Late Season Weed Escapes in Indiana Soybean Fields

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Late-Season Weed Escapes in Indiana Soybean Fields

Bill Johnson, Assistant Professor, Jeff Barnes, Post Doctoral Research Associate, and Kevin Gibson, Assistant Professor, Department of Botany and Plant Pathology, and Steve Weller, Professor, Department of Horticulture and Landscape Architecture, Purdue University, W. Lafayette, IN 47907-2054

Crop advisors and extension personnel have observed a recent increase in the number of complaints regarding weed control with glyphosate (Fig. 1). Since glyphosate-resistant soybeans are grown in 89% of soybean acres in Indiana, we were interested in determining what weeds are escaping glyphosate treatments by recording identity, occurrence, and distribution of weed species in soybean fields just prior to harvest.

Fig. 1. Soybean field in southeast Indiana with giant ragweed protruding through the canopy.

A field survey of 389 fields in 15 counties was conducted in September and October 2003. Survey sites were randomly selected by examining NASS Cropland Data Layer and United States Geological Service digitized imagery. The number of sites in each county was based upon a target of one field per every 3500 acres of cropland (Fig. 2). At each site, identity and field coverage (%) of each weed protruding above the soybean canopy was recorded. Southeastern Indiana was sampled more extensively due to widespread failures of glyphosate to control horseweed in 2002 and 2003. Due to time constraints, we were not able to sample a sufficient number of fields in southwestern Indiana.

Fig. 2. State map of Indiana showing the counties where soybean fields were sampled and the number of fields sampled in each county.

Weed diversity was highest in southeastern and northwestern Indiana with 57% and 54%, respectively, followed by northeastern Indiana with 42% of the fields containing at least 3 weed species.

The predominant weeds in northeastern Indiana were giant ragweed (28% of the fields sampled), giant foxtail (Setaria faberi) (12%), and horseweed, common lambsquarter (Chenopodium album), cocklebur (Xanthium strumarium), and waterhemp (Amaranthus tuberculatus) (4% each). In northwestern Indiana, the predominant weeds were giant ragweed (49%), giant foxtail (8%), barnyardgrass (Echinchloa crus-galli) (6%), fall panicum (Panicum dichotomiflorum) (5%), and horseweed, common lambsquarter, redroot/smooth pigweed (Amaranthus spp.), and volunteer corn (Zea mays) (4% each). In southeastern Indiana, the predominant weeds were horseweed (47%), giant ragweed (17%), giant foxtail (7%), and common lambsquarter and fall panicum (5% each).

Weeds found in all three regions included horseweed (30% of all fields), giant ragweed (26%), giant foxtail (8%), common lambsquarter and fall panicum (5%), cocklebur (4%), barnyardgrass (3%), waterhemp (2%), and volunteer corn (2%).

Visual estimates of weed density suggest horseweed infestations were high enough (10% or more of field infested) to cause economic losses in 0, 0, and 28%, respectively, of fields in northeastern, northwestern, and southeastern Indiana. Giant ragweed densities were high enough (10% or more of field infested) to cause economic losses in 19%, 19, and 20% of fields in northeastern, northwestern, and southeastern Indiana. Although giant foxtail was present in numerous fields, field coverage was low.

Conclusions and Recommendations

In 2003, weed management programs provided poor control of horseweed in 28% of southeastern Indiana soybean fields, where glyphosate-resistance is widespread (1). Growers should not plant soybean into fields containing emerged horseweed. To control horseweed before planting, management tactics could include tillage, 2,4-D, dicamba, cloransulam, or chlorimuron tankmixed with glyphosate, or paraquat + 2,4-D or dicamba. Cloransulam, chlorimuron, metribuzin, flumetsulam, or flumioxazin will provide soil residual activity for seedlings that emerge in the spring. If horseweed is present after soybean emergence, cloransulam or chlorimuron should be tankmixed with postemergence glyphosate. If the population is resistant to ALS inhibitors, cloransulam, chlorimuron, and flumetsulam will not be effective.

Economic infestations of giant ragweed were observed in approximately 20% of all fields. Although giant ragweed is poorly controlled by most soil-applied soybean herbicides except cloransulam, many provide suppression. Effective tactics for managing this weed in soybean will include utilization of soil-applied herbicides for early season suppression and postemergence glyphosate or glyphosate plus cloransulam.

Literature Cited

1. Barnes, J. W., Johnson, W. G., and Gibson, K. D. 2003. Developing a field survey for the estimation of glyphosate-resistant horseweed. Proc. North Central Weed Sci. Soc. 58:176.