Volunteer Corn in Northern Indiana Soybean Correlates to Glyphosate-Resistant Corn Adoption

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Vince M. Davis, Graduate Research Assistant, Paul T. Marquardt, Research Associate, and William G. Johnson, Associate Professor, Department of Botany and Plant Pathology, Purdue University, West Lafayette, IN 47907-2054

Crop producers and advisers have expressed concern about the increasing prevalence of volunteer corn (Zea mays) in soybean fields, which can reduce crop quality and yield (1) (Fig. 1). Cropping sequences throughout the Midwest routinely alternate between corn and soybean. In Indiana, herbicide-resistant corn acres increased from 4% in 2000 to 47% in 2007 (3) and the increase was driven by adoption of glyphosate-resistant varieties. Glyphosate-resistant soybean have been grown on greater than 88% of soybean acres since 2003. Since glyphosate-resistant corn is rapidly increasing in cropping systems that already rely heavily on glyphosate for postemergence weed control, it was hypothesized that volunteer corn in soybean may increase in conjunction with increasing frequency of glyphosate-resistant corn.

Fig. 1. Volunteer corn in Indiana soybean fields. Field on left represents volunteer corn densities at < 10% field coverage and field on right represents > 70% field coverage.

A total of 505 randomly-selected northern Indiana soybean fields (156 in 2003, 147 in 2004, and 202 in 2005) were surveyed to assess the frequency and distribution of weed species prior to crop harvest. Data were collected from individual fields each year in the northern half of the state. The frequency of soybean following corn in fields sampled was not different (P = 0.27) at 93, 93, and 96% for 2003, 2004, and 2005, respectively. Furthermore, tillage practices in fields where volunteer corn was present were not different across years (P = 0.96) with 50, 57, and 52% of fields tilled, respectively. In fields with volunteer corn, field coverage was estimated by visually dividing acres in 100 cell grids and estimating the average number of cells with volunteer corn across the entire field (Fig. 1).

In northern Indiana, volunteer corn was present in 3% of soybean fields sampled in 2003 and increased to 5% in 2004, and 12% in 2005 (P = 0.0007). Volunteer corn was found more frequently in systems with tillage (10%) verses no-tillage (5%) (P = 0.04) across all years. Counties surveyed account for 1.8 million acres of soybean in 2004 (2). Based on the frequency of fields infested with volunteer corn at the end of the season in this survey, the presence of volunteer corn was detected in fields that represented approximately 54,000 acres in 2003, 90,000 acres in 2004, and 217,000 acres in 2005. Herbicide-resistant corn in Indiana, primarily glyphosate-resistant, increased to 6, 8, and 10% of planted acres in 2002, 2003, and 2004, respectively (3), and was strongly correlated (r = 0.95) to the frequency of volunteer corn found in soybean fields the following year.

In fields where volunteer corn was present, it was the only weed escape in 26% of fields sampled and was correlated with the presence of common lambsquarters (Chenopodium album) in 31% (P = 0.03) of fields sampled. Common lambsquarters is a broadleaf weed difficult to control with postemergence glyphosate and these results suggest most volunteer corn escapes were glyphosate-resistant given the broadspectrum postemergence weed control efficacy of glyphosate. Density of volunteer corn plants was <10% field coverage in 89% of the fields sampled; however, in 2005 density in one field was estimated above 70% field coverage (Fig. 1).

Conclusions and Recommendations

The frequency of volunteer corn in soybean fields in northern Indiana increased each year concurrently with the adoption of glyphosate-resistant corn. Since (i) glyphosate was used on the majority of soybean acres and (ii) volunteer corn was commonly found either by itself or with common lambsquarters which is notably difficult to control with glyphosate, volunteer corn was most often found in soybean rotated with glyphosate-resistant corn. Growers using both glyphosate-resistant corn and soybean in a cropping sequence, especially when combined with tillage, should scout soybeans for volunteer corn prior to postemergence applications. In soybeans, the addition of clethodim, imazamox, fluazifop, fluazifop + fenoxaprop, or quizalofop in tank-mix combinations with glyphosate will help control volunteer corn in glyphosate-resistant soybean.

Literature Cited

1. Beckett, T. H., and Stoller, E. W. 1988. Volunteer corn (Zea mays) interference in soybeans (Glycine max). Weed Sci. 36:159-166.

2. CTIC. 2004. National Crop Residue Management Survey Conservation Tillage Data. Conservation Tillage Info. Center (CTIC), West Lafayette, IN.

3. USDA-ERS. 2008. Adoption of genetically engineered crops in the US: Corn varieties. Online. Data sets, USDA-ERS, Washington, DC.