Effects of increasing CA:P ratio in diets containing phytase on growth performance of grow-finish pigs

We used 144 growing-finishing pigs (72 barrows and 72 gilts; initially 85 lb) to determine the effects of calcium to total phosphorus (Ca:P) ratio on growth performance. Pigs were housed in an environmentally regulated finishing building with two pigs per pen and nine pens per sex per treatment in a randomized complete block design. Pigs were blocked by initial weight and sex, and then allotted to one of four dietary treatments. The dietary treatments were corn-soybean meal-based diets fed in three phases. In each phase, diets consisted of a 1:1; 1.25:1; 1.5:1, and 2:1 Ca:P ratio. Diets were formulated to contain 0.44%, 0.39%, and 0.34% phosphorus from 70 to 130, 130 to 190, and 190 to 250 lb, respectively. All diets contained 0.05% phytase, providing 300 FTU/kg of feed. For the overall experiment, increasing Ca:P ratio decreased ADG (quadratic P<0.03) and ADFI (linear P<0.05). However, the greatest decrease in ADG and ADFI was observed when Ca:P increased from 1.5:1 to 2:1. Feed to gain was not affected by Ca:P ratio. These results suggest that in growing-finishing diets containing 300 FTU/kg phytase, a Ca:P ratio greater then 1.5:1 will decrease ADG and ADFI.