Refined methodology for identification of bitterness aversion in mature rams through quantification of fluid intake and behavioral response to phenylthiocarbamide

Abstract

Sheep have a sensitive threshold for bitterness, which may affect feed selection. Previous research attempted to classify sheep according by bitter sensitivity by measuring fluid intake of the compound phenylthiocarbamide (PTC) at various concentrations over a 14-d period. Although successful, the study was laborious, lengthy, required multiple layers of acclimation and testing, and accommodated a limited number of animals. Accordingly, a more efficient test is needed to classify bitter-taster status of sheep. The objective of this experiment was to develop a more rapid and refined method to assist in identification of PTC-taster status in rams utilizing only two concentrations of PTC-mixture over a 2-day test period. Mature Targhee, Rambouillet, Polypay, and composite-breed rams (n = 44), were equally stratified into three barns and subjected to 4-d acclimation, 2-d sham, and 2-d testing phases. The sham phase was conducted to determine if ethanol (delivery vehicle for PTC) influenced voluntary fluid intake or behavior; no effect (P ≥ 0.13) was observed. The 2-d test phase was a side-by-side preference study between water and one of two concentrations (0.20 and 2.03 mM) of PTC-mixture delivered on alternate days. Video surveillance was conducted to evaluate the number and duration of approaches to the PTC-mixture and subsequent sniffing, head bobbing, jerking, shaking, and lip movements, smacking, and licking behaviors. Overall, intake of PTC-mixture was less (P < 0.001) than water intake. The PTC-taster classifications were determined based on average PTC intake relative to ± 1.0 standard deviation of the study population mean and used to sort rams into super- (n = 9), intermediate (n = 29), or non- (n = 6) PTC-taster classifications. The amount of time spent drinking either PTC-mixture or water was different (P < 0.001) amongst PTC-taster classifications and correlated with both water (r = 0.23, P = 0.028) and PTC-mixture (r = 0.46, P < 0.001) intake. Head bobbing (P = 0.035) and lip smacking (P < 0.001) were associated with PTC intake across all classes. Super-tasters appeared to utilize sense of smell (sniffing) more frequently to identify PTC-mixture than the intermediate or non-tasters (P ≤ 0.012). Non-tasters did not exhibit behavioral differences (P ≥ 0.12) regardless of solution type. The streamlined methodology outlined in this study will help facilitate future studies aimed at better understanding forage selection and consumption in sheep.