Effects of High Heat Load Conditions on Body Weight, Dry Matter Intake, and Blood Constituent Levels of Dorper, Katahdin, and St. Croix Sheep from Different Regions of the USA

Thirty-seven Dorper, 35 Katahdin, and 31 St. Croix ewes (57, 58, and 44 kg; SEM = 2.2) from 45 commercial farms in the Midwest (MW), Northwest (NW), Southeast (SE), and central Texas (TX), between 2.2 and 3.4 yr of age, were used to evaluate responses to high heat load index (HLI) conditions. There were four sequential 2-wk periods with target HLI during day/nighttime of 70/70, 85/70, 90/77, and 95/81. A 15% CP and 50% concentrate pelleted diet was fed at 120% of the ME requirement for maintenance, and water was offered free-choice. Body weight was measured three times each week, and blood was sampled at 1300 h on the last day of each period. There was an interaction (P < 0.001) between period and week within period in BW, with slightly greater values in wk 2 vs. 1 of periods 3 and 4 and a greater difference between period 1 and 4 values in wk 2 than 1 (53.1, 54.1, 54.9, and 55.4 kg in wk 1, and 53.0, 54.2, 55.4, and 56.1 kg in wk 2 in periods 1, 2, 3, and 4, respectively; SEM = 0.85). There was an interaction (P = 0.037) in DMI (g/kg BW^0.75) among region, period, and week, with values generally similar between weeks in periods 1 and 2 relative to those in periods 3 and 4 (51.0, 52.4, 51.0, and 51.2 in period 3 and wk 1, 49.5, 52.1, 50.8, and 51.6 in period 3 and wk 2, 49.6, 52.1, 50.6, and 49.4 in period 4 and wk 1, and 48.9, 52.0, 49.7, and 46.3 in period 4 and wk 2 for MW, NW, SE, and TX, respectively; SEM = 1.09). Neither blood glucose nor lactate concentration was affected by breed (P > 0.05), but there were breed differences (P < 0.02) in serum concentrations of creatinine (0.91, 0.81, and 0.77 mg/dL; SEM = 0.023), total protein (6.13, 6.42, and 6.81 g/dL; SEM = 0.156), and urea N (17.4, 18.0, and 20.0 mg/dL for Dorper, Katahdin, and St. Croix, respectively; SEM = 0.54). In conclusion, some blood constituent levels suggest breed differences in resilience to high HLI. Differences among periods and weeks in BW presumably relate to increased water consumption with high HLI. The interaction in DMI may reflect differences among regions in rate of adaptation to high HLI and the contribution of decreased feed intake to coping with high HLI.