Evaluation of serum parameters to predict the dietary intake of calcium and available phosphorus in growing pigs

Adequate provision of calcium (Ca) and phosphorus (P) is essential for bone formation and high growth performance in pigs. Nevertheless, reliable serum biomarkers for pig’s Ca and P intake are still missing. Here, we used phytase supplementation to alter the dietary available P (aP) level in order to investigate the effect of differences in dietary aP levels on serum parameters related to the Ca and P homeostasis in pigs. Moreover, we assessed whether serum parameters can be used to predict the Ca, total P (tP), and aP intake in barrows and gilts throughout the fattening period. In total, 216 pigs (115 gilts and 101 barrows) were randomly allotted to one of the two diets in three replicate batches, each lasting 56 d (n = 108/diet). Pigs had free access to the diets without (Con) or with phytase (Phy; 650 phytase units/kg) via a transponder-based feeding system. Blood samples were collected on days 2, 23, and 52, and serum parameters were correlated with the daily Ca, tP, and aP intake. The intake of tP, aP, and Ca was overall 14.2%, 13.8%, and 14.2% higher in barrows compared with gilts, respectively (P < 0.001). Concurrently, phytase decreased the intake of tP and Ca by 8.4% and 6.7%, respectively, whereas it raised the intake of aP by 16.3% compared with the Con diet (P < 0.001). Serum levels of fibroblast growth factor 23, alkaline phosphatase (ALP), vitamin D (VitD), and osteocalcin (OCN) decreased with age (P < 0.05). The higher aP intake of pigs fed the Phy diet increased serum P on days 2 and 23 but decreased it on day 52 compared with the Con diet (P = 0.004). Pigs fed the Phy diet had higher serum ALP compared with pigs fed the Con diet on days 23 and 52 (P < 0.05). Correlation analysis between serum parameters and Ca, tP, and aP intake showed age- and sex-related associations. With 12 wk of age, serum P in both sexes, serum VitD in barrows, and serum OCN and ALP in gilts correlated with aP intake (|r| > 0.38), whereas serum OCN correlated with Ca in both sexes’ intake (r > 0.50). At 20 wk, serum Ca and ALP in gilts correlated with aP intake, whereas serum P, Ca, and VitD correlated with Ca intake in both sexes (|r| > 0.39). In conclusion, the present results showed that the daily Ca and aP intake could be most reliably estimated from serum parameters for an approximate age of 12 and 20 wk. Serum P and the Ca:P ratio at 12 wk of age and serum VitD at 20 wk of age may be used to predict pig’s daily aP intake in both sexes.