The interactive effects of high-fat, high-fiber diets and ractopamine HCl on finishing pig growth performance, carcass characteristics, and carcass fat quality1,2

A total of 576 mixed-sex pigs (PIC 327 × 1,050; initial BW = 55.8 ± 5.5 kg) were used to determine the effects of corn dried distillers grains with solubles (DDGS) and wheat middlings (midds) withdrawal 24 d before harvest in diets without or with ractopamine HCl (RAC) on growth performance, carcass characteristics, and carcass fat quality. From d 0 to 49, pigs were fed a corn-soybean meal–based diet (CS) or a diet high in unsaturated fat and crude fiber provided by 30% DDGS and 19% wheat midds (HFF) and not balanced for energy. On d 49, pens of pigs previously fed CS diets remained on the CS diet. Half of the HFF-fed pigs were switched to the CS–based diets, which served as the withdrawal regimen. Finally, half of the HFF-fed pigs remained on the same HFF diet. All 3 regimens were fed without or with 10 mg/kg RAC. There were 12 pens per treatment with 8 pigs per pen. No significant diet regimen × RAC interactions were observed. From d 0 to 49, pigs fed the CS diet had increased (P< 0.001) ADG and G:F compared with pigs fed the HFF diet. Overall (d 0 to 73), pigs fed the CS diets throughout had greater (P< 0.001) ADG and G:F than those fed the HFF diets throughout. Pigs fed the withdrawal diets had greater (P= 0.014) ADG, but similar G:F to those fed the HFF diets throughout. Pigs fed the CS diets throughout had greater (P= 0.025) carcass yield compared with pigs fed the HFF diets throughout, with those fed the withdrawal diets intermediate. Pigs fed RAC had greater (P< 0.001) ADG, G:F, and carcass yield (P= 0.061than pigs not fed RAC. Jowl, backfat, belly, and leaf fat iodine value (IV) were lowest (P< 0.001) for pigs fed the CS diets, highest (P< 0.015) for those fed HFF diets throughout, and intermediate for pigs fed the withdrawal diet. There were no differences in either full or rinsed intestine or organ weights between pigs that were fed CS diets throughout and pigs fed the withdrawal diet; however, pigs fed the HFF diets throughout the study had increased (P =0.002) rinsed cecum and full large intestine weights (P= 0.003) compared with the pigs fed the withdrawal diets. Withdrawing the HFF diet and switching to a CS diet for the last 24 d before harvest partially mitigated negative effects on carcass yield and IV often associated with high-fat, high-fiber ingredients such as DDGS and wheat midds. Feeding RAC for the last 24 d before market, regardless of dietary regimen, improved growth performance and carcass yield.