Conditioned feeding responses of sheep towards flavoured foods associated with the administration of ruminally degradable and/or undegradable protein sources

Abstract

AbstractThe main objective of the experiment was to investigate the conditioned responses of sheep towards food flavours associated with the administration of ruminally degradable protein (RDP) and ruminally undegradable, but readily digestible protein (DUP) sources given either alone or in combination. The experiment consisted of three consecutive periods during which sheep were conditioned to associate a flavoured food with a nutritive stimulus (or water, W). Two foods (basal and novel test) with different crude protein (CP; 92 and 64 g/kg dry matter (DM) respectively) and similar metabolizable energy (≊ 9 MJ/kg DM) contents were used on a total of 48 Texel ✕ Greyface female sheep. The basal food was offered during non-experimental (rest) days whereas the test food was used in combination with two flavours, orange and aniseed, during experimental days. Food was presented for 8 h (09:00 to 17:00 h) daily throughout the experiment. Two nutritive stimuli (casein, C, and formaldehyde treated casein, FC) were chosen such as to provide major contrasts in their RDP and DUP contents, on an isonitrogenous basis. Each dose (50 g) of a particular nutritive stimulus was administered by gavage through a stomach tube twice daily (at 10:00 and 14:00 h). Sheep were randomly assigned to one of four (C v . W, FC v . W , C v . FC, C v . FC + C) treatments (no. = 12 per treatment). For the first 2 days (days 1 + 2) of each conditioning period half of the sheep within each treatment were offered one flavoured food paired with the administration of C (treatments C v . W , C v . FC and C v. FC + C) or FC (treatment FC v. W). The other half were offered the opposite flavoured food paired with the administration of water (treatments C v . W and FC v . W), FC (treatment C v . FC) or C + FC (treatment C v . FC + C). There followed 2 days (days 3 + 4) of rest and for the 2 days subsequently (days 5 and 6) received the opposite flavoured food and the opposite stimuli to that received earlier. In the morning of day 7 sheep were offered a choice between the two flavoured foods for 20 min. After the completion of the preference test sheep were offered the basal food. The same procedure was followed for each of three conditioning periods (i.e. each animal followed the same flavour/stimulus association throughout the experiment). The design was balanced for order of flavour and stimulus presentation. Sheep preferred the flavoured food associated with C (P < 0·05) or FC (P < 0·01) over the opposite flavoured food associated with water in C v . W and FC v . W treatments respectively. In the C v . FC treatment sheep showed a strong preference for food flavours associated with the administration of FC to those associated with C (P < 0·05). In the C v. FC + C treatment sheep showed equal preference towards the food flavours associated with either stimuli. These results: (i) support the view that sheep are able to form learned preferences for food flavours associated with the administration of protein, and (ii) suggest that sheep are able to distinguish between food flavours associated with the administration of both RDP and DUP sources. Sheep preferred flavours associated with DUP administration only over flavours associated with RDP administration only; however, such preferences did not develop when DUP was administered concurrently with RDP. Given the learned responses of sheep towards flavours associated with RDP and DUP the expectation is that they may be able to select their diet on the basis of these qualities when they are offered a choice.