Avoidance of Phenylthiocarbamide in mature Targhee and Rambouillet rams

Authors: HENSLEE, DILLON - University Of Idaho; YELICH, JOEL - University Of Idaho; Taylor, Joshua - Bret; ELLISON, MELINDA - University Of Idaho

Submitted to: Translational Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/7/2019
Publication Date: 10/5/2019
Citation: Henslee, D., Yelich, J., Taylor, J.B., Ellison, M. 2019. Avoidance of Phenylthiocarbamide in mature Targhee and Rambouillet rams. Translational Animal Science. 3(4):1194-1204. https://doi.org/10.1093/tas/txz125.
DOI: https://doi.org/10.1093/tas/txz125

Interpretive Summary: Sheep that like bitter foods. Some people like bitter-tasting foods and drinks, like broccoli and coffee, but others don’t. Genetics drive our acceptance of bitter foods, and sensitivity to bitterness affects what we choose to put on our plates and in our cups. Based on this, we asked, can sheep distinguish between plants that do or do not taste bitter? Phenylthiocarbamide (PTC) is a compound, not found in nature, that mimics bitter tastes found in food. Researchers have demonstrated that people that can taste PTC avoid bitter-tasting foods and drinks. We offered drinking water spiked with various levels of PTC to sheep in a side-by-side presentation with regular tap water to see if some sheep can taste or not taste the PTC. We, indeed, observed that some sheep refused the PTC-spiked water, while others consumed the PTC-spiked water as freely as the regular tap water. We further determined that, from individual to individual, sheep vary in their sensitivity to PTC. Our next step is to determine if the degree of sensitivity to PTC can be used as an indicator of a sheep’s preference for certain plants. Ultimately, a greater understanding of sheeps’ preference or avoidance of certain plants will enable us to develop groups of sheep that can target specific plants while avoiding others.

Technical Abstract: Shrub encroachment on grasslands is a worldwide issue and sheep are a potential tool for mitigating shrub encroachment. Many shrubs, however, contain bitter-tasting compounds that may deter grazers. Cattle and sheep commonly graze rangelands, but of the two, sheep have a greater tolerance for bitter compounds and would be expected to consume more bitter-tasting vegetation. We hypothesized that sheep could detect (i.e., taste) bitter-tasting compounds and the sensitivity to these compounds would vary from animal to animal. The objective of this study was to determine whether sheep could detect the bitter-tasting compound phenylthiocarbamide (PTC), and if so, what PTC concentration would elicit an avoidance response. Using a crossover study design, mature Rambouillet and Targhee rams (n = 30) were subjected in randomized order to various PTC concentrations mixed in the drinking water (PTC-solution). In trials 1 and 2 (n = 15/trial), 0.20, 0.56, 1.57, 4.39, and 12.29 mM and 0.20, 0.43, 0.94, 2.03, and 4.39 mM of PTC were tested, respectively. On test days, PTC-solution (trial 1: 1.5 kg; trial 2: 3.0 kg) and water (same amounts) were offered for ad libitum intake in a side-by-side presentation for 1 h in trial 1 and 2 h in trial 2. Each test day was followed by a rest day where PTC-solution was replaced with water to limit potential carry over effects into the next test day. Consumption of PTC-solution for each PTC concentration was expressed as the percentage of PTC-solution intake of total morning fluid intake. There was no effect (P > 0.74) of sequence that rams received PTC-solutions on PTC consumption during either trial. As PTC concentration increased, percentage of PTC-solution intake decreased (P 0.05) from each other in percentage intake. This research suggests rams could taste the PTC, and the concentration at which PTC-solution was avoided varied across rams. It may be possible to select sheep, based on demonstrated avoidance of PTC, for targeted grazing applications to manipulate vegetation towards range management goals.