Reducing dietary crude protein (CP) in pig production can improve sustainability by increasing nitrogen (N) utilization, but low-protein (LP) diets must be fully balanced in essential amino acids and fed under stress-free conditions to maintain pig growth and carcass quality, otherwise performance may suffer. Therefore, fourty-eight Swiss Large White female pigs (20 ± 2.2 kg BW) were reared in pens cleaned daily (D) or twice a week (W) and fed standard (ST) or LP diets according to a 2×2 factorial design. The ST grower and finisher diets were formulated to be isocaloric and contain 16.4 and 12.5% CP, respectively. The LP diets were obtained by reducing CP and digestible Lys, Met, Thr, Trp, and Val levels by about 20% compared to the respective ST diets. Weekly BW, feed intake (FI), and carcass composition by dual-energy x-ray absorptiometry were assessed for each pig. Data were analysed by linear mixed models (R, v.4.4.1), with diet, pen cleaning protocol and the interaction as fixed effects, and the litter of origin as random effect. The pen cleaning protocol influenced average daily feed intake and average daily gain. Compared to D, W pigs ate 8.4% more feed and grew 8.7% faster (p < 0.01), but total FI was similar as D pigs spent 5.9 more days on feed (p < 0.01), resulting in comparable feed efficiency. Regardless of the pen cleaning status, LP pigs were 7.5% less feed efficient, consuming 9.1% more feed and growing 4.9% less per day (p ≤ 0.03). Carcass protein deposition efficiency was 5.4% lower in W than D pigs and 10.4% higher in LP than ST pigs (p ≤ 0.02). Carcass fat content was 6.3 and 10.7% higher in W and LP pigs compared to D and ST pigs, respectively (p ≤ 0.04). In conclusion, partially balanced LP diets improved protein efficiency but impaired growth and carcass characteristics. Despite growing faster, W pigs had fatter carcasses and lower protein deposition efficiency which tended to be more evident when pigs were fed partially balanced LP diets.
