Phosphorus utilization and characterization of excreta from swine fed diets containing a variety of cereal grains balanced for total phosphorus1

Abstract

Intrinsic phytase in swine feeds may increase phytate utilization and alter the solubility of the excreted P. The objective of this experiment was to quantify changes in fecal P composition from swine fed a variety of cereal grains containing a range of phytate concentrations and endogenous phytase activities. Twenty-five crossbred barrows (89.3 +/- 6.8 kg) were fed 1 of 5 dietary treatments that were based on wheat, corn, barley, low-phytate barley, or high-fat-low-lignin oats. Experimental diets were formulated to contain 75% of the test grain and were fed for a 7-d acclimation period followed by a 3-d fecal collection period. Total-tract apparent digestibility coefficients were determined for DM, P, and phytate using an indicator method. Fecal P was characterized using solution-state (31)P nuclear magnetic resonance spectroscopy. Water-soluble P (WSP) and WSP-to-total P (TP) ratio were determined in the feces. Apparent total-tract digestibility coefficients for P and phytate ranged from 0.33 (barley) to 0.45 (low-phytate barley) and from 0.20 (corn) to 0.79 (oats), respectively. The majority of P excreted in the feces was in the form of phosphate (>47% of TP), and phytate degradation was not related to the endogenous phytase activity in the diet. There was a positive linear relationship between dietary NDF and apparent total-tract phytate digestibility (r(2) = 0.82; P = 0.03), indicating that greater dietary fiber content may enhance microbial breakdown of phytate in the hindgut. There was a negative relationship between the fecal WSP-to-TP ratio and the percentage of TP that was in the form of phytate in the feces. In summary, our results indicate that the majority of P in the feces of pigs fed diets based on cereal grains is present in the form of phosphate and relatively small amounts of phytate were contained in the excreta. The exception to this was the corn diet, for which 45% of the total fecal P was in the form of phytate. Hydrolysis of phytate in the gut did not appear to be related to the content of either phytate or phytase in the grain, but was related to dietary fiber concentration.