Cattle avoidance of leafy spurge: a case of conditioned aversion.

Abstract

Leafy spurge (Euphorbia esula L.) represents a serious threat to the productivity and profitability of many cattle ranches in the midwestern U.S. Sheep and goats will graze the weed, but cattle generally do not. We hypothesized that cattle avoid leafy spurge because it contains 1 or more chemicals that elicit a conditioned flavor aversion when consumed. First, we tested cattle to determine if they reduced their intake of a novel feed on subsequent days if we modestly increased rumen fill by introducing additional feed or additional feed plus an aversive agent (lithium chloride, LiCl) after they had consumed the novel feed. We observed that cattle became averted (P = .0001) to the novel feed only when LiCl was administered with additional feed. Simply increasing rumen fill by a small amount did not cause cattle to reduce their intake of the novel feed the following day. Secondly, we tested cattle to determine if they reduced their intake of a novel feed on subsequent days if we introduced leafy spurge into their rumina following consumption of the novel feed. We also tested cattle to determine if a spurge-induced aversion to a novel feed was preventable by inoculation with rumen microbes from sheep with spurge in their diets. We found that introducing spurge into cattle after their intake of novel feed reduced (P < .01) their intake of the novel feed on subsequent days. Cattle inoculated with rumen microbes from spurge-adapted sheep had similar (P > .40) aversions to a novel feed paired with spurge introductions. Apparently, cattle avoid leafy spurge partly or wholly because they develop a conditioned aversion after first ingesting some threshold amount of it.