Woolly Cupgrass Management in Sethoxydim-Resistant Corn

Field studies were conducted from 1995 to 1997 to evaluate the potential for sethoxydim to control woolly cupgrass [Eriochloa villosa (Thunb.) Kunth.] in sethoxydim-resistant (SR) corn (Zea mays L.) and to identify potential interactions among sethoxydim and selected broadleaf herbicides. A single preemergence (PRE) application of acetochlor did not provide adequate season-long control of woolly cupgrass at 60 d after the postemergence (POST) timing. Sequential applications of acetochlor followed by sethoxydim or nicosulfuron improved woolly cupgrass control and plant density reduction. Sethoxydim controlled woolly cupgrass equal to or greater than acetochlor or nicosulfuron in all years. Split POST applications of sethoxydim provided 87% or greater woolly cupgrass control and 93% or greater plant density reduction. Dicamba, atrazine plus dicamba, bromoxynil, or halosulfuron did not reduce woolly cupgrass control by sethoxydim. However, atrazine plus bentazon and primisulfuron plus pro-sulfuron reduced woolly cupgrass control by sethoxydim resulting in greater plant densities compared with sethoxydim applied alone. These studies suggest that sethoxydim can be used as an effective tool in SR corn for control of woolly cupgrass. Research QuestionTraditional methods of woolly cupgrass management using chloroacetamide herbicides and nicosulfuron have not provided consistent control. Sethoxyim controls woolly cupgrass, and use of sethoxydim-resistant (SR) corn with sethoxydim could be a viable management tool for control of woolly cupgrass. The objectives of this research were to evaluate the potential for the use of sethoxydim for woolly cupgrass control in SR corn as well as to identify potential interactions among sethoxydim and selected broadleaf herbicides. Literature SummaryWoolly cupgrass has developed into a major weed throughout the corn growing areas of the Midwest. Competitive ability, seed size and dormancy, and multiple emergence events throughout the growing season contribute to the challenges associated with woolly cupgrass management. Additionally, studies have suggested woolly cupgrass is more tolerant to the soil-applied herbicides commonly used for annual grass control in corn. The commercialization of SR corn allows for sethoxydim to be applied for postemergence control of grasses. Previous research suggests that sethoxydim controls of giant foxtail in SR corn. Other research indicates the efficacy of sethoxydim was antagonized when applied with several broadleaf herbicides. Information on the effectiveness of sethoxydim for woolly cupgrass management in SR corn is limited. In addition, the compatibility of sethoxydim with broadleaf herbicides applied in corn has not been evaluated on woolly cupgrass. Study DescriptionStudies were conducted on a Clarence silty clay loam soil near Dwight, IL, in 1995, 1996, and 1997. Asgrow RX 620SR seed corn was planted in rows spaced 30 in. apart at a population of 28 000 seeds/acre. Herbicide treatments consisted of acetochlor at 1.9 lb a.i./acre applied PRE alone or followed by sethoxydim (0.19 lb a.i./acre) or nicosulfuron (0.031 lb a.iVacre) applied POST. Nicosulfuron and sethoxydim were also applied alone. Sethoxydim was applied in a tank mixture with dimethenamid (0.94 lb a.i./acre), dicamba (0.5 lb a.i./acre), atrazine (0.92 lb a.i./acre) + dicamba (0.5 lb/acre), atrazine (0.52 lb/acre) + bentazon (0.52 lb a.i./acre), bromoxynil (0.25 lb a.i./acre), halosul-furon (0.031 lb a.i./acre), and primisulfuron (0.018 lb a.i./acre) + prosulfuron (0.018 lb a.iVacre). Additional treatments included sequential applications of sethoxydim at 0.094 lb/acre followed by 0.094 lb/acre and 0.19 lb/acre followed by 0.19 lb/acre. A nontreated control was included. Visual estimates of percentage woolly cupgrass control were recorded 60 d following POST applications using a scale 0% (no control) to 100% (complete control). Woolly cupgrass plant numbers per square feet were determined by randomly placing two 5.4 sq ft quadrats in the two middle rows of each plot 60 DAT. Applied QuestionsWhat herbicide treatment resulted in the greatest level of woolly cupgrass control? A single POST application of sethoxydim provided equal or greater control of woolly cupgrass than a single application of acetochlor or nicosulfuron. However, a two-pass herbicide system involving a sequential treatment of acetochlor followed by either sethoxydim or nicosulfuron as well as split EPOST and LPOST application of sethoxydim controlled woolly cupgrass more effectively than a one-pass herbicide system. Do corn broadleaf herbicides have the potential to antagonize the activity of sethoxydim on woolly cupgrass? No reduction in woolly cupgrass control was observed when sethoxydim was tank-mixed with dicamba, atrazine plus dicamba, bromoxynil, and halosulfuron compared with sethoxydim applied alone. However, sethoxydim applied in combination with atrazine plus bentazon and primisulfuron plus prosulfuron reduced woolly cupgrass control compared with sethoxydim applied alone. RecommendationThe best approach for woolly cupgrass management is a sequential grass herbicide program that includes PRE followed by POST applications or EPOST followed by LPOST applications. However, light infestations of woolly cupgrass might allow for a single herbicide application to provide adequate season-long control of woolly cupgrass. Due to the flexibility of sethoxydim having a longer application window for control of taller woolly cupgrass compared with nicosulfuron, the use of sethoxydim is recommended when a timely application of nicosulfuron is not possible. However, SR corn hybrids are required when using sethoxydim for woolly cupgrass control in corn.