Cyclic Heat Stress Affects Carcass Characteristics and Fresh Pork Quality of Pigs Despite Zinc Supplementation at a High or Low Level from Inorganic and Organic Sources.

Commercial crossbred mixed-sex pigs (n=120; initially 72.0 kg) were housed under either thermoneutral (TN) or cycling heat (HS) conditions simulating chronic summer heat with acute heat waves. Thermoneutral conditions were 18.9–16.7°C while HS consisted of chronic heat (30°C/26.7°C for 12h:12h) on days 24-72 except during periods of acute heat (32-33°C/29-30°C for 12h:12h) on days 21-24, 42-45, and 63-66. Treatments were arranged in a 2×2×2 factorial with main effects of environment (HS vs. TN), added Zn level (50 vs. 130 mg/kg available Zn added to corn-soybean meal based diet), and added Zn source (100% inorganic from ZnO vs. predominantly organic from Availa^®Zn (Zinpro Corp, Eden Prairie, MN) consisting of 100% Zn from Availa^®Zn at 50 mg/kg level or 62% Zn from Availa^®Zn and 38% from ZnO at the 130 mg/kg level). Pens of 5 pigs each were blocked on initial bodyweight (BW) then randomly allotted to 1 of 8 temperature and diet treatments across a total of 10 complete replicate blocks. After a fixed time of 65 days, a representative pig closest to pen mean BW was selected from each pen and slaughtered at Purdue University Meats Laboratory (n=80). The HS pigs were lighter BW (P=0.039), yielded lighter carcasses (P=0.011; 96.1 kg HS vs. 99.3 kg TN), had less (P=0.032) backfat at the last rib, and tended to have less (P=0.054) leaf fat, smaller (P=0.062) loin eye area (LEA), and lower (P=0.088) dressing percentage. Carcasses of HS pigs exhibited improved carcass quality over TN pigs with higher (P=0.001) 24-hour loin pH, decreased (P=0.034) drip loss, and greater (P<0.050) subjective color and firmness scores. No main effects of Zn source, level, or interactions were observed (P>0.050). Forty additional HS pigs were slaughtered on day 72 after attaining a common weight compared to that of TN pigs previously slaughtered on day 65 (126.4 kg TN vs. 125.8 kg HS, P=0.674). Considered on a fixed weight schedule, 130 mg/kg Zn improved both live BW (P=0.017) and HCW (P=0.014) compared to 50 mg/kg Zn. Compared to 65-day TN pigs, 72-day HS pigs had a lower dressing percentage (P=0.012), tendency for lower (P=0.088) subjective marbling score, but more (P=0.033) BF at the last rib. Also, LEA under 72-day HS conditions was numerically greater when pigs received inorganic Zn but under 65-day TN conditions organic Zn numerically increased LEA (source×temperature, P=0.039). In conclusion, cyclic HS affected carcass quality but Zn level and source imparted negligible benefits.