Herbicide Timing and Combination Strategies for Woolly Cupgrass Control in Corn

Field studies in 1992 and 1993 evaluated woolly cupgrass [Eriochloa villosa (Thunb.) Kunth] control in imidazolinone-resistant field corn (Zea mays L.) hybrids with imazethapyr, nicosulfuron, and dimethenamid. Woolly cupgrass control from combinations of cyanazine and alachlor, metolachlor, or dimethenamid applied preemergence was less than 78%. Postemergence and sequential applications (early-postemergence / late-postemergence) of nicosulfuron controlled more than 87% of woolly cupgrass. Dimethenamid plus cyanazine applied preemergence and followed by nicosulfuron postemergence provided greater than 88% woolly cupgrass control. Woolly cupgrass control from imazethapyr alone averaged only 72%. Sequential applications of pendimethalin followed by imazethapyr controlled 87% or more of woolly cupgrass. Corn grain yields from single applications of soil-applied herbicides preemergence, imazethapyr early-postemergence, or sequential soil-applied herbicides were all less than from sequential treatments that included nicosulfuron postemergence. This research suggests that sequential applications or those that included nicosulfuron were essential to many of the successful treatments and that no one treatment timing is always effective for controlling woolly cupgrass. Research QuestionWoolly cupgrass infestations continue to spread to more acres in the north central corn growing region of the USA. Woolly cupgrass is difficult to manage due to prolonged seed germination, tolerance to many commonly used herbicides, rapid growth, and prolific seed production. Recently introduced herbicides such as dimethenamid and nicosulfuron have given new options for woolly cupgrass control. Another option is the availability of imidazolinone-resistant (IR) corn hybrids, which allows use of imazethapyr for woolly cupgrass control. The objective of this study was to compare three relatively new herbicide options available to corn growers for woolly cupgrass control in corn. Literature SummarySeason-long woolly cupgrass control is often difficult to achieve. Corn producers have traditionally controlled woolly cupgrass with high rates of soil-applied herbicides and inter-row crop cultivations. Herbicides commonly used in corn production have not consistently controlled woolly cupgrass. Applications of soil-applied herbicides often gave acceptable early-season control, but late-season control is often inadequate. Season-long woolly cupgrass control from sequential applications of soil-applied and postemergence herbicides is typically superior to single herbicide applications. Nicosulfbron and imazethapyr give growers the option of sequential postemergence applications. Relatively little research has been reported on woolly cupgrass control with these herbicides. Study DescriptionThe study was conducted in 1992 on a Troxel silt loam and in 1993 on a Dodge silt loam near Madison, WI. Both locations were on grower fields infested with woolly cupgrass. ‘Pioneer IR’ corn hybrids were planted in 30-in. rows at a population of 30 000 seeds/acre. A. Herbicide treatments were applied at four timings: preplant incorporated, preemergence, early postemergence, and postemergence. B. Herbicides used were: AlachlorImazethapyrCyanazineMetolachlorDimethenamidNicosulfuronEPTCPendimethalin C. Woolly cupgrass control was estimated visually twice during the growing season. Corn grain yields were harvested from each plot. Applied QuestionsAre dimethenamid, nicosulfuron, and imazethapyr effective for woolly cupgrass control in IR corn? Two years of field research show that single applications of these herbicides are not always effective. Woolly cupgrass control from dimethenamid was equal to alachlor or metolachlor, Treatments that included nicosulfuron provided the most consistent woolly cupgrass control. Herbicide treatments that-included sequential applications (PREPOST) gave season-long control of woolly cupgrass. Dimethenamid applied preemergence followed with a postemergence application of nicosulfuron provided 87% or more woolly cupgrass control (Table 1). Sequential treatments of nicosulfuron applied early postemergence and postemergence gave 89% or more woolly cupgrass control. Sequential applications of pendimethalin preemergence followed with imazethapyr early postemergence provided 87% or greater woolly cupgrass control. A single application of nicosulfuron provided 87% or greater woolly cupgrass control. Imazethapyr applied alone, as well as single or sequential applications of dimethenamid provided 71% or less woolly cupgrass control. What are the concerns with sequential herbicide applications? The high cost and high herbicide application rates associated with the more effective treatments identified by this study may be a concern to growers and environmentalists. Other options, such as crop rotation and cultivation, should be included in complete weed management programs. This may allow growers to reduce herbicide application rates. Frequent use of sequential applications of nicosulfuron or imazethapyr may also contribute to development of acetolactase synthase herbicide-resistant weeds. Table 1Effect of herbicide treatments on late-season woolly cupgrass control for 1992 and 1993. ApplicationControlHerbicide treatment†Timing‡Rate 1992 1993 lb/acre%ImazethapyrEPOST0.067155Pendimethalin / imazethapyrPRE/EPOST1.5/0.068793Dimethenamid + cyanazinePRE1.3 + 2.03557Dimethenamid + cyanazine /PRE/1.0 + 2.0 dimethenamidEPOST0.55365NicosulfuronPOST0.038792Dimethenamid + cyanazine /PRE1.3 + 2.0 nicosulfuronPOST0.028798Nicosulfuron / nicosulforonEPOST/POST0.02/0.028996†Imazethapyr treatments included 28% N at 4.0% v/v plus 1.0 lb ai/acre of Sun-it ll, a multicomponent methylated seed oil adjuvant by Agsco, Inc., Box 458, Grand Forks, ND 58206. Nicosulfuron treatments included a nonionic surfactant (alkylaryl polyoxyethylene ether and free fatty acids) plus 28% N at 0.25% and 4.0% v/v, respectively. ‡EPOST, early postemergence; POST, postemergence; PPI, preplant incorporated; PRE, preemergence.