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The shrub Flourensia cernua (tarbush) has rapidly increased in dominance within Chihuahuan Desert grasslands, and is comparable to alfalfa in nutrient content. Increasing tarbush in livestock diets may improve diet quality, but reduces tarbush dominance. We examined dietary selection for tarbush by sheep, and the effect of previous exposure on selection. Thirty-eight ewe lambs received either tarbush or alfalfa in a sorghum-based pellet 120 days postparturition, after which dietary selection was assessed. Previous exposure to tarbush deterred lambs from tarbush consumption. Lambs without previous exposure maintained greater intakes initially, but this declined with time, which corresponds with the onset of tarbush toxicosis. (C) 2000 Academic Press.