The Effect of Dietary Protein Restriction in Phase Feeding Systems on Nitrogen Metabolism and Excretion in Pig Production

In recent years, much attention has been paid to environmental protection, not only by reducing emissions of harmful gases from industry, but also by reducing the excretion of biogenic compounds or ammonia emissions from agriculture, including animal production. The aim of this study was to determine the effects of complete diets with reduced inclusion levels of crude protein and limiting essential amino acids, fed to pigs in two- and three-phase feeding systems, and the feeding system on crude protein digestibility, nitrogen retention and utilization, fecal and urinary pH, fecal and urinary nitrogen and ammonia levels, and nitrogen excretion. Digestibility-balance trials were performed on 24 growing–finishing pigs housed in individual metabolism crates, in three groups, in two- and three-phase feeding systems. The pigs were fed the following diets: C-control diet; L-low-protein diet where the levels of crude protein and essential amino acids (lysine, methionine + cystine, threonine, and tryptophan) were reduced by 15% relative to diet C; L+AA-low-protein diet supplemented with crystalline lysine, methionine, threonine, and tryptophan to the standard levels (as in diet C). Diets L fed to pigs in two- and three-phase feeding systems significantly decreased crude protein digestibility and nitrogen retention, particularly in the three-phase system. The supplementation of diets L+AA with crystalline essential amino acids improved crude protein digestibility and nitrogen retention and utilization, especially in the two-phase system. Reduced concentrations of crude protein and essential amino acids in diets L contributed to a significant increase in feces and urine acidity in both two- and three-phase feeding systems. The supplementation of diets L+AA with essential amino acids resulted in a significant increase in urinary pH and a non-significant increase in fecal pH. This experimental factor had no effect on fecal ammonia concentration in group L+AA. The values of pH and total fecal nitrogen were somewhat higher in the two-phase system than in the three-phase system. Fecal ammonia concentration was similar in both systems. The three-phase feeding system contributed to a decrease in urinary pH and total urinary nitrogen. The analyzed feeding systems had no significant effect on urinary ammonia concentration. It was estimated that a reduction in crude protein (by 20–25 g/kg) and essential amino acid levels in pig diets, relative to the standard levels, reduced nitrogen excretion by 18.7% and 15.6% in two- and three-phase feeding systems, respectively. The supplementation of low-protein diets (L) with lysine, methionine, threonine, and tryptophan induced a further reduction in nitrogen excretion. A comparison of the effects of feeding systems (two-phase system vs. three-phase system) on crude protein digestibility and nitrogen retention and utilization revealed that better results were obtained in the two-phase feeding system.