Influence of dietary crude protein and phosphorus on ileal digestion of phosphorus and amino acids in growing pigs1

A study was conducted to investigate the effect of dietary CP levels on ileal P digestion in growing pigs. A total of 18 ileal-cannulated pigs (44.2 ± 3.2 kg initial BW) were used in a duplicated 9 × 3 incomplete Latin square design, with 9 treatments and three 7-d experimental periods giving 6 replicates per treatment. The 9 treatments consisted of 1 N-free diet to estimate basal endogenous loss of AA and 8 corn–soybean meal–based diets in a 2 × 4 factorial arrangement, which included 2 CP levels (6.9 or 13.4%) and 4 apparent total tract digestible P (ATTDP) levels (0.09, 0.16, 0.24, or 0.32%). Soybean meal and monocalcium phosphate were used to adjust the CP level and ATTDP level, respectively. Limestone was included in diets to maintain a Ca:ATTDP ratio of 2.5 across treatments. Ileal digesta was continuously collected for 10 h on the last 2 d of each 7-d experimental period. The ileal digesta samples were lyophilized and analyzed to calculate ileal digested N and P. Data were analyzed using PROC MIXED of SAS (9.3) and contrasts were used to test the linear and quadratic effects of increasing levels of P within each CP level. In the low-CP group (6.9%), the ileal digested P was 0.71, 1.16, 1.64, and 2.03 g/kg DMI for diets that contained 0.09, 0.16, 0.24, and 0.32% ATTDP, respectively, and was 0.70, 1.54, 2.03, and 1.99 g/kg DMI, respectively, for their counterparts in the high-CP group (13.4%). A low dietary CP level decreased (P< 0.05) ileal digested P (g/kg DMI). The ileal digested P (g/kg DMI) linearly increased (P< 0.01) with increasing ATTDP levels in the low-CP group, but the pattern was linear (P< 0.01) and quadratic (P< 0.01) in the high-CP group. In the low- and high-CP diets, the determined true ileal digestibility of P in monocalcium phosphate was 54.4 and 75.6%, respectively. In conclusion, this research indicated that the ileal digestion of P could be limited by protein deficiency. The dietary CP level should be considered in P digestibility–related studies.