Effects of feeding graded levels of protein in diets adequate in indispensable amino acids on performance of weanling pigs

Experiments were conducted to determine if the crude protein (CP) concentration of diets adequate in indispensable amino acids (IAA) could be reduced below recommended concentrations, and if so, attempts were made to quantify the dietary CP concentration at which nonspecific nitrogen became limiting for weanling pigs. In Exp. 1, average daily gain (ADG; P .05), but increased the plasma (alpha)-amino N concentration (PAN; P .05) but G/F (P \u3c .05) and plasma urea N concentration (PUN; P \u3c .001) increased linearly with increasing protein (16, 18, 20, and 22% CP diets). Changing amino acid ratios associated with increasing dietary protein did not affect performance in Exp. 4. With increasing protein (15, 18, and 21% CP), increased linearly. In Exp. 5, increasing protein (12, 15, and 18% CP diets), linearly increased ADG (P \u3c .06) G/F (P \u3c .05), PUN (P \u3c .001), and daily gains of water (P \u3c .05) and protein (P \u3c .01), and ether extract concentration of the empty body decreased linearly (P \u3c .05). Adding three CP percentage units (GG) to 12 and 15% CP diets decreased ADG (P \u3c .01), ADF (P \u3c .001), ether extract concentration of the empty body (P \u3c .001), and daily gains of empty body protein (P \u3c .01), ether extract (P \u3c .001) and ash (P \u3c .01). Plasma somatomedin-C was higher (P \u3c .01) with the 12 and 12+GG than with the 15 and 15+GG diets. The nonspecific N requirement varied depending on the response criterion measured. Fifteen to 16% CP diets adequate in IAA were sufficient for maximum gains, but G/F improved as dietary CP increased from 12 to 22%, and carcass leanness increased over the range from 12 to 18% dietary CP. Because the GG and GG+IAA mixtures reduced feed intake, they are not good replacements for natural proteins