Microbial phytase combined with amino acid supplementation reduces P and N excretion of growing and finishing pigs without loss of performance

The objective of this study was to evaluate the effect of a combined low-protein (CP) low-phosphorus (P) diet, supplemented with limiting amino acids and microbial phytase, on performance, nutrient utilization and carcass assessment of grower-finisher barrows. 4 x 6 crossbreed barrows were continuously housed in metabolism cages from 25 to 100 kg live weight and were fed diets, either conventional according to recent nutrient recommendations (A) or protein reduced (B) or protein and phosphorus reduced diets (C) based on barley (59-80%), maize (15%) and soybean meal (23-2%). Diet A (positive control) contained in fresh matter 18.5%, 15.6% and 13.6% CP and 0.59%, 0.51%, 0.50% P (growth phases I: 25-50 kg, II: 50-75 kg, III: 75-100 kg weight). Diet B was low in CP (14.2%, 13.4%, 11.0%), diet C low in CP and low in P (CP: as B, P: 0.46%, 0.41%, 0.32%). Diet D served as a negative control (CP and P as C). To diet B the limiting amino acids lysine, methionine, threonine and trypthophan were added to meet the levels in diet A, to diet C the limiting amino acids and 800 FTU Aspergillus-phytase per kg were supplemented. At the end of the balance periods of about 100 (A,B,C) or 133 (D) days, the barrows were slaughtered at 100 kg live weight, the carcasses scored and loin, biceps femoris, os femoris and phalanx prima IV were analysed for their nutrient (dry matter, protein, fat) or mineral (ash, Ca, P, Mg, Fe, Zn) composition. In contrast to the basal diet D, the CP or P reduction in diets B and C did not negatively affect growth, feed efficiency, nitrogen retention or overall carcass performances of the pigs. With the low CP diets B and C, N excretion via the urine was significantly reduced and thus N excretion per unit weight gain was decreased by about 30%. The addition of microbial phytase (C) increased apparent digestibility of P significantly between 25% (growth phase III) and 50% (growth phase II). In spite of 25% reduction of P intake (C), the absolute daily P retention did not differ between treatments. Thus phytase supplementation in diet C reduced P excretion per unit weight gain by about 35% (I, II) to 44% (III) as compared with A and B. Similarly, phytase raised apparent digestibility of Ca by about 25% and Zn by about 80%. Generally, the carcass and meat characteristics were not affected by any of the diet strategies employed in this experiment. Mineralization (bone weight, ash, Ca, P, Mg) of the os femoris and phalanx prima IV were also similar in all treatment groups. However, phytase supplementation led to significantly increased zinc concentration in bones, indicating considerably improved zinc release from the phytate complex. In contrast, Fe incorporation into the os femoris was significantly reduced in the phytase group. In general, the low CP and low P feeding regimen introduced in this experiment offers substantial benefits in maintaining a sustainable environmentally-friendly pork production.