Flavour preference conditioned by postabsorptive propionate and acetate in wethers

Previous studies have indicated that intraruminal infusions of acetate and propionate condition flavour preference in sheep. Three experiments were conducted to evaluate and compare the preference-reinforcing effects of postabsorptive acetate and propionate. Eight wethers were used in Experiment 1, and a different eight wethers were used in Experiment 2 and 3. In Experiment 1, wethers were divided into two groups, and a half of each group (n =2) was given clove-flavoured straw and the other half was given cardamom-flavoured straw on odd-numbered days during a 12-d conditioning period. During straw ingestion, wethers in one group received intramesenteric infusion of sodium propionate (0.56mmol/kg) and the other group received saline infusion. On even-numbered days, flavours and infusate were switched, so that each group received a new flavour and a new infusate. When wethers were offered a choice between the two flavoured straw, they preferred (P <0.01) the flavoured straw that had been paired with propionate infusion. In Experiment 2 and 3, two trials were conducted using four animals in each trial. In Experiment 2, wethers were given clove-flavoured (n =2) or nutmeg-flavoured (n =2) straw paired with intramesenteric infusion of sodium acetate (1.69 and 0.85mmol/kg in Trial 1 and 2, respectively) on odd-numbered days and the other flavoured straw paired with saline infusion on even-numbered days during a 10-d conditioning period. In subsequent two-choice tests, wethers in Trial 1, but not in Trial 2, showed a preference (P <0.05) for the flavoured straw paired with the acetate infusion. Wethers used in Trial 2 were then given 10 additional days of conditioning with intrajugular infusion of 1.69mmol/kg of sodium acetate and saline, but preferences for the flavour paired with acetate infusion were not developed. In Experiment 3, wethers were given fennel-flavoured (n =2) or nutmeg-flavoured (n =2) straw paired with sodium propionate infusion (1.69 and 0.96mmol/kg in Trial 1 and 2, respectively) into the ruminal vein on odd-numbered days and the other flavoured straw paired with saline infusion on even-numbered days during a 10-d conditioning period. In subsequent two-choice tests, wethers in Trial 1 tended to prefer (P <0.10) the flavoured straw paired with propionate infusion to that with acetate infusion. In Trial 2, wethers avoided nutmeg-flavoured straw more than fennel-flavoured straw, but the intake of each flavoured straw tended to be higher (P <0.10) when it was paired with propionate. These results suggest that acetate and propionate generate preference-reinforcing signals in the portal-hepatic area, but the signal generated by acetate is weaker than that by propionate.