparent absence of the herbicide has complicated diagnosing this problem.   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 will summarize results of controlled studies on the effect of dicamba on soybean yields to help evaluate situations that occur in the field.

One of the best studies was conducted by Behrens and Leushen at the University of Minnesota (Behrens, R. and W.E. Lueshcen.   1979.  Dicamba volatility.  Weed Sci.  27:486-493.)   Quarter acre plots of corn were planted within soybean fields and treated with the dimethylamine salt of dicamba (Banvel).   They reported that significant injury to soybean due to volatilization from corn fields could occur up to three days after application.  In one out 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.  I suspect that volatilization could occur at longer periods after application when large acreages of corn is treated (rather than a quarter acre as in these studies), but the greatest risk is in the first few days after application.

The researchers reported that low levels of foliar injury (leaf cupping) did not influence yield potential (Table 1).  Soybean injury was evaluated three weeks after dicamba drift 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.

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

¹ Yield in parenthesis indicates increased yield compared to untreated control.
Source:  Behrens and Lueschen.  Weed Sci.  27:486-493.

Weidenhamer et al. ( 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% 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-70's by Auch and Arnold.  (Dicamba use and injury on soybeans in South Dakota.  1978.  Weed Sci. 26:471-475).  Similar experiments were conducted during three years, although soybean stage at dicamba application varied among the experiments.   Dicamba was applied at rates of 0.001, 0.011 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.  The important points in this study are that the yield response varied widely from year to year, and that exposure of soybeans to dicamba during the bloom stage is more likely to affect yields than exposures made during the vegetative stage of growth.

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

Source:  Auch and Arnold, 1979.   Data in red significantly different than untreated control.

The most recent study was conducted in Kansas (Al-Khatib, K. and D. Peterson.  1999.  Soybean response to simulated drift from selected sulfonylurea herbicides, dicamba, glyphosate and glufosinate.  Weed Technol. 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 oz per acre).  Experiments were conducted in 1997 and 1998, data presented in Table 3 are averaged over the two years since 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 rates.   The lowest rate resulted in 35% visual injury 30 DAA, but yields were reduced only by 2%.  The 1/33 rate (0.5 oz Banvel) resulted in a 10% yield loss.  Several other herbicides (Beacon, Basis, Exceed, Roundup and Liberty) were evaluated at equivalent fractions of their label rates (data not presented).  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% yield loss at this rate.  Exceed was the second most damaging herbicide, but the yield loss differed significantly between the two years.  In 1997 the 1/3 rate of Exceed reduced soybean yields approximately 35% whereas in 1998 an 85% loss occurred.

Table 3.  Response of soybeans to simulated dicamba drift¹.

¹Source:  Weed Technol. 1999.   13:264-270.
²Label rate:  16 oz Banvel/acre; 0.5 lb dicamba/acre

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 will 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.  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 research with dicamba, although I am unaware of any research on this response.

Prepared by Bob Hartzler, extension weed management specialist, Department of Agronomy, Iowa State University