Determining the phosphorus release curve for Smizyme TS G5 2,500 phytase from 500 to 2,500 FTU/kg in nursery pig diets.

A total of 320 pigs (Line 241 × 600, DNA, Columbus, NE; initially 11.9 ± 0.22 kg) were used in a 21-d growth study to determine the available P () release curve for Smizyme TS G5 2,500 (Barentz, Woodbury, MN). At approximately 19 d of age, pigs were weaned, randomly allotted to pens, and fed common starter diets. Pigs were blocked by average pen body weight () and randomly allotted to one of eight dietary treatments on day 18 postweaning, considered day 0 of the study. Dietary treatments were derived from a single basal diet and ingredients including phytase, monocalcium P, limestone, and sand were added to create the treatment diets. Treatments included three diets containing increasing inorganic P from monocalcium P (0.11%, 0.20%, and 0.28% aP), or five diets with increasing phytase (500, 1,000, 1,500, 2,000, or 2,500 FTU/kg) added to the diet containing 0.11% aP. All diets were corn-soybean meal-canola meal-based and were formulated to contain 1.24% standardized ileal digestibility Lys, 0.30% phytate P, and an analyzed Ca:P ratio of 1.10:1. Prior to the beginning of the study, all pigs were fed a diet containing 0.11% aP for a 2-d period (days 16 to 18 postweaning). At the conclusion of the study, one pig, closest to the mean weight of each pen, was euthanized and the right fibula, rib, and metacarpal were collected to determine bone ash, density, and total bone P. Bones were weighed while suspended in a vessel of water and the weights used to calculate bone density (Archimedes' principle). For bone ash, bones were processed using the non-defatted method. For the overall experimental period, pigs fed increasing inorganic P had increased (quadratic,  ≤ 0.033) average daily gain (), average daily feed intake (), and final BW and a tendency for increased (quadratic,  ≤ 0.090) gain:feed ratio (). Pigs fed increasing phytase had increased (quadratic,  ≤ 0.004) ADG, G:F, and final BW and increased (linear,  = 0.019) ADFI. For fibula, rib, and metacarpal characteristics, pigs fed increasing aP from inorganic P had increased (linear,  < 0.001) bone ash weight, percentage bone ash, bone density, and bone P concentration. Additionally, pigs fed increasing phytase had increased (linear or quadratic,  < 0.05) bone ash weight, percentage bone ash, bone density, and bone P. TheaP release curve generated for Smizyme TS G5 2,500 for percentage bone ash using data generated from all three bones is aP = (0.228 × FTU/kg) ÷ (998.065 + FTU/kg).