Abstract
Abstract
Bitter taste perception has been studied in various species, including humans, using the synthetic compound phenylthiocarbamide (PTC). Human perception of PTC is linked to the ability to taste bitter foods. Sheep express varying levels of aversion to PTC delivered in drinking water. The objective of this study was to determine the effects of PTC delivered in feed on dietary intake and behavior in mature rams. We hypothesized that rams previously classified into PTC-taster categories using PTC-spiked drinking water would rank similarly when PTC was added to feed. Mature Targhee, Rambouillet, Polypay, and composite-breed rams, previously classified as either super- (n = 3), intermediate- (n = 5), or non-PTC tasters (n = 4), were subjected to 4-d acclimation and 6-d test phases. Alfalfa (70%), corn (15%), and beat pulp (15%) meal were mixed with ethanol (150 g/kg, as fed) only (control) or PTC solubilized in ethanol (150 g/kg, as fed) to create three pelleted (6.3 mm) test diets (110, 220 and 330 mg PTC/kg feed, 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 PTC concentration. Feeding duration, number of bucket approaches and switches, sniffs, drinks, lip smacks and licks, and head bobs, jerks, and shakes were recorded throughout the trial using video surveillance. For each PTC concentration, consumption of PTC and control diets was similar (P = 0.22) when measured as a percentage of total diet (control + PTC) consumed. Individual PTC diet intakes ranged from 1.5 to 89.8% of total diet consumed. The CV for intake of 110, 220 and 330 mg/kg diets increased from 22.4 to 34.1 to 36.1%, respectively. Intake of total diet offered 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). Rams were classified by average PTC diet consumption relative to ± 1.0 SD of the population mean (50.2 ± 7.5%). Using this method, only 7 of the 12 rams were classified similarly to the former classification established when PTC was added to drinking water. Total feeding duration was less (P ≤ 0.03) when rams were offered 330 mg/kg (805.4 ± 18.2 s) compared with 110 (858.1 ± 19.3 s) and 220 mg/kg (862.7 ± 19.5 s). Rams switched between the control and PTC diets more (P ≤ 0.02) when offered 110 mg/kg (21.2 ± 10.0) than the 220 (16.4 ± 7.8) and 330 mg/kg (18.4 ± 8.4). These results suggest that sheep perceive and exhibit variable responses to PTC when incorporated into a pelleted feed. More research is needed to identify mechanisms by which sheep perceive bitter-tasting feeds.