This study investigated the effects of low-protein (LP) diets on growth, nutrient digestibility, and carcass composition in pigs reared under environmental stressors (ES). LP diets are a well-established strategy for enhancing the sustainability of pig production. However, when only partially balanced for essential amino acids and fed in the presence of ES, these diets may fail to sustain efficient growth. Forty-eight Swiss Large White female pigs, with an initial body weight (BW) of 20±2.2 kg, were used. Pigs were allocated within BW and litter to pens cleaned daily (D) or twice a week (W). Pigs (N=12) assigned to either the D or W pen, had ad libitum access to isocaloric grower and finisher standard (ST) or a LP diets. The ST diets were formulated according to the Swiss feeding recommendations for pigs and contained 16.4 and 12.5% crude protein (CP), respectively. In the LP diets, CP, digestible Lys, Met, Thr, Trp and Val levels were on average 20% lower than the ST diets. The individual BW and feed intake (FI) was monitored weekly and at each feeder visit, respectively. Energy and nitrogen (N) digestibilities were determined from fecal samples collected at 41.9±2.5 and 71.5±2.3 kg BW. Carcass composition was assessed with dual-energy x-ray absorptiometry. Data were analyzed with linear models (R, v4.4.1), with diet, cleaning protocol and the interaction as main effects, and litter as a random effect. Compared to pigs in D pens, average daily FI and average daily gain of pigs in W pens were 8.4 and 8.7% higher (p<0.01), respectively, but total FI did not differ. The latter was due to pigs in D pens spending 5.6 more days on feed (p<0.05), resulting in comparable gain-to-feed ratios. Pigs fed the LP diets were 7.5% less efficient, with 9.0% higher feed intake and 5.1% lower growth rate (p≤0.02 for each). In LP pigs, gross energy and N digestibilities were on average 2.4 and 3.1% lower than ST pigs, respectively (p≤0.04 for each). Pen cleaning status as well as dietary CP level had an impact on carcass fat content. Pigs in W pens and LP pigs had 6.3 and 10.7% higher carcass fat content (p≤0.03 for each). In conclusion, partly balanced LP diets impaired growth traits, nutrient digestibility and carcass quality. Pigs reared in W pens exhibited improved growth without compromising efficiency but showed also increased carcass fatness. This suggests a trade-off between enhanced growth performance and increased carcass fat under less strict hygiene conditions.
