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Effect of dicamba on soybean yields
by Bob Hartzler

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July 28, 2003 - 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 (see related article).   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.)   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. 

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.  Yields expressed as
percent of non-damaged plants. 

Soybean injury rating ( 3 WAA) Corsoy Hodgson
0 100 100
10 100 102
20 104 102
30 98 102
40 102 95
50 92 89
60 104 84
70 - 77
LSD NS 14

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.

  Soybean height (cm) Soybean Yield (% of control)
  0.001 kg/ha 0.01 kg/ha 0.056 kg/ha 0.001 kg/ha 0.01 kg/ha 0.056 kg/ha
1974            
Control 92 92 92 100 100 100
1-2 trifol. 93 88 77 100 108 100
3-4 trifol. 80 69 59 110 102 80
6-7 trifol. 93 56 45 118 103 79
Early bloom 80 46 36 114 91 46
1975            
Control 49 49 49 100 100 100
Early bloom 47 40 40 95 66 64
Early pod 54 47 50 97 102 20
1976            
Control 61 61 61 100 100 100
Early bloom 46 38 36 58 40 33
Mid bloom 46 43 38 82 72 37
Early pod 56 51 53 94 55 42

SourceAuch and Arnold, 1979.   Data in bold significantly different than untreated control.

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 it is suspected 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

For more information contact:
ISU Extension Agronomy
2104 Agronomy Hall
Ames, Iowa 50011-1010
Voice: (515) 294-1923
Fax: (515) 294-9985
http://www.weeds.iastate.edu
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