Increased dietary protein or free amino acids supply for heat stress pigs: effect on performance and carcass traits

Heat stress (HS) pigs reduce their voluntary feed intake (VFI) and ingestion of indispensable amino acids (AA). Increasing the dietary crude protein (CP) content may help to correct the reduced AA intake by HS pigs, but it may further increase their body heat load. Increasing the AA intake by adding free AA to the diet does not affect the heat load of HS pigs. Two 21-d experiments were conducted. In Exp. 1, 30 pigs (31.1 ± 1.2 kg initial body weight) were used to determine the performance depression because of HS. Treatments were: thermo neutral pigs fed a 22% CP control diet (TN-C); HS pigs fed the control diet (HS-C); HS pigs fed a 14% CP, AA supplemented diet (HS-AA). HS pigs had lower ADG and Lys utilization efficiency, and consumed 20 and 25% less Lys and Thr, respectively, than the TN-C pigs (P < 0.05). In Exp. 2 (comparative slaughter), 25 pigs (33.6 ± 0.65 kg initial body weight) were used to evaluate the effect of extra dietary AA either as protein-bound or free AA on the performance and carcass traits of HS pigs. Treatments were: control wheat-SBM-free Lys, Thr and Met diet (CON); diet with 30% more CP than CON (HSxP); diet added with free AA to contain at least 25% more of each AA than the recommended level (HSxAA). Ambient temperature (AT) ranged from 27.7 to 37.7°C, and body temperature (39.9 to 41.2°C) followed a similar daily pattern as the AT did. There was no dietary treatment effect on daily feed and NE intake (P > 0.10), but the Lys, Thr, and Met intake was higher in pigs fed the HSxP or HSxAA diets than in pigs fed the CON diet (P < 0.05). The daily weight gain (ADG) was not affected (P > 0.10) but G:F tended to be higher and the Lys utilization efficiency (ADG, g/g Lys intake) tended to be lower in HSxP pigs than in CON pigs (P < 0.10). The HSxAA pigs had higher ADG (P < 0.05), and tended to have higher weight of hot carcass and leg muscle, and the weight gain of hot carcass and leg muscle than the CON pigs (P < 0.10). The weight and daily weight gain of loin muscle was higher in the HSxAA than in the HSxP pigs (P < 0.05). Kidney weight and serum urea in HSxP pigs were higher than in CON and HSxAA pigs, but spleen weight was higher in HSxAA pigs than in CON and HSxP pigs (P < 0.05). These results confirm that HS reduces the VFI, and show that increased levels of AA either as free or protein-bound do not additionally reduce the VFI of HS pigs. These also show that extra free AA supply rather than protein-bound AA better ameliorate the reduced growth performance of HS pigs.