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

Authors: FOYIL, CLAIRE - University Of Idaho; Taylor, Joshua; YELICH, JOEL - University Of Idaho; ELLISON, MELINDA - University Of Idaho

Submitted to: Applied Animal Behaviour Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/11/2025
Publication Date: 5/12/2025
Citation: Foyil, C.N., Taylor, J.B., Yelich, J.V., Ellison, M.J. 2025. Feed intake and behavioral responses of sheep provided phenylthiocarbamide internally incorporated into feed pellets. Applied Animal Behaviour Science. 250. Article 107528. https://doi.org/10.1016/j.smallrumres.2025.107528.
DOI: https://doi.org/10.1016/j.smallrumres.2025.107528

Interpretive Summary: We set out to determine if bitter flavors added to feed would alter sheep’s voluntary diet intake. We learned that the acceptance or rejection of bitter-flavored diets varies from sheep to sheep and the ability of sheep to taste bitter flavor may not directly influence its dietary intake. Regardless, bitter taste perception is crucial to the formation of learned flavor aversions. Its influence on sheep’s dietary selection may depend on the formation of flavor aversions that commensurate with grazing experiences. Accordingly, further research is needed to understand the dynamic relationship between a sheep’s dietary intake and its ability to taste bitter flavor and how this information may be strategically used in grazing management of rangeland vegetation.

Technical Abstract: 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 the ram lamb experiment, three treatment diets were prepared to include 0 (control), 180 and 280 mg PTC/kg. For the mature ram experiment, four treatment diets were prepared to include 0 (control), 110, 220, and 330 mg PTC/kg. Treatment diets were prepared using a common ground ration (70% alfalfa hay, 15% corn, 15% dehydrated beet pulp, as-fed basis), which was pelleted after inclusion of PTC. For both experiments, rams received control and PTC diets (0.87% BW) daily for 30 min in a side-by-side presentation and replicated for each concentration. Feed intake duration, number of bucket approaches and switches, sniffs, drinks, lip smacks and licks, and head bobs, jerks, and shakes 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%). Total intake of feed offered to mature rams decreased (P = 0.01) between 110 mg/kg (80.9 ± 3.2%) and 330 mg/kg (72.3 ± 3.2%), while 220 mg/kg (78.4 ± 3.2%) was intermediate (P = 0.06). 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 × 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 ram 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.