True digestible phosphorus requirement of 10- to 20-kg pigs

The objective of this study was to determine the true digestible P requirement of 10- to 20-kg pigs by broken-line regression analysis of growth performance using 6 levels of true total-tract digestible P. In Exp. 1, 48 barrows were used to determine the true total-tract digestibility (TTTD) of P in monocalcium phosphate using the regression method. The dietary treatments included a negative control (3.30 g/kg total P) and 7 additional dietary treatments with incremental addition of 0.74 g/kg P through monocalcium phosphate, resulting in a dietary range of 3.30 to 8.45 g/kg total P. Limestone was added accordingly to maintain a constant Ca to P ratio of 1.25:1 across all diets. A 5-d adjustment period preceded a 5-d total collection of feces. Ferric oxide was used to time the initiation and termination of fecal collection. In Exp. 2, 108 barrows and gilts were used in a 3-wk growth performance study. Monocalcium phosphate was added to a corn-soybean meal-based diet to set up 6 levels of TTTD-based digestible P, ranging from 1.55 to 4.21 g/kg at an increment of 0.53 g/kg digestible P. Limestone was added accordingly to maintain a constant Ca level across all diets. The results of Exp.1 showed dietary P intake, fecal P output, and digested P increased linearly (P < 0.001) with the increasing P level, whereas the apparent total-tract digestibility of P increased both linearly and quadratically (P < 0.01). The regression of daily digested P against daily P intake gave a TTTD of 67.5% for P in monocalcium phosphate. In Exp. 2, ADG increased linearly (P < 0.001) with the increasing P level in wk 1. In wk 2, wk 3, and the overall 3-wk duration, both linear and quadratic effects of P level (P < 0.05) were observed for ADG. Average daily feed intake increased with the addition of monocalcium phosphate linearly in wk 2 (P < 0.05) and during the overall period (P < 0.001) and both linearly (P < 0.001) and quadratically (P < 0.05) in wk 3. For any period, the G:F was improved in both a linear (P < 0.001) and quadratic (P < 0.05) fashion. The true digestible P requirement, defined as the breakpoint using G:F of the overall duration, was estimated to be 3.22, 2.81, and 3.45 g/kg of diet with dietary DM averaged at 887 g/kg for pigs of mixed sex, barrows, and gilts, respectively. Using ADG, the corresponding P requirement was 3.19, 3.40, and 2.97 g/kg of diet. In conclusion, the true digestible P requirement of pigs from 10 to 20 kg BW was determined to be 3.2 g/kg of diet.