Increasing standardized ileal digestible arginine to lysine ratio increases growth performance with limited effects on measured gut functions in 8.5- to 12-kg pigs

Limited Arg availability may impact growth performance in the early post-weaning period. Arginine, in addition to its main role as a building block for protein, is a substrate for nitric oxide production and polyamines, which could have implications for gut functionality in nursery pigs. The objective of this study was to determine the impact of increasing the ratio of dietary standardized ileal digestible (SID) Arg to Lys on nursery pig growth performance, nutrient digestibility, and gut function markers. A total of 240 pigs (initial body weight, 8.88 ± 1.15 kg) were weaned at 28 days and assigned to one of four dietary treatments (net energy, 2,515 kcal/kg; crude protein, 22.7%; SID Lys, 1.34%; five mixed-sex pigs per pen): 1) R75 (SID Arg: Lys, 0.75; n = 12); 2) R90 (SID Arg: Lys, 0.90; n = 12); 3) R105 (SID Arg: Lys, 1.05; n = 12); and 4) R120 (SID Arg: Lys, 1.20; n = 12). Arginine, above that in the R75 diet, was supplied as crystalline L-Arg; diets were made isonitrogenous with L-Ala. Pigs had free access to diets for 14 d; body weight and pen feed disappearance were measured weekly and pen fecal samples were collected over the last 3 d of the study. At day 7 post-weaning, 24 pigs (n = 6 per treatment) were removed from the growth performance study and used for a parallel gut function study. A catheter was placed in the jugular vein for serial blood sampling. After recovering from surgery for 3 d, pigs were administered lactulose and mannitol to assess gut permeability and an intravenous injection of L-alanyl-L-glutamine to evaluate gut citrulline production. These pigs were subsequently euthanized and gut tissue was collected for morphology and digestive enzyme analysis. Increasing SID Arg: Lys ratio increased average daily gain, final body weight, and gain: feed ratio linearly (P < 0.05) without affecting average daily feed intake (P > 0.10). Apparent total tract digestible gross energy and crude protein also increased linearly (P < 0.05) and quadratically (P < 0.01) with increasing SID Arg: Lys ratio. Although increasing SID Arg: Lys ratio increased plasma nitric oxide concentration, SID Arg: Lys ratio did not affect gut morphology, digestive enzyme activity, permeability, and citrulline production. In summary, increasing SID Arg: Lys ratio up to 1.20 increases nursery pig growth performance in the early post-weaning period. The increase in performance, however, appears to be largely independent of measured impacts on gut function.