Expression of conditioned preference for low-quality food in sheep is modulated by foraging costs.
Sheep are more resistant than cattle to larkspur poisoning and thus may be used as a biological tool to graze larkspur to reduce cattle poisoning. Sheep readily graze larkspur in its mature stages, but if they are to be an effective management tool, they must graze it in the early growth stages before cattle enter the allotment. The objective of this study was to determine if sheep could be positively conditioned to graze duncecap larkspur (Delphinium occidentale (S.Wats) S. Wats) early in its growth stages. Eighteen ewes were divided into 3 groups of 6 ewes each. During conditioning, group 1 was offered potted larkspur plants then were gavaged with glucose, the second group was exposed to larkspur plants together as a group (social facilitation), and the third group was an untreated control. In the preference test, the glucose group ate more duncecap larkspur than the social facilitation and control groups. The glucose and control groups were taken to duncecap larkspur-infested mountain rangeland to test the conditioning. In the field grazing trial, the glucose group consumed more larkspur than the control group, but it occurred later in the grazing trial when larkspur was in flower and after desirable forages had been consumed. High levels of diterpenoid alkaloids in larkspur and other alternative palatable forages may have caused ewes to reject larkspur at the beginning of the trial. The sheep were positively conditioned to graze larkspur, but the amount consumed and the timing of consumption was not sufficient to prevent potential cattle poisoning.