Effects of dietary digestible calcium on growth performance and bone ash concentration in 50-to 85-kg growing pigs fed diets with different concentrations of digestible phosphorus

A 30-day experiment was conducted to determine the response of 50-85 kg pigs to standardized total tract digestible (STTD) Ca at 3 levels of STTD P. Fifteen corn-soybean meal based diets were formulated using a 3 x 5 factorial design with diets containing 1.4, 2.7, or 4.1 g/kg STTD P and 1.3, 2.5, 3.8, 5.0, or 6.3 g/kg STTD Ca. Ninety barrows (BW: 50.2 +/- 2.1 kg) were allotted to 15 diets and 5 blocks in a randomized complete block design. At the conclusion of the experiment, weights of pigs and feed left in feeders were recorded. On d 31, pigs were euthanized and the right femur was removed. Data were analyzed using a response surface model. Results indicated interactions (P < 0.10) between dietary concentrations of STTD Ca and STTD P for final body weight (BW), average daily gain (ADG), gain to feed (G:F), and the concentration (g) of bone ash, Ca, and P. The full model was used to predict final BW and ADG, whereas for G:F, the model only included the linear terms for STTD Ca and STTD P and the interaction between the linear terms. The predicted maximum BW (87.20 kg) and ADG (1.23 kg) at 2.7 g/kg STTD P was obtained at STTD Ca:STTD P ratios of 1.20:1 and 1.25:1, respectively. However, a predicted maximum G:F was not obtained because of the linear nature of the equation. A decrease in ADFI (main effect of Ca, P < 0.05) regardless of the concentration of dietary STTD P was observed as STTD Ca increased. The model to predict bone ash (g) was not reduced and the estimated maximum values at STTD P concentrations of 2.7 and 4.1 g/kg were 55.8 and 60.1 g. These values were obtained at STTD Ca:STTD P ratios of 2.03:1 and 1.59:1, respectively. In conclusion, excess Ca is detrimental to growth performance of 50- to 85-kg pigs if the concentration of P is at or below the requirement, whereas growth performance of pigs fed diets containing STTD P above the requirement is maximized only if STTD Ca is also above the requirement. The amount of STTD Ca needed to maximize bone ash is greater than the amount needed to maximize growth performance. The STTD Ca to STTD P ratio needed to assure adequate bone mineralization without affecting growth performance is about 1.23:1 if the concentration of P is at the requirement.