PHOSPHORUS STUDIES IN PIGS .3. EFFECT OF PHYTASE SUPPLEMENTATION ON THE DIGESTIBILITY AND AVAILABILITY OF PHOSPHORUS IN SOYBEAN-MEAL FOR GROWER PIGS

Two experiments were conducted (1) to determine the effects of phytase (EC3.1.3.26) on the digestibility and availability of P in soya-bean meal for growing pigs and (2) to compare growth v. digestibility varia bles for assessing the availability of P. In the first experiment the effect of phytase on P availabitity was assessed in a growth assay usi ng a slope-ratio design of treatments. Two different levels of either monosodium phosphate (MSP) or soya-bean meal were added to a basal sug ar-soya-bean-meal diet (2-5 g P/kg) to give two levels of P (g/kg): 3. 25 and 4.0 for each source. An additional five diets were supplemented with phytase. The ten diets were offered ad lib. for 35 d to female p igs initially weighing 20 kg live weight. In addition, the relative ef fectiveness of different variables for assessing P availability were c ompared: bone bending moment, ash in various bones, and ash and P in t he empty body. The addition of phytase increased growth rate (g/d) (74 1 v. 835; P < 0.05), lowered the food conversion ratio (2.37 v. 2.16; P < 0.01), and increased protein deposition (g/d) (108 v. 123; P < 0.0 5), protein retention (kg/kg) (0.33 v. 0.36; P < 0.05), energy retenti on (MJ gross energy/MJ digestible energy) (0.36 v. 0.38; P < 0-05) and the availability of P in soya-bean meal from 0.11 to 0.69 when bone b ending moment was the criterion of availability. Atl other criteria fo r assessing availability were unsuitable. In the second experiment the availability of (P) in soya-bean meal was assessed in a digestibility experiment with grower pigs using diets 1-5 as for Expt 1 arranged in a slope-ratio design of treatments. In addition, the effects of phyta se supplementation on the apparent digestibility of P, dry matter, cru de protein (N x 6.25) and energy were determined. The diets were offer ed at three times maintenance energy requirements to male pigs initial ly weighing approximately 30 kg live weight and total collection of fa eces was conducted over a 10 d period. The availability of P in the so ya-bean meal was 0.66 using digestible P intake as the criterion of re sponse. The apparent digestibility of P in soya-bean meal was 0.42. Ph ytase supplementation increased the apparent digestibility of soya-bea n meal P to 0.69 (P < 0-01) but had no effect on the faecal digestibil ity of dry matter or crude protein. Overall these experiments indicate that (1) estimates of P digestibility and availability were unlikely to be interchangeable and (2) phytase was effective in releasing much of the bound P in soya-bean meal.