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 avoi d the plant entirely or graze it reluctantly. We hypothesized that: (1 ) a diterpene diester that can occur in leafy spurge was an aversive a gent, and (2) diet selection differences among ruminant species may be partly a function of differential ruminal metabolism of aversive phyt ochemicals, and further that cattle and sheep may be reluctant to graz e leafy spurge because their ruminal microbes do not metabolize certai n leafy spurge chemicals as do ruminal microbes in goats. Sheep did no t develop an aversion to a novel food when its consumption was followe d by an intravenous injection of ingenol 3,20-dibenzoate (P = 0.34). S heep did develop an aversion to a novel food when its intake was follo wed by a dose of leafy spurge fermented with sheep ruminal digesta, bu t not when followed by a dose of leafy spurge fermented with goat rumi nal digesta (P = 0.03). This suggests that goat ruminal microbes may m odify leafy spurge such that it does not elicit an aversion in sheep.