Conditions to evaluate differences among individual sheep and goats in resilience to high heat load index

Thirty-three yearling Katahdin sheep (KAT, 38.9 kg) and Boer (BOE, 28.6 kg) and Spanish goat wethers (SPA, 22.7 kg) were used to determine conditions appropriate for evaluating resilience to high heat load index (HLI). Grass hay (69% NDF and 9.5% CP) was consumed ad libitum with concentrate supplemented at 0.5% BW. Period 1 was 2 wk and periods 2-5 were each 1 wk. Target HLI for the five periods during the day/night was 70/70, 80/70, 90/76.5, 95/80.75, and 100/85, and measured HLI was 66/66, 80/75, 92/84, 97/86, and 101/89, respectively. Respiration rate increased with advancing period except from period 4-5 when there was a smaller decline for KAT than for BOE or SPA. Rectal temperature also increased as the experiment progressed until period 4 and was similar among animal types in period 5 when values for BOE and SPA were lower than in period 4, in contrast to similar values for KAT. Respiration rate at 13:00 and 17:00 h increased with advancing period up to a plateau at 150-155 breaths/min converse to much lower rates (i.e., 32-83) at 06:00 in periods 2-5. Respiration rate at 06:00 h differed more among days of period 5 than at 13:00 or 17:00 h, with values increasing from day 1-3 and thereafter generally declining from 118 to 37 breaths/min on day 7. Rectal temperature for KAT was lower than for goats early in period 5 but similar among animal types on days 6 and 7. In conclusion, a HLI in the range of 95/80.75 and 100/85 seems appropriate, periods longer than 1 wk appear necessary for full adaptation, and measures should occur during both night and day.