Effects of Increasing Calcium-to-Phosphorus Ratio in Diets Containing Phytase on Finishing Pig Growth Performance

A total of 288 finishing pigs were used to evaluate the effects of increasing the calcium-to-total phosphorus ratio (Ca:P) in diets containing phytase on growth performance. In Exp. 1, 144 pigs were fed diets with 300 phytase units (FTU)/kg of phytase and Ca:P ratios of 1.0:1, 1.25:1, 1.5:1, or 2.0:1 from 38.6 to 113 kg. Pigs were fed in three phases (d 0 to 28, 28 to 57, and 57 to 76) with total P levels of 0.45, 0.40, and 0.35%, respectively. Increasing Ca:P decreased ADG (quadratic; P<0.03), ADFI, and carcass weight (linear; P<0.05). However, the greatest decrease was observed when Ca:P increased from 1.5:1 to 2.0:1. In Exp. 2, 144 pigs (initially 38.6 kg) were fed diets containing 300 FTU/kg of phytase with Ca:P ratios of 0.75:1, 1.0:1, 1.25:1, 1.5:1, or 2.0:1. Diets were formulated to contain 0.44, 0.38, and 0.32% total P from d 0 to 28, 28 to 56, and 56 to 73, respectively. A sixth treatment group (negative control) was fed a diet containing 77% of the available P equivalent in other treatment diets to determine whether diets were formulated above the pig's P requirement. Increasing Ca:P decreased (quadratic; P<0.04) ADG and gain-to-feed ratio (G:F), with the greatest decrease observed as Ca:P increased from 1.5:1 to 2.0:1. Pigs fed the negative control diet had numerically decreased ADG and decreased (P<0.05) fat thickness. These studies suggest that finishing diets containing 300 FTU/kg of phytase should not have a Ca:P of >1.5:1.