Operant analysis of chronic locoweed intoxication in sheep

Five sheep were fed a 10% locoweed (Oxytropis sericea) pellet or alfalfa pellets for 3- to 5-wk periods to determine the effects of intermittent locoweed ingestion on operant responding; three controls were fed alfalfa pellets for 22 wk. Sheep were trained to respond to a multiple schedule with a fixed ratio (FR) 5 and fixed interval (FI) 50 s as major elements; performance was reinforced with rolled barley. Locoweed-treated sheep decreased (P < .05) FR response rate after 4 wk of locoweed feeding, but this decrease first appeared during the first recovery period (wk 6). The FR response rate of intoxicated sheep did not return to baseline during the remainder of the study and differed from controls during most of the study. Controls did not deviate (P > .05) from their FR baseline except during wk 2. Sheep did not stabilize on the FI component. As locoweed-treated sheep became progressively more intoxicated, they altered their pattern of FR responses, with longer post-reinforcement pauses, and a slower overall FR rate. Intoxicated sheep ingested an average of .21 mg swainsonine.kg-1.d-1. Overt signs of intoxication were noted when two sheep were stressed on wk 17. These two sheep had neuroviseral vacuolation typical of locoweed poisoning, whereas the three remaining locoweed-treated sheep that were euthanatized 5 wk later showed little histologic evidence of intoxication. Our findings indicate that "on-off" or cyclic grazing of locoweed ranges should be approached cautiously. Such a grazing program may be feasible because of the rapid resolution of histologic pathology; however, an initial toxic insult of 4 wk seems to be excessive, even at low doses, because sheep may exhibit persistent behavioral abnormalities that require > 6 wk to resolve.