Effects of total calcium-to-total phosphorus ratio on the calcium and phosphorus digestibility and growth performance of growing pigs

We investigated the impact of total Ca:total P on the Ca and P digestibility and growth performance of pigs. Two experiments were performed. In experiment 1, a total of 2688 Duroc × Landrace × Yorkshire barrows with initial body weight (BW) of 50.1 ± 0.8 kg were blocked by BW and randomly allocated into eight treatment groups using a randomised complete block design with 12 replicate groups containing 28 pigs per group. The eight experimental treatments consisted of two phytase levels (0 and 800 FTU/kg) and four total Ca:total P levels (0.80, 1.05, 1.30 and 1.55) in a 2 × 4 factorial arrangement. In experiment 2, forty Duroc × Landrace × Yorkshire barrows (initial BW = 35.6 ± 0.3 kg) were housed individually in 20 stainless steel metabolism cages. The barrows were blocked by BW and randomly allocated into four treatment groups using a randomised complete block design with 10 replicate pigs per group. The experimental treatments consisted of two phytase levels (0 and 800 FTU/kg) and two total Ca:total P levels (0.80 and 1.55) in a 2 × 2 factorial arrangement. All faeces and urine were collected to determine the digestion and retention of Ca and P. The performance results of growing pigs in response to increasing total Ca:total P showed that adding phytase to diets increased (P < 0.05) final BW (FBW), average daily gain (ADG) and average daily feed intake during days 0–42. Increasing total Ca:total P quadratically decreased FBW and ADG (P < 0.05) and tended to interact with supplemental phytase for FBW and ADG (P < 0.10). Total Ca:total P quadratically decreased gain:feed (P = 0.044) not affected by interaction effects. In experiment 2, the results of Ca and P digestibility analysis showed that adding phytase to the diet increased Ca and P digestibility and retention (P < 0.05). Total Ca:total P decreased P digestibility (P < 0.001). The tendency of interaction effects between phytase and total Ca:total P was observed on P digestibility (P = 0.082) and apparent total tract digestible P content (P = 0.091). Interactions between phytase and total Ca:total P were observed (P = 0.033) on Ca retention, with total Ca:total P increasing Ca retention in diets without phytase and decreasing Ca retention when phytase was added. Digestibility of Ca tended to be influenced (P = 0.075) by interactions between phytase and total Ca:total P, with the total Ca:total P increasing digestibility of Ca in diets without phytase and decreasing digestibility of Ca when phytase was added. In summary, total Ca:total P may negatively affect the digestibility of P and the efficacy of phytase. Increasing the total Ca:total P may decrease the growth performance of growing pigs. Adding phytase to the diet can ameliorate the negative impact of Ca.