Effect of Cyclic Heat Stress and Supplemented Inorganic and Organic Zinc Source Levels on Grow-Finish Pig Growth Performance, Estimated Body Composition, Intestinal Morphology and Gene Expression

Heat stress is a physiologic state where an animal can no longer properly regulate it’s body temperature to prevent hyperthermia. It has been well documented that heat stress can negatively impact feed intake, growth performance, carcass characteristics, gut morphology and integrity in swine. Finishing swine are particularly sensitive to heat stress which costs the U.S. swine industry millions of dollars annually. There are several ways heat challenges can be mitigated to reduce the negative impacts of heat stress during warmer temperatures, one of which being nutrition. Knowing that heat stress alters intestinal barrier function and morphology, we took a holistic approach in assessing how supplementing finishing pigs with an inorganic zinc source and an organic/inorganic zinc blend during cyclic heat stress could potentially improve gut function and thus growth performance and carcass characteristics. 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. Thermoneutral conditions were 18.9–16.7 °C throughout the study. Heat stress pigs were housed at the same temperature as TN from d 0–18, then increasing temperatures d 18–21 (28 °C/24 °C for 12h:12h, 50.0% humidity) followed by 30 °C/26.7 °C (12h:12h, 52.4% humidity) d 24–70 except during acute heat (32–33 °C/29–30 °C, 12h:12h, 51.5% humidity) on d 21–24, 42–45, and 63–66. Treatments were arranged in a 2×6 factorial with main effects of environment (HS vs. TN) and six dietary zinc supplementations, from either an inorganic (ZnO) or organic (Availa®Zn) source. Dietary Zn treatments were: 1) 50 mg/kg ZnO; 2) 130 mg/kg ZnO: 3) 50 mg/kg organic Zn; 4) 50 mg/kg ZnO + 40 mg/kg organic Zn; 5) 50 mg/kg ZnO + 60 mg/kg organic Zn; and 6) 50 mg/kg ZnO + 80 mg/kg organic 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. Body weight and feed intake were determined at the beginning/end of each acute and chronic heat event. All pigs were ultrasonically scanned at the 10th rib for loin eye area (LEA) and backfat (BF) to calculate percent lean on d 63. Pigs were marketed on d 64 (TN) and d 71 (HS). Growth performance, carcass characteristics, and gut morphology were analyzed by the MIXED procedure and gene expression was anazlyzed with the GLM procedure in SAS 9.4. At d 63, HS pigs were lighter (124.11 vs. 128.34 kg; P \u3c 0.001), had lower overall ADG (826 vs. 901 g/d; P \u3c 0.001), ADFI (2.740 vs. 3.032 kg/d; P \u3c .001), but tended to have greater G:F (0.301 vs. 0.297; P = 0.1204). A diet by environment interaction was noted for overall ADG ( P = 0.0039) and ADFI (P \u3c 0.001). The interaction is an order of magnitude in the reduction of ADG as a result of diet 5 having only a 3.9% reduction in ADG whereas diet 6 had 14.4% reduction in ADG under HS conditions. Pigs under HS had less BF at the 10th rib (16.9 vs. 18.0 mm; P \u3c 0.001) and a smaller LEA (53.5 vs. 55.2 cm2; P \u3c 0.001), but a greater calculated percent lean (54.6 vs. 54.1%; P = 0.039). Overall, HS pigs used 6.13 L more water per day than TN pigs (P = 0.0007) with the biggest difference in water utilization during the d 42–45 acute heat wave (6.17 vs. 15.6 L/pig/day; P \u3c 0.0001). Cyclic heat stress simulating summer conditions reduced growth performance by approximately 8.3%, resulting in 4 kg lighter pigs after 63d. To explore how zinc supplementation impacted gut morphology and jejunal gene expression during cyclical HS, one pig/pen (n = 80) from diets 1, 2, 3, and 6 from each environment was harvested on d 65 at the Purdue University meats lab. Intestinal tissue was collected and analyzed for gene expression (heat shock proteins (HSP) 27, 70, and 90, occludin, and mucin-2) and morphology (n = 80). Ileal villi were shorter in HS pigs (P = 0.020) and jejunal HSP70 (P \u3c 0.080) also tended to increase with HS. A 3-way interaction (Environment x Zn Source x Zn Level) was observed for villus height (P \u3c 0.02) and HSP27 (P = 0.05) expression in the jejunum. The villus height 3-way interaction was primarily caused by the 50 mg/kg inorganic Zn treatment increasing villus height under HS and all other treatments decreasing villus height under HS. (Abstract shortened by ProQuest.