Invasive plants are viewed as a universal threat to ranching enterprises because invasive plants can alter forage quantity or quality for livestock and therefore reduce net revenue. These concerns are almost entirely based on how invasive plants alter forage production in small plots rather than direct measures of invasive plant impacts on livestock production. In this study we quantify impacts of the invasive annual grass medusahead (Taeniatherum caput-medusahead [L.]) on livestock production on California annual rangeland and evaluate how management buffers or exacerbates impacts to net revenue. To do this, we tracked changes in yearling steer weights across the growing season (March to May) for 2 yr in eleven 2.1-ha pastures where medusahead abundance was experimentally manipulated. We then integrated weight gain results with market data to illustrate how changing grazing season duration and animal density in response to either a 10% increase or decrease in medusahead impacts net revenue. Holding grazing season duration and end-of-season residual forage constant across pastures, pastures with more medusahead supported a lower density of animals than pastures with less medusahead. However, this decrease in livestock production was significantly less than impacts reported from indirect estimates in the literature. Medusahead impacts on net revenue depend on livestock management decisions. Responding to a 10% increase in medusahead cover by shortening the grazing season reduced net revenue 102.97 USD per hectare, whereas responding to a 10% increase in medusahead cover by reducing animal density by selling animals early increased net revenue 2.71 USD per hectare. Our study shows that the impacts of medusahead on livestock production may be lower than those estimated from indirect measures of invasive plant impacts on livestock production and that relatively simple changes in livestock grazing management could further reduce how invasive plant impacts on livestock production scale to impacts on net revenue.
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Vol. 82 • No. 1