Reliability of dietary Ca and P levels and bone mineral content as predictors of bone mechanical properties at various time periods in growing swine.

Seventy-two growing pigs were fed one of nine dietary treatments that provided a range of Ca (0.4%-1.2%) and P (0.4%-1.2%) levels arranged in a two-factor central composite design. Two pigs per treatment after either 5 (T5) or 10 (T10) wk and four pigs per treatment after 15 (T15) wk were slaughtered, and the femur (F) and third (MT3) and fourth (MT4) metatarsal bones were collected for evaluation of mechanical properties (force, stress and modulus of elasticity), ash weight (ASHW), percent ash (PASH) and bone mineral content (BMC) using photon absorptiometry. Stepwise regression analysis utilized to generate best-fit equations predicted that dietary Ca and P levels could reliably predict force (R2 = 0.83), ASHW (R2 = 0.93) and BMC (R2 = 0.89) of the MT3 bones but the prediction of force differed at the various time periods. Stress (R2 = 0.41), the modulus of elasticity (R2 = 0.29) and PASH (R2 = 0.17) were not reliably predicted by dietary mineral levels. The amount of force withstood by the MT3 bone could be reliably predicted by BMC (R2 = 0.90), but stress was not reliably predicted by BMC (R2 = 0.40) even if dietary Ca levels were considered. These results indicated that dietary Ca levels between 0.6% and 1.2% and dietary P levels between 0.6% and 0.8% did not adversely affect growth, efficiency or bone development. The BMC, as determined by photon absorptiometry, was not a reliable predictor of bone strength.