The effect of cyclical and mild heat stress on productivity and metabolism in Afshari lambs.

To investigate the effect of heat stress (HS) on production and metabolism of Afshari sheep, 32 intact male lambs (33.2 ± 4.5 kg) were used in a completely randomized design using 2 experimental periods. In period 1 all 32 lambs were housed in thermal neutral (TN) conditions (25.6 ± 2.6°C and a temperature-humidity index [THI] of 72.0 ± 2.6) and fed ad libitum for 8 d. In period 2 (P2; 9 d), 16 lambs were subjected to cyclical HS (29.0 to 43.0°C and a THI ≥80 for 24 h/d) and the other 16 lambs were maintained in TN conditions but pair fed (pair-fed thermal neutral [PFTN]) to the HS lambs. During each period DMI and water intake were measured daily. Respiration rate, rectal temperature, and skin temperature at the shoulder, rump, and front and rear leg were recorded at 0700 and 1400 h daily. Water intake increased (P < 0.05) during P2 in both HS and TN lambs (88 and 35%, respectively). Heat stress increased the 0700 and 1400 h surface temperature at the shoulder (3.0 and 10.6%), rump (2.7 and 12.7%), rear leg (3.1 and 13%), and front leg (3.0 and 13%) and respiratory rates (72 and 124%; P < 0.01, respectively, for 0700 and 1400 h) but only the 1400 h rectal temperature was increased (P < 0.01; 0.54°C) in HS lambs. Plasma glucose concentration decreased in P2 (P < 0.01) in both the HS and PFTN lambs. Basal insulin concentrations decreased in PFTN controls but increased in HS lambs (environment × period interaction; P < 0.05). Blood urea nitrogen concentration was not affected by environment or period, but NEFA levels were slightly elevated (P < 0.01) in both PFTN and HS lambs during P2. Interestingly, HS did not affect DMI, but ADG was reduced (36%; P < 0.01) compared to the PFTN lambs. These results indicate that the direct effects of heat (not mediated by reduced DMI) are partially responsible for reduced growth in heat-stressed lambs.