Mild Conditioned Food Aversions Developed by Sheep Towards Flavors Associated with Plant Secondary Compounds

Abstract

The objectives of the experiment were to test whether sheep develop conditioned flavor aversions (CFAs) towards a food flavor associated with the administration of a plant secondary compound, and providing that such CFAs develop, to determine how long they persist. For experimental purposes, two natural secondary compounds were used—quebracho (mainly condensed tannins) and oxalic acid—at doses previously known to produce negative postingestive consequences in sheep. The experiment consisted of four conditioning periods (each eight days long) during which the novel flavored foods (flavored hay) were offered concurrently with oral administration by gavage of the secondary compounds. In the conditioning period four groups of sheep (N = 6 each) were arranged such that each group received different flavored feeds (orange and aniseed) for two-day sequential periods. One flavor was paired with one of the model secondary compounds. Between the two sets of two days and after the second two-day period there was a period of two “rest” days, to avoid residual physiological effects of secondary compound administration. The intakes of flavored hay were recorded during each conditioning day and the effects of the secondary compounds were quantified in rumen (for volatile fatty acids and ammonia concentrations) and blood samples (for plasma calcium levels). At the end of each conditioning period, the preference for the two flavors was measured by a two-choice short-term preference test (20 min each); the persistence of the CFAs was similarly measured at 0, 10, 30 and 60 days after the completion of the conditioning with two-choice longterm preference tests (3 hr each). Results of the experiment indicated that quebracho administration did not lead to development of CFAs at the level administered. This does not necessarily reflect the sheep's inability to develop CFAs towards tannins but may reflect possible trade-offs between the ingestion of nutrients and secondary compound concentration that might have been imposed on the sheep in this experiment. In other words, it might reflect the increased familiarization with the hay (i.e., increase in its intake and hence of nutrients), which overcame the negative effects of quebracho administration. However, oxalic acid caused CFA in sheep: hay intake tended to be reduced during the days that the secondary compound was administered, and importantly the secondary compound associated flavored hay was partially avoided during the two-choice preference tests. The latter avoidance was not affected by time (consistent across conditioning periods). This CFA persisted for up to 60 days after the completion of conditioning in the absence of intervening oxalic acid exposure. Thus, sheep were able to form CFAs towards a flavor associated with the administration of a naturally occurring organic plant secondary compound.