Feed preferences and habituation of sheep poisoned by locoweed.

Livestock addiction to locoweed is commonly cited as a clinical sign of locoweed poisoning. However, there have been no physiological or pathological explanations for the reported addiction. Four hypotheses were tested to evaluate locoweed's (Astragalus lentiginosus) addictive nature: 1) animals become psychologically addicted to locoweed; 2) animals have an innate preference for locoweed; 3) animals become habituated to locoweed in the sense of acquiring an acceptance or liking for it; and 4) intoxicated animals lose the ability to discriminate between feeds and eat whatever is present. Two feeding trials were conducted to evaluate these hypotheses. In 1987, 20 ewes were fed a pelleted basal feed consisting of alfalfa with 20% locoweed to induce locoweed poisoning. Five control ewes were fed pelleted alfalfa. In 1988, seven ewes were fed a basal feed consisting of alfalfa and 15% locoweed, and five control ewes were fed pelleted alfalfa. At 2- to 3-wk intervals, ewes from both trials were placed on cafeteria trials and offered a choice of four feeds (100% locoweed pellets, 15% or 20% locoweed pellets, mixed grain pellets and alfalfa pellets) to evaluate their preferences at increasing levels of intoxication. Sheep did not seek out locoweed, which would have indicated a psychological addiction. Locoweed was not innately palatable to sheep. One ewe habituated to 100% locoweed and two ewes habituated to the 15% locoweed diet. The rest of the ewes rejected the 100% locoweed pellets and the 15% or 20% locoweed basal feed when the choice of feeds was offered, even though they were severely intoxicated. Sheep show neither initial preference nor addiction to locoweed but may acquire preference (habituation).