Feed intake and behavioral responses of sheep provided phenylthiocarbamide internally incorporated into feed pellets

The objective of this study was to determine the effects of phenylthiocarbamide (PTC), a bittering agent, delivered in feed on dietary intake and behavior in PTC-naive ram lambs (n = 26) and PTC-experienced mature rams (n = 12) previously classified as either super- (n = 3), intermediate (n = 5), or non-PTC tasters (n = 4). For mature rams, four treatment diets were prepared to include 0 (control), 110, 220, and 330 mg PTC/kg. For ram lambs, three treatment diets were prepared to include 0 (control), 180 and 280 mg PTC/kg. Treatment diets were prepared using a pelleted common ground ration (70 % alfalfa hay, 15 % corn, 15 % dehydrated beet pulp, as-fed). Rams received control and PTC diets (0.87 % BW) daily for 30 min in a side-by-side presentation, which was replicated for each concentration. Feed intake duration, number of bucket approaches and switches, sniffs, drinks, head bobs, jerks, shakes, lip smacks and licks were recorded using video surveillance. Consumption of PTC and control diets were similar for all rams (P ≥ 0.22) when measured as a percentage of total diet (control + PTC) consumed. While mature rams began to exhibit signs of PTC aversion at 220 mg/kg (CV = 34.1 %; P = 0.16), ram lambs were indifferent (P = 0.61) to concentrations up to 280 mg/kg (CV = 13.9 %). For the mature rams, total intake of feed decreased (P = 0.01) from 80.9 ± 3.2 % to 72.3 ± 3.2 %, with respect to increasing concentrations of PTC. Total feed intake duration was less (P ≤ 0.01) when mature rams were offered 330 mg/kg (805.6 ± 440 s) compared with 110 (858.9 ± 44.3 s) and 220 mg/kg (862.9 ± 44.0 s). Although ram lambs consumed total test feed to near entirety, ram lambs took longer (P < 0.01) to consume 280 mg/kg PTC (438.4 ± 26.5 s) compared to control (332.2 ± 27.4 s) pellets. Head bobbing and lip smacking observations increased after mature rams (solution type $\times$ replication, P ≤ 0.01) and ram lambs (solution type, P ≤ 0.01) consumed PTC pellets. The rejection threshold appears to be different for mature rams versus lambs. These results suggest that sheep perceive PTC concentrations between 180 and 280 mg/kg when PTC is incorporated into feed pellets. More research is needed to understand the influence of PTC tasting ability on dietary selection in sheep.