Comparison between hair (St. Croix) and wool (Targhee and Dorset) lambs regarding the effect of heat stress on food and water intake, digestive function, and nitrogen balance

Abstract

Three experiments were conducted to evaluate the effects of heat stress on food and water intake, digestive function, and nitrogen balance in St. Croix (hair sheep), and Targhee and Dorset (wool sheep) lambs. In Exp. 1 four 6-months-old St. Croix and four Targhee lambs were sequentially exposed for 34-d to each of three air temperatures (AT) 8, 18 and 30°C, at which humidity indices (THI) were 48, 62, and 92, respectively. Feed was offered ad libitum and then restricted to 38.5 g/kgW0.75 from d 15 to 24 and d 25 to 34, respectively, at each AT. St. Croix lambs consumed more feed (P<0.05) at 8 and 30°C and less water (P=0.07) at 30°C AT than Targhee lambs. In Exp. 2, four 4-month-old St. Croix and Dorset lambs were kept at 15 and 34°C AT (THI 59 and 88) for two consecutive periods de 45-d (14-d adaptation and 31-d comparison). Both breeds consumed more water (P<0.05), but Dorset lambs consumed less feed (P<0.05) at higher AT. The weight gain by Dorset lambs at lower AT was similar to that of St. Croix at higher AT. Apparent digestion coefficients for dry matter (DM), crude protein (CP), and neutral detergent fiber (NDF) were higher (P<0.05) at 34°C than 15°C AT in both breeds. Nitrogen balance was not affected by AT in St. Croix lambs but was lower (P<0.05) at 34°C than at 15°C AT in Dorset lambs. In Exp. 3, eight 7-month-old Dorset lambs were kept in the natural environment at 5.8 to 15.6°C AT (THI ranged from 46 to 60) for 48 d. Another group was held at heat stress in a temperature-controlled room, where a 12 h dark cycle at 24.5°C and 12 h light cycle at 31.1°C were maintained (THI ranged from 73 to 83) for 35 d.