Phosphorus studies in pigs

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Abstract
Two experiments were conducted (1) to determine the effects of phytase (EC 3.1.3.26) on the digestibility and availability of P in soya-bean meal for growing pigs and (2) to compare growth v. digestibility variables for assessing the availability of P. In the first experiment the effect of phytase on P availability was assessed in a growth assay using a slope–ratio design of treatments. Two different levels of either monosodium phosphate (MSP) or soya-bean meal were added to a basal sugar–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 pigs initially weighing 20 kg live weight. In addition, the relative effectiveness of different variables for assessing P availability were compared: bone bending moment, ash in various bones, and ash and P in the empty body. The addition of phytase increased growth rate (g/d) (741 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 < 0·05), protein retention (kg/kg) (0·33 v. 0·36; P < 0·05), energy retention (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 bending moment was the criterion of availability. All other criteria for 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 phytase supplementation on the apparent digestibility of P, dry matter, crude protein (N × 6·25) and energy were determined. The diets were offered at three times maintenance energy requirements to male pigs initially weighing approximately 30 kg live weight and total collection of faeces was conducted over a 10 d period. The availability of P in the soya-bean meal was 0·66 using digestible P intake as the criterion of response. The apparent digestibility of P in soya-bean meal was 0·42. Phytase supplementation increased the apparent digestibility of soya-bean meal P to 0·69 (P < 0·01) but had no effect on the faecal digestibility 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.