Optimizing diets for finishing pigs: balancing ideal protein to enhance growth performance and reduce environmental impacts

To investigate the performance of pigs offered diets with reduced crude protein (CP), but supplying ideal protein with different limiting amino acids (AAs), 280 pigs (PIC 337 × (Landrace × Large White)) were randomly allocated to 1 of 4 treatments (with 7 pen replicates per treatment) balanced for weight and sex. The pigs were penned in mixed-sex (5 females and 5 males) pen groups of 10 and offered treatment diets from 12 wk of age (≈40 kg liveweight) to slaughter (≈112 kg liveweight). The control diet had 175 g/kg CP formulated to contain ideal protein to lysine (Lys), methionine (Met), and threonine (Thr) and the remaining 3 diets contained 150 g/kg CP. Ideal diet 1 had ideal protein to Lys, Met, Thr, tryptophan (Trp), valine (Val), isoleucine (Ile) and arginine (Arg). Ideal diet 2 had ideal protein to Lys, Met, Thr and Trp, and Ideal diet 3 had ideal protein to Lys, Met and Thr. All diets had 13.85 MJ/kg of digestible energy. Feed intake, average daily gain (ADG), and feed conversion ratio (FCR) were measured. ADG was higher (<i>P</i> = 0.021) between 12 and 15 wk of age in pigs fed Ideal 1 (786 g/d) and 2 (755 g/d) diets than pigs fed the control diet (682 g/d), resulting in a heavier body weight (≈2 kg) at 15 wk. The FCR was improved (<i>P</i> = 0.033) between 12 and 15 wk of age in pigs fed Ideal 1 diet (2.08) than that of the control diet (2.41). Diet had no effect (<i>P</i> > 0.05) on overall pig performance between 40 and 112 kg liveweight. Cold carcass weight was heavier (<i>P</i> = 0.006) in pigs fed Ideal diet 1 than pigs fed other diets. A dietary reduction in CP by 25 g/kg fed to boars and gilts from 40 kg reduced N intake (<i>P</i> = 0.003) and excretion (<i>P</i> < 0.001) by 15% and 24%, respectively, with percentage nitrogen retention (average = 41.9%) remaining unaffected across treatments (<i>P</i> = 0.891). Formulating feed down to Ile and Val had production benefits (higher ADG and better FCR) in the early finishing period, although no effect was observed during the late finishing stages. Economically, Ideal 3 diet with 150 g/kg CP balanced for ideal protein to Lys, Met, and Thr was the most cost-effective. We suggest that, while formulating diets down to 6 AAs is not essential, there is need to pay attention to early finishing diets to yield production benefits. We provide further evidence in support of reducing dietary CP without sacrificing overall pig production performance, at the same time, lowering the negative environmental impact.