by Keith Harmoney, range scientist, Hays
Western ragweed (Ambrosia psilostachya) is one of the most common native forbs found throughout Kansas rangelands. Western ragweed is a perennial plant that can form dense colonies from growth of lateral creeping rootstalks. It’s used by wildlife for cover and for food, and the high protein content of western ragweed seed makes it a valuable staple of bobwhite quail and other grassland bird diets. Cattle also graze on western ragweed and utilized up to 50% of the western ragweed produced in moderately stocked pastures when measured in a long-term grazing trial at the Ag. Research Center in Hays.
In that trial, more western ragweed was also found in light stocked pastures compared to heavy stocked pasture. However, producers still question if western ragweed affects grass production and if cattle achieve adequate gains in pastures with high densities of western ragweed. We recently finished a grazing trial on short to midgrass native pastures at the KSU Ag. Research Center in Hays in 2021 and 2022 to determine if controlling western ragweed in pasture could improve animal gains.
We had eight, 35-acre study pastures that averaged 11.1 western ragweed plants/ft2 the year before starting the study, which is a rather dense stand of ragweed. The year of starting the study in 2021, we treated four of the pastures to control the western ragweed with dicamba at 6 oz/acre shortly after animal stocking in all pastures. The spray treatment significantly reduced western ragweed density in 2021 in sprayed pastures compared to unsprayed pastures by mid-season (1.2 vs. 4.5 ragweed plants/ft2, respectively). However, almost no ragweed was left in sprayed pastures at the end of the season because plants that initially survived the spray treatment had eventually died by the late season.
In October, at the end of the season, available ragweed yield (317 vs. 0 lb/acre, respectively) and ragweed as a percentage of total dry matter available (14.4 vs. 0%, respectively) was greater in unsprayed pastures. But, the western ragweed in unsprayed pasture had no effect on grass yield, as available grass yield (1976 lb/acre) and total yield (2135 lb/acre) were not different between the pasture spray treatments. Animal gains between sprayed and unsprayed pastures was also the same throughout the grazing season, and gains averaged 211 lb/hd.
Ragweed density and ragweed yield were both greater in unsprayed pastures in 2022, but drought conditions also lowered ragweed densities and yield compared to the prior year in the unsprayed pastures. Grass yield, total pasture yield, and animal gains again were similar between sprayed and unsprayed pasture in 2022. We also directly compared western ragweed and grass nutritive value in the unsprayed pastures at both mid-season and late-season, and western ragweed had greater crude protein and greater total digestible nutrient content than the harvested grass at all periods both years (CP 11.7 and TDN 67.2% vs. 7.2 and 47.2%, for western ragweed vs. grass, respectively). Producers often view western ragweed as a weedy forb, but this study showed ragweed had no effect on grass yield, total pasture yield, or animal gain.
Past research in Kansas has shown that western ragweed needed to be 35% or more of the dry matter composition in a pasture stand to reduce grass growth. Density of ragweed was high in the current study, but ragweed yield was still well below contributing 35% of the total pasture yield and had little impact on pasture growth. Even though high densities of ragweed were present in this current study, the total amount of ragweed yield contributing to the stand wasn’t a high enough percentage to cause reductions in grass yield.
The ultra-low-cost treatment to control the dense ragweed stand was an added cost with no significant financial return in our study. Other pastures in Kansas may be in the same situation, ragweed may be present, but not at a great enough level to cause significant reductions in plant or animal production. In some cases, western ragweed may be dense with enough yield percentage to reduce grass growth in localized areas of a pasture, but it’s unlikely that a whole pasture would have enough ragweed to reach the level that overall production is reduced. In those cases, localized spot treatments may be adequate to reduce ragweed if drought conditions don’t reduce the population naturally.
The results of our recent study suggests that producers should evaluate what percentage of the dry matter ragweed may be contributing to their pasture yield composition before deciding to treat a whole pasture for ragweed alone. The sight of western ragweed in pastures may be undesirable to many folks, but it is a natural and native component in our rangelands that does provide benefits to wildlife and has potential to be a higher quality forage for late season cattle diets.