Nutritional strategies for reducing the environmental impact of manure phosphorus and nitrogen excretion by pigs and poultry

Excretion of nitrogen and phosphorus in animal manure has been implicated as potential environmental pollutants. In the first study, the relative bioavailability (RBV) of P from a genetic mutant, low-phytate (LP) soybean meal for broiler chicks was estimated. The RBV of P from LP soybean meal was 15 to 25 percentage points higher than normal soybean meal. In the second study, RBV of P, growth performance and nutrient balance was evaluated in pigs fed a LP corn-based diet with or without phytase (PT). Estimates of P RBV from LP and normal corns when plasma P was regressed on supplemental P intake were 46 and 33%, respectively. The percentage of P digested and retained was improved and fecal P excretion lowered by feeding LP corn. The addition of PT to a LP corn-based diet improved P digestibility and reduced P excretion by pigs. In the third study the effects of graded levels of PT in low-crude protein (CP), low-P diets on N and P balance and growth performance of growing pigs was examined. Phytase improved retention of N in low-CP diets, and absorption and retention of P regardless of the CP level. In the fourth study the response to supplemental microbial PT in pigs fed diets containing different levels of intrinsic phytate was evaluated. Intrinsic phytate concentration did not influence growth performance or nutrient digestibility. Phytase supplementation improved weight gain and P digestibility, but had no effect on amino acid or N digestibility. In the fifth study, possible interactions between dietary N and P in broiler chicks were investigated. A high level of P reduced feed intake in chicks fed low or normal CP levels, but weight gain and feed efficiency were only reduced at the low CP level. Feeding low-CP, high-P diets, reduced intake and increased nutrient outputs. Retention of dry matter, energy and N was higher at the low CP level, but P retention was higher at the normal CP level. These results provide evidence of interactions between dietary N and P and support for an integrated strategy for reducing manure N and P excretion by pigs and poultry