RUMINAL METABOLISM OF LEAFY SPURGE IN SHEEP AND GOATS - A POTENTIAL EXPLANATION FOR DIFFERENTIAL FORAGING ON SPURGE BY SHEEP, GOATS, AND CATTLE

Leafy spurge (Euphorbia esula) is an introduced forb that is invading western rangelands. Goats (Capra hircus) readily graze the plant, but cattle (Bos tarus) generally and sheep (Ovis aries) locally appear to develop conditioned flavor aversions to leafy spurge. They either avoid the plant entirely or graze it reluctantly. We hypothesized that: (1) a diterpene diester that can occur in leafy spurge was an aversive agent, and (2) diet selection differences among ruminant species may be partly a function of differential ruminal metabolism of aversive phytochemicals, and further that cattle and sheep may be reluctant to graze leafy spurge because their ruminal microbes do not metabolize certain leafy spurge chemicals as do ruminal microbes in goats. Sheep did not develop an aversion to a novel food when its consumption was followed by an intravenous injection of ingenol 3,20-dibenzoate (P = 0.34). Sheep did develop an aversion to a novel food when its intake was followed by a dose of leafy spurge fermented with sheep ruminal digesta, but not when followed by a dose of leafy spurge fermented with goat ruminal digesta (P = 0.03). This suggests that goat ruminal microbes may modify leafy spurge such that it does not elicit an aversion in sheep.