Nitrogen retention in weanling and finishing pigs fed corn–soybean meal-based diets is greater than 50%, regardless of diet crude protein concentration

The objective was to test the hypothesis that nitrogen retention by weanling and finishing pigs, measured as a percentage of nitrogen intake, is greater than 50%, regardless of the dietary protein level and the body weight of pigs. Six diets containing corn and soybean meal were formulated. Diets were fed to pigs in two phases (i.e., weanling and finishing). Within each phase, a high-protein, medium-protein, and a low-protein diet were formulated. Twenty-four weanling pigs and 24 finishing pigs were allotted to the three dietary treatments. Pigs were housed individually in metabolism crates, and feces and urine were collected quantitatively for 5 days. Nitrogen retention relative to intake ranged from 60.9% to 71.9% and increased ( P < 0.001) as dietary protein was reduced, but nitrogen retention measured as g/day was reduced ( P < 0.05) as dietary protein was reduced. In conclusion, the hypothesis that pigs retain more than 50% of consumed nitrogen was accepted, but pigs fed a low-protein diet have reduced nitrogen retention measured as g/day compared with pigs fed diets with greater protein concentrations.