Conditioned feeding responses of sheep towards flavoured foods associated with casein administration: the role of long delay learning

The objective of two experiments was to investigate whether a delayed type of learning could account for the conditioned feeding responses of sheep towards novel food flavours associated with post-ingestive consequences (PIC) created from the administration at different points in time of a nutritive stimulus (casein). The doses of casein were low (15 g) and high (75 g) for experiments 1 and 2, previously known to result in positive and negative PIC respectively. Each experiment consisted of three conditioning periods, during which sheep were trained to associate one of two novel flavours with either casein or water (placebo) administration. During each conditioning, a novel flavoured food low in protein and relatively high in energy, was offered for 3 h (08:00 to 11:00 h) and was followed by an unflavoured, nutritionally similar food for the rest of the feeding time (11:00 to 17:00 h). Sheep were randomly assigned to one of three treatments that were defined by the time when casein or water doses were administered, in relation to the presence of the flavoured food (A= 08:30 and 10:00, B= 11:30 and 13:00 and С = 14:30 and 16:00 h respectively). At the end of each conditioning period preference tests were performed, where sheep were offered a choice between the two flavoured foods. There was no effect of time of casein administration on the conditioned responses towards flavoured foods in either experiment. In both experiments, the proportion of the flavoured food selected was significantly affected by the interaction between preference tests and casein association. For experiment 1 this was due to an increasing preference for the casein associated food accompanied by a decreasing preference for the water associated flavoured food as a result of repeated conditioning. The degree of such preference was different between flavours used for association with casein or water. For experiment 2 avoidance of the casein and preference for the water associated food were established after the completion of the second and reinforced by the third conditioning period. Flavours used had a lesser effect on the conditioned responses of this experiment. The results support the view that sheep develop conditioned responses towards novel food flavours associated with the administration of a nutritive stimulus, even when the PIC resulting from its administration are significantly disassociated in time from the presence of the flavoured food.