The effect of phenylthiocarbamide topically applied to alfalfa pellets on feed intake and behavioral responses of sheep

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

Submitted to: Translational Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/5/2026
Publication Date: 1/30/2026
Citation: Foyil, C.N., Taylor, J.B., Yelich, J.V., Ellison, M.J. 2026. The effect of phenylthiocarbamide topically applied to alfalfa pellets on feed intake and behavioral responses of sheep. Translational Animal Science. 10. Article txag008. https://doi.org/10.1093/tas/txag008.
DOI: https://doi.org/10.1093/tas/txag008

Interpretive Summary: We set out to determine whether bitter tasting ability in young sheep (< 1-year-old) could be identified by measuring sheep's voluntary intake of alfalfa feed pellets topically treated with a bitter flavor. We found that voluntary intake of alfalfa pellets treated with bitter favor was not a conclusive indicator of a sheep's acceptance or rejection of bitter flavor. This is contrary to what we previously observed with voluntary intake of drinking water spiked with bitter flavor. Furthermore, compared with previous studies, young sheep seem less likely to demonstrate absolute aversions to bitter flavor, which is commonly expressed in older sheep. Other intake measurements, such as increased consumption of non-bitter feed after exposure to bitter feed paired with behavioral responses (e.g, lip smacking, head shaking), may be more indicative of young sheep's bitter tasting ability.

Technical Abstract: Voluntary fluid intake studies utilizing phenylthiocarbamide (PTC) have demonstrated variation among sheep in their willingness to consume bitter compounds. Bitter sensitivity has been linked to dietary preferences in other species, leading researchers to inquire whether this ability impacts foraging decisions. A series of experiments was conducted on ram lambs to determine whether bitter tasting ability could be identified using an oral drench or measuring intake of alfalfa pellets topically sprayed with a PTC-solution. For the oral drench study, 2.5 mM PTC or water were applied directly to the tongue. Lambs (n = 62) were monitored (30 sec) for any aversive response. Reactive behaviors were similar (P = 0.20) between water and PTC drenches, indicating the reaction to the drenching procedure masked any responses that may be due to the bitter flavor of PTC. A pilot study was then conducted to determine 1) if group-housed lambs (n = 6) would exhibit an aversive response to topical applications of PTC-treated (LOW: 20 mM; HIGH: 30 mM) feed and 2) the maximum duration of feed consumption (981.4 ± 70.6 sec; P = 0.011). Lambs exhibited a greater (P = 0.002) number of feeding events, positional changes around the feed bunk, and increased (P = 0.002) reliance on olfaction when offered PTC. Based on the results of the pilot study, two back-to-back feed trials were conducted with 28 lambs (Polypay: n = 10; Rambouillet: n = 10; Targhee: n = 8), which were offered alfalfa pellets sprayed with either LOW or HIGH PTC solutions (PTC + EtOH + water) followed by control (EtOH + water) pellets for 15 min each. Feed intake was not affected by PTC (P = 0.18). Control pellet intake was greatest (P = 0.001) after consuming HIGH PTC pellets and was directly correlated (P <0.001, r = 0.45) to intake of PTC-treated feed. Lambs exhibited more (P = 0.001) behavioral reactions to PTC and demonstrated a more continuous feeding pattern when offered control pellets (P = 0.005). Classifications were devised based on the range of behaviors observed over both concentrations of PTC (low: 0-3, n = 14; moderate: 4-6, n = 30; high: 7-10, n = 11). Highly and moderately reactive lambs consumed the greatest amount of PTC and control pellets (P = 0.006). Differences were observed between the classes (P = 0.035) for the sniffing, head shaking and butting, biting, pawing, and fenceline behaviors which may aid in the identification of bitter sensitivity. Further research is needed to determine the effects of bitter tasting ability on the willingness of sheep to consume bitter feeds.