The effects of increasing dietary total Ca/total P ratios on growth performance, Ca and P balance, and bone mineralization in nursery pigs fed diets supplemented with phytase.

The objective of this study was to investigate the effects of increasing dietary total Ca/total P ratios on growth performance, digestibility of Ca and P, bone mineralization, and concentrations of Ca and P in urine and plasma in nursery pigs. There were six diets in a randomized complete block design, including one positive control and five diets corresponding to five total Ca/total P ratios: 0.55, 0.73, 0.90, 1.07, and 1.24 (analyzed as 0.58, 0.75, 0.93, 1.11, and 1.30). These five diets were deficient in P but supplemented with 1,000 phytase units/kg feed. Each diet was fed to six pens of eight pigs (four barrows and four gilts per pen). All diets contained 3 g/kg TiO2, and fecal samples were collected from each pen on days 5-7 of trial. At the end, one pig per pen was sacrificed to collect the right tibia and urine in the bladder. The results showed that increasing dietary Ca/P ratio to 0.93 increased gain:feed but then gain:feed decreased as the Ca/P ratio was increased to 1.30 (linear and quadratic, P < 0.05). Although average daily gain and final BW were unaffected by changing Ca/P ratio in diet, dry bone weight; weights of bone ash, Ca and P; and bone Ca/P ratio increased linearly (P < 0.001) with increasing dietary Ca/P ratio. The percent bone Ca showed a tendency to increase (P = 0.064). Increasing dietary Ca/P ratio decreased apparent total tract digestibility (ATTD) of Ca and P linearly (P < 0.05) and the concentration of digestible P linearly (P < 0.001), but increased the concentration of digestible Ca (linear and quadratic effects: P < 0.01) and the digestible Ca/P ratio (linear effect: P < 0.001). In plasma, the concentration of Ca increased both linearly (P < 0.01) and quadratically (P = 0.051), whereas the concentration of P tended (linear and quadratic, P < 0.10) to decrease with increasing dietary Ca/P ratio. Similarly, in urine, the concentration of Ca increased both linearly and quadratically (P < 0.05), whereas the concentration of P decreased linearly (P < 0.01). In conclusion, increasing the dietary Ca/P ratio reduced feed efficiency but increased bone mass and the amounts of Ca and P deposited in bone of nursery pigs fed diets supplemented with 1,000 FYT/kg phytase. The increases in bone growth led to a reduction of urinary P excretion that exceeded the decreased digestible P supplied in diet with the widening dietary Ca/P ratios.