Effect of Cyclic Heat Stress and Supplemented Inorganic and Organic Zinc Source Levels on Grow-Finish Pig Growth Performance and Estimated Body Composition.

Six hundred cross-bred pigs were housed under thermoneutral (TN) or cycling heat (HS) conditions simulating summer heat with acute 3-day heat waves for a 70-day study. TN conditions were 18.9–16.7°C throughout the study. HS pigs were housed at the same temperature as TN from d0-18, then increasing temperatures d18-21 (28°C/24°C for 12h:12h, 50.0% humidity) followed by 30°C/26.7°C (12h:12h, 52.4% humidity) d24-70 except during acute heat (32-33°C/29-30°C, 12h:12h, 51.5% humidity) on days 21-24, 42-45, and 63-66. Treatments were arranged in a 2×6 factorial with main effects of environment (HS vs. TN) and dietary zinc supplementation: 1) 50 mg/kg ZnO; 2) 130 mg/kg ZnO: 3) 50 mg/kg of organic Zn from Availa^®Zn); 4) 50 mg/kg ZnO + 40 mg/kg Availa^®Zn); 5) 50 mg/kg ZnO + 60 mg/kg Availa^®Zn; and 6) 50 mg/kg ZnO + 80 mg/kg Availa^®Zn. Pigs (5/pen) were blocked by initial BW (72.2 kg) and randomly allotted to 1 of 12 temperature and diet treatment combinations across 10 replicates. There were 5 rooms per thermal environment, each containing 2 blocks of dietary treatments. BW and feed intake were determined at the beginning and end of each acute heat event. All pigs were ultrasonically scanned at the 10^th rib to predict loin eye area (LEA), backfat (BF), and percent lean. Data were analyzed by the GLM procedure in SAS with pen being the experimental unit. Pigs were marketed on d63 (TN) and d70 (HS). At d63 HS pigs were lighter (124.11 vs. 128.34 kg; P<.001), had lower overall ADG (826 vs. 901 g/d; P<.001), ADFI (2.740 vs. 3.032 kg/d; P<.001), but tended to have greater G:F (0.301 vs. 0.297; P=0.059). A diet by environment interaction was observed for ADG (P=0.043) and approached a tendency for ADFI (P=0.141) and G:F (P=0.135).ADG interaction is an order of magnitude in the reduction of ADG is a result of diet 5 having only a 3.8% reduction in ADG whereas diet 6 had 9.2% reduction in ADG. Pigs under HS had less BF at the 10^th rib (16.9 vs. 18.0 mm; P<0.001) and a smaller LEA (53.5 vs. 55.2 cm²; P<0.001), and a greater percent lean (54.6 vs. 54.1%; P=0.039). Cyclic heat stress simulating summer conditions reduced growth performance by approximately 8.3%, resulting in 4kg lighter pigs after 63d. A blend of supplemental zinc at 110mg/kg may have helped mitigate the reduction in growth performance due to a summer heat stress and warrants further research.