Performance responses and indicators of gastrointestinal health in early-weaned pigs fed low-protein amino acid-supplemented diets

The effects of low-protein AA-supplemented diets on piglet performance, visceral organ mass, incidence of diarrhea, intestinal microbial population, and fermentation were studied in a 3-wk trial. After a 7-d adaptation period, 96 piglets (similar to 6.2 kg of initial BW) were assigned to 4 corn-wheat, soybean meal-based dietary treatments in a completely randomized design to give 6 replicate pens per treatment (n = 4 piglets per pen). The treatments were a control wheatcorn-soybean meal-based phase I diet containing 23% CP, or the same diet with CP reduced to 21%, 19%, or 17% and supplemented with crystalline AA to achieve equal standardized ileal digestible contents of Lys, Met plus Cys, Thr, and Trp in all diets. Diets were formulated to similar nutrient levels and provided ad libitum. Blood from all pigs was taken on d 0, 7, 14, and 21 for determining plasma urea N. Weekly feed intake, BW changes, and G:F were determined. On d 21, 2 pigs per pen were randomly selected and killed to determine small intestinal morphology, digesta pH and ammonia levels, and luminal microbial counts. Average daily feed intake, ADG, and GY were not affected (P > 0.10) by reducing CP to 21%, but a reduction to 19% or 17% decreased ADFI (P < 0.001) and ADG (linear, P < 0.001; quadratic, P < 0.05) over the 3-wk study period. Reducing CP to 19% had no effect (P > 0.10) on G:F; however, this response criterion was decreased linearly (P < 0.001) over the 3-wk study period as dietary CP declined. Water usage was only numerically decreased (P > 0.10) with dietary CP reduction. Plasma urea N was decreased linearly (P < 0.01) with CP reduction. Reducing CP from 23 to 17% had a linear (P < 0.05) and cubic effect on stomach and liver weights, respectively. Although histological data showed some differences among diets, no distinct trend was evident. Ammonia N in ileal digesta was reduced linearly (P < 0.01) as dietary CP was decreased. With the exception of valeric acid, VFA levels in ileal digesta of piglets fed low-protein diets were generally lower (P < 0.05) compared with the control diet. Diet had no effect on intestinal microbial counts (P > 0.10). The results show that piglet performance may suffer when dietary CP is reduced by 4 or more percentage units from 23% and support the hypothesis that low-CP diets help maintain enteric health in pigs by lowering toxic microbial metabolites such as ammonia.