Devaluation of low-quality food during early experience by sheep

Abstract

A ruminant's early experience with low-quality food (LQF) is expected to increase its acceptance and preference in adulthood. Contrarily, we found that experienced sheep (ES) exposed to mature oat hay early in life ate less of this LQF than inexperienced sheep (IS). A possibility is that ES might have devaluated the LQF through continuous comparisons against high-quality food (HQF) supplements (sunflower meal and ground corn) that were simultaneously available during early experience. In this study, we tested the devaluation hypothesis with a successive negative contrast (SNC) procedure. In a consummatory SNC procedure, ‘shift’ subjects are unexpectedly changed from HQF to LQF, and their consumption is then compared against the consumption of ‘unshift’ subjects that receive LQF all throughout the SNC procedure. The magnitude of the difference in consumption between preshift and postshift is regarded as a measure of the degree to which both foods (HQF and LQF) are perceived to differ hedonically. When sheep from our previous study were 300 days old, both ES and IS were randomly assigned to either shift (ES-S and IS-S) or unshift conditions (ES-U and IS-U; n = 6 in each group). Groups ES-S and IS-S were fed HQF (alfalfa hay) during the preshift phase, and then suddenly changed to LQF (oat hay) in the postshift phase. Groups ES-U and IS-U (controls) were fed only LQF throughout the SNC procedure. Subjects in ES-S showed a significantly lower intake of LQF than those in ES-U in the first postshift session (i.e. they showed an SNC effect), which was not observed in IS. These results agree with ES subjects having devalued LQF during early experience. We discuss the possibility that high levels of nutrient supplementation can result in devaluation of LQF (i.e. decrease in preference and acceptance), whereas restricted levels of supplementation may promote a positive experience with LQF.