| Abstract | Forty-six Dorper, 46 Katahdin, and 43 St. Croix female sheep (initial BW of 58, 59, and 46 kg, respectively, SEM = 1.75; 3.3 ± 0.18 yr of age, 2.6 – 3.7) from 45 commercial farms in four regions of the USA (Midwest, Northwest, Southeast, and central Texas) were used to evaluate responses to high heat load index (HLI) conditions in a central facility. The four trials entailed sequential 2-wk periods with target HLI during day/nighttime of 70/70 (thermoneutral zone conditions during the day and night), 85/70, 90/77, and 95/81, with weekly measures at 0700 (before increased daytime HLI), 1300, and 1700 h (preceding lower nighttime HLI). Rectal temperature (ºC) in period 3 (38.64, 38.66, and 38.48 at 0700 h, 39.08, 39.23, and 38.84 at 1300 h, and 39.20, 39.15, and 38.99 at 1700 h) and period 4 (38.71, 38.90, and 38.51 at 0700 h, 39.18, 39.12, and 38.83 at 1300 h, and 39.45, 39.34, and 39.07 at 1700 h for Dorper, Katahdin, and St. Croix, respectively) (SEM = 0.054, 0.037, and 0.038 at 0700, 1300, and 1700 h, respectively) ranked St. Croix < Katahdin < Dorper. Nonetheless, there were corresponding differences in respiration rate (breaths/min) in period 3 (63.1, 56.8, and 49.6 at 0700 h, 133.8, 125.8, and 115.7 at 1300 h, and 125.5, 114.7, and 106.8 at 1700 h) and period 4 (81.6, 72.2, and 54.5 at 0700 h, 149.2, 143.6, and 137.3 at 1300 h, and 147.0, 141.4, and 134.2 at 1700 h for Dorper, Katahdin, and St. Croix, respectively) (SEM = 3.12, 4.11, and 3.62 at 0700, 1300, and 1700 h, respectively). In conclusion, based on rectal temperature and respiration rate the overall impact of region was minimal and resilience to high HLI was greatest for St. Croix and lowest for Dorper sheep. |
