Crop Rotation and Tillage System Influence Late-Season Incidence of Giant Ragweed and Horseweed in Indiana Soybean

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Jeff Barnes, Post Doctoral Research Associate, Bill Johnson, Assistant Professor, 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

Many crop advisors and farmers have expressed concern regarding the performance of glyphosate-based weed management practices on horseweed (Conyza Canadensis) and giant ragweed (Ambrosia trifida) control (Fig. 1). Corresponding increases in conservation tillage and utilization glyphosate-resistant soybean throughout the 1990s make it difficult to access impact either is having on glyphosate effectiveness. The objective of this survey was to determine the relationship between crop rotation, tillage system, and prevalence of horseweed and giant ragweed in Indiana soybean fields.

Fig. 1. Soybean field in Indiana with horseweed protruding through the canopy.

Field surveys of 389 soybean fields in 15 counties were conducted in September and October 2003. Survey sites were randomly selected from NASS Cropland Data Layer and USGS digitized aerial imagery. The number of sites in each county was based upon a target of one field per every 3500 acres of cropland (Fig. 2). Weed species protruding above the soybean canopy, tillage system, and previous crop were recorded in each field. Tillage systems were defined as no-till (greater than 30% residue coverage), mulch-till (tilled with greater than 30% residue coverage), reduced-till (tilled with 15 to 30% residue coverage), and conventional tillage (tilled with less than 15% residue coverage) (1). Southeastern Indiana (232 fields) was sampled more intensively than northern Indiana (157 fields) due to widespread reports of horseweed escapes following glyphosate.

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

Corn:soybean rotations were observed in 85% of northern Indiana fields and 50% of southeastern Indiana fields. Continuous soybeans were grown in 6% of fields in northern Indiana, compared to 29% in southeastern Indiana. Double-cropped soybean after wheat was present in 11% of fields in southeastern Indiana but less than 1% in northern Indiana. No-till was more predominant in southeastern Indiana than in northern Indiana (66 versus 50%); however, mulch-till and reduced-till systems were practiced in 23 and 15%, respectively, of northern Indiana fields compared to only 18 and 6% in southeastern Indiana.

Horseweed was found in 47% of southeastern Indiana soybean fields but less than 4% of northern Indiana fields. Sixty three percent of double-crop soybean fields contained horseweed compared to 51% for full-season continuous soybean and 47% for soybean following corn. Increases in tillage intensity reduced horseweed incidence. Sixty one percent of no-till fields contained horseweed compared to 27% for mulch till, 21% for reduced till, and 8% for conventional-till fields.

Giant ragweed in northern Indiana was observed more frequently in fields rotated with corn (42%) than in continuous soybean (30%), but this trend was not observed in southeastern Indiana. Giant ragweed was observed more frequently in mulch-tilled fields (49%) than no-till (37%) or conventional-till fields (32%).

Conclusions and Recommendations

Horseweed was most commonly found in southeastern Indiana in no-till fields with either wheat or soybean as the previous crop. Increased tillage intensity reduced prevalence of horseweed by 30% or more. Rotating corn with soybean did not greatly reduce horseweed prevalence. Growers that have had problems with horseweed control in southeastern Indiana should consider utilization of more effective herbicides or tillage to reduce horseweed prevalence.

Giant ragweed was most commonly found in northern Indiana in fields rotated with corn utilizing intermediate levels of tillage. Utilization of no-till or more aggressive conventional tillage practices reduced giant ragweed incidence by 10 to 15%. Strict no-till practices would leave giant ragweed seed on the soil surface and more prone to predation by insects and other soil organisms. Fall chisel or moldboard plowing would bury some of the seed deep enough to reduce emergence the following spring.

Literature Cited

1. CTIC. 2003. National crop Residue Management Summary: 2002 Results. Online. Conservation Tillage Information Center. West Lafayette, IN.