Quackgrass(Agropyron repens)Control in Established Alfalfa(Medicago sativa)With Pronamide

Abstract

The degree and longevity of quackgrass [Agropyron repens(L.) Beauv.] control provided by autumn applications of pronamide [3,5-dichloro(N-1,1-dimethyl-2-propynyl)benzamide] was influenced greatly by alfalfa(Medicago sativaL.) stand density, as well as by rate of application and soil type. When applications were made to a field with a sparse alfalfa stand (10 to 20 crowns/m2), quackgrass yields were reduced the year following pronamide application, but quackgrass reinfested the plots the second year after treatment. Quackgrass yields one and two seasons following autumn treatment with 2.2 kg/ha pronamide were 1480 and 3890 kg/ha compared to 4480 and 4870 kg/ha for control plots. Annual applications of pronamide were necessary to maintain quackgrass control in the sparse alfalfa stand. Pronamide application to a vigorous, dense alfalfa stand (40 to 50 crowns/m2) provided quackgrass control which persisted into the second year after treatment. Quackgrass control ratings one and two seasons after application of 1.1 kg/ha pronamide were 100 and 90%. Pronamide treatments reduced first cutting quackgrass yields, and increased first cutting alfalfa yields. Treatment of the sparse alfalfa stand with 2.2 kg/ha pronamide resulted in first cutting alfalfa and quackgrass yields of 2420 and 990 kg/ha compared to 1430 and 3940 kg/ha for controls. Treatment of a dense alfalfa stand with 1.1 kg/ha pronamide resulted in first cutting alfalfa and quackgrass yields of 3850 and 300 kg/ha compared to 1710 and 1660 kg/ha for controls. Total herbage yields from all cuttings were either slightly decreased when fields with sparse alfalfa stands were treated or remained constant or slightly increased when more dense alfalfa stands were treated. Pronamide applied with potash as the carrier was as effective as when applied in water. Pronamide treatment resulted in increased crude protein concentration in first cutting herbage at all locations in all years. Crude protein concentration in first cutting control herbage ranged from 13.3 to 16.1%. Crude protein concentration in first cutting herbage from plots treated with 1.1 kg/ha pronamide ranged from 17.7 to 20.6%, depending on year and location. In vitro digestible dry matter (IVDDM) concentrations were increased by most rates of pronamide in first, second, and third cutting herbage. IVDDM for first cutting control herbage was 51.4% compared to 61.3% for herbage treated with 2.2 kg/ha pronamide. Acid detergent fiber concentration in first cutting herbage was not affected by pronamide.