Vegetation and animal production in pastures sprayed for western ragweed control

Western ragweed (Ambrosia psilostachya DC) is one of the most common perennial, native forbs of western US rangelands. Cattle consume small quantities of western ragweed in their diets when present in the forage sward, and forage nutritive value tends to be greater than the associated grass component in those swards. Yet, many producers view western ragweed as an undesirable forb in rangelands. We conducted a study over two years to compare vegetative and stocker animal productivity in pastures that were either sprayed with dicamba at 6 oz/acre (0.19 lb a.i./acre) for western ragweed control or were left unsprayed. Pastures started the study with high western ragweed densities (>9 western ragweed plants/ft(2)) before pasture treatment, and spraying with dicamba significantly reduced western ragweed density and yield compared to unsprayed pastures (0.2 vs. 3.5 ragweed plants/ft(2), and 0 vs. 206 lb/acre, respectively). Grass production and total vegetation production were similar between treatments. Crude protein and total digestible nutrients of western ragweed was greater than grass at all mid-season and end of season sampling dates both years. Stocker animal gain was not different between pastures sprayed for ragweed control and pastures left unsprayed in either year, nor when averaged over the two years (189 vs. 188 lb/head, sprayed vs. unsprayed, respectively). Beef production per acre was also similar. The spray treatment was an added expense that did not result in greater total forage, grass, or animal production. With natural fluctuations in western ragweed population densities due to weather patterns, producers will likely experience little benefit from spraying for western ragweed alone in pasture. Plain Language Summary Western ragweed is a common pasture plant that is often viewed as a weedy species and sprayed with herbicides because it is not a pasture grass. However, cattle consume small quantities of western ragweed, and grassland birds greatly use western ragweed for cover and a food source, so ragweed can be beneficial to pastures and wildlife. We compared pastures with high densities of western ragweed that were either sprayed for western ragweed control with dicamba (6 oz/acre, or 0.19 lb a.i./acre) or were left unsprayed. Herbicide spraying greatly reduced the amount of western ragweed in the pastures for two seasons (0.2 vs. 3.5 ragweed plants/ft(2) and 0 vs. 206 lb/acre, respectively), but the spray treatment was an expense that did not produce more total forage, more grass, or better animal gains compared to unsprayed pastures. Unless pastures have a much greater western ragweed density than measured in this study, spraying ragweed will not benefit cattle production nor wildlife.