Effect of Cobra^® Herbicide on Soybean Yield in the Absence of White Mold or Weed Pressure¹

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Introduction

White mold, also known as sclerotinia stem rot, has spread in recent years, partly due to cultural practices that accelerate soybean canopy development and increase yield potential. The disease develops in humid conditions beneath the soybean canopy in seasons with wet weather conditions and moderate temperatures during flowering.

Application of Cobra^® herbicide has been shown to reduce white mold incidence and increase yield of susceptible soybean varieties in some studies (Oplinger et al., 1999). However, yield results have been highly variable overall (Nelson et al., 2002). In fact, studies have shown yield losses in the absence of the disease, and researchers have therefore advised caution when considering use of this product for white mold control (Dann et al., 1999).

Cobra Effects on Soybeans - Cobra is a herbicide for postemergence weed control in soybeans. Herbicides with PPO-inhibiting sites of action such as Cobra usually cause moderate levels of leaf necrosis. The reduction in leaf area from this necrosis reduces or "opens" the soybean canopy and may be a contributing factor in white mold suppression with Cobra. On the other hand, yield losses may result in the absence of the disease.

White Mold Management - Few chemical solutions have been available for white mold management, which may lead some growers to consider application of Cobra to their soybeans for this purpose. However, the downside risk may be significant. For this reason, Pioneer researchers conducted extensive studies to clarify the possible yield effects due to Cobra application in seasons when white mold is not an issue for growers and in the absence of weed pressure.

Study Description

Locations - 5 locations in 2012, 8 locations in 2013, and 7 locations in 2014 (see map below). Plots were 15 feet long and (2) 30-inch rows wide (4 rows wide at Princeton in 2012).

Map: Research locations in Illinois, Iowa and Wisconsin. — The study included locations in Illinois, Iowa and Wisconsin.

Plot Design - Randomized complete block arranged in a split-plot layout with 3 to 4 replications; variety was the main plot and Cobra application was the subplot.
Varieties - 4 to 6 adapted Pioneer^® brand soybean varieties per location in 2012 and 3 varieties per location in 2013 and 2014.
Cobra - Applied at the rate of 8 ounces per acre with crop oil concentrate (COC) using a CO₂ backpack sprayer.
- Application was made at the V3 to V4 stage of crop development.
Planting - Plots were planted in late April in 2012 (except Delmar on 5/8), the second week of May in 2013, and the first week of May in 2014 (except Delmar, Iowa and Janesville, Wis., which were planted in third week of May).
- Seeding rate was 168,000 seeds/acre in 2012 and 210,000 seeds/acre in 2013 and 2014.
- Seed was treated with a fungicide-insecticide combination.
Measurements - Yield was measured at all locations. Lodging, plant height, and days to maturity were measured at some locations. At Princeton in 2012, plants in the outside rows were sampled to quantify the number of nodes on the main stem and lateral branches.

Yield Results

**Figure 1.** Untreated check vs. Cobra-treated plots in Iowa, Illinois and Wisconsin.
(IA1-Delmar, IA2-Maysville, IA3-Rochester, IL1-Princeton, IL2-Troy Grove, IL3-Eureka, IL4-Kirkwood, IL5-Lena, IL6-Milledgeville, IL7-Wyoming, WI1-Janesville). *Significantly diff. at P=0.05.

Photo: Cobra study at Princeton, Illinois, in 2012. — **Figure 2**. Cobra study at Princeton, Ill., in 2012 showing plots at 1, 4, 7 and 11 Days After Treatment (DAT) with Cobra herbicide.

Results and Discussion

None of the locations were significantly infected with white mold in any year; the disease was basically not present in 2012 and 2013, and at very low levels in 2014.
There was no interaction of Cobra^® treatment and soybean variety. All varieties lost yield when Cobra was applied.
Similarly, there was no interaction of Cobra treatment and growing environment. All locations suffered yield loss when Cobra was applied in the absence of white mold in all 3 years.
Lodging counts were taken at sites where lodging was present and variety differences were apparent. Only in 2014 did Cobra^® treatment significantly decrease lodging; no change in lodging was observed in 2012 or 2013. Plant height was also reduced significantly by Cobra^® treatment in 2014.
Averaged across locations, the untreated check outyielded the Cobra-treated plot by 4.7 bu/acre (7.3%) in 2012, 7.8 bu/acre (11%) in 2013 and 10.6 bu/acre (15%) in 2014 (Figure 1).
Averaged across locations and years, the untreated check outyielded the Cobra-treated plot by 8 bu/acre (11.5%).
The locations with the highest mean yield often showed the greatest negative effect from Cobra treatment.
Cobra treatment significantly increased days to maturity by 2.6 days (average of multiple locations).

Conclusion

In the absence of white mold pressure and weed pressure in 2012, 2013, and 2014, Cobra application was highly detrimental to soybean yield when applied at 8 oz per acre at the V3 to V4 stage of development. These results were similar across soybean varieties and test locations.

References

Agrios, G. N. 1988. How plants defend themselves against pathogens. 97-115. In Plant Pathology, third edition. Academic Press, Inc. San Diego, CA.
Bradley, C. A. 2009. Soybean white mold fungicide trial results from northern Illinois agronomy research center. The Bulletin: December 4th, 2009.
Bradley, C. A. 2009a. Conditions favorable for sclerotinia stem rot (white mold) on soybean. The Bulletin July 24th, 2009.
Dann, E. K., B. W. Diers, and R. Hammerschmidt. 1999. Suppression of sclerotinia stem rot of soybean by lactofen herbicide treatment. Phytopathology 89:598-562.
Hao, J., D. Wang, and R. Hammerschmidt. 2010. Using biological agents to control soybean white mold. 2010 Michigan Soybean Checkoff.
Ma, B. L., L. M. Dwyer, C. Costa, E. R. Cober, and M. J. Morrison. 2001. Early prediction of soybean yield from canopy reflectance measurements. Agron. J. 93:1227-1234.
Mueller, D. S., A. E. Dorrance, R. C. Dersken, E. Ozkan, J. E. Kurle, C. R. Grau, J. M. Gaska, G. L. Hartman, C. A. Bradley, and W. L. Pederson. 2001. Efficacy of fungicides on Sclerotinia sclerotium and their potential for control of sclerotinia stem rot on soybean. Plant Disease 86:26-31.
Mueller, D. S., C. A. Bradley, C. R. Grau, J. M. Gaska, J. E. Kurle, W. L. Pederson. 2004. Application of thiophanate-methyl at different host growth stages for management of Sclerotinia Stem Rot in Soybean. Crop Protection 23:983-988.
Nafziger, E. D. 2009. Soybean. In Illinois Agronomy Handbook, 24th edition 27-36.
Nelson, K. A., K. A. Renner, and R. Hammerschmidt. 2001. Effects of protoporphyrinogen oxidase inhibitors on soybean (Glycine max L.) response, Sclerotinia sclerotium disease development, and phytoalexin production by soybean. Weed Technology 16:353-359.
Nelson, K. A., K. A. Renner, and R. Hammerschmidt. 2002. Cultivar and herbicide selection affects soybean development and the incidence of Sclerotinia Stem Rot. Agron. J. 94:1270-1281.
Oplinger, E. S., C. R. Grau, J. E. Kurle, J. M. Gaska, and N. Kurtzweil. 1999. Foliar treatments for control of white mold in soybean.
Pedersen, P. 2009. Soybean growth and development. Iowa State University Extension.
University of Wisconsin – Madison. 2008. White mold in Wisconsin.