Digestive utilization of phosphorus in cereal grains by pigs
This thesis is an investigation of the digestive utilization of phosphorus (P) in major cereal grains by pigs. Methodological aspects of measuring intrinsic phytase activity and phytate-P content in grains were studied. The results demonstrated that the intrinsic phytase hydrolysis of phytate was sample-specific and the complete removal of phosphate groups from phytate could be achievable by microbial phytase-catalyzed hydrolysis. Multiple time point experiments need to be conducted in order to determine valid intrinsic phytase activity, and phytate-P content measured by microbial phytase hydrolysis. The potential of barley, corn, oats and wheat in providing true digestible P for weaned and growing pigs was measured according to the simple regression analysis method. Pigs were fed semi-purified diets formulated from these test cereal grain ingredients according to a 4 x 4 Latin square design. True P digestibility and fecal endogenous P outputs were estimated for growing pigs, although these measurements were affected by reduced feed intake for some cereal grains in weanling pigs. True fecal P digestibility values were determined to be 64.7, 59.8, 83.5 and 41.2% for barley, corn, oats and wheat, respectively. Fecal endogenous P output accounted for a significant portion of the daily P requirements in pigs. Phytate degradation associated with cereal grain feeding contributed substantially to the digestion and absorption of P at the pre-cecal level. The large intestine did not play a significant role in the absorption of P from the major cereal grains in the pig. In order to understand the relative regional importance of the apical transcellular phosphate (Pi) transport along the intestinal longitudinal axis, kinetics of sodium-Pi co-transport activity and apparent Pi diffusion were examined 'in vitro' with the fast-filtration technique by using [32P]phosphate tracer and gut mucosal apical membrane vesicles prepared by the Mg2+-precipitation and the differential centrifugation procedures from gut tissues collected from weaned pigs. The apical Na+Pi co-transporter activity is expressed along the entire intestinal longitudinal axis in the pig. The whole gut apical Na+-Pi co-transporter capacity is important to absorb luminal Pi to maintain the epithelial metabolism, however, contribute little to the whole body P requirement and homeostasis. In summary, pigs can utilize P from conventional cereal grains with a digestive efficiency likely much higher than previously reported in the literature.