Response of broilers to varying dietary calcium and phosphorus.

Calcium (Ca) and phosphorus (P) are essential for bone mineralisation in broilers, but their deposition is often assumed to be proportional to BW or protein growth in current requirement models. Given the intense genetic selection for rapid growth, the balance between soft tissue and skeletal development may have shifted, potentially leading to underestimation of mineral requirements and impaired skeletal health. This study aimed to evaluate the effect of different levels of Ca and P on growth performance and kinetics of soft tissue and bone deposition in broilers. A total of 360 Ross-308 broilers were randomly divided into 10 replicate cages (0.46 m2) of four diets. During each experimental phase (0 to 9 d, 10 to 21 d, and 22 to 35 d), broilers were fed either a low Ca and digestible P diet (L), a moderate diet (M), or a high diet (H), the latter formulated to maximise bone mineralisation based on a mechanistic requirement model. Bone mineral content (BMC), body lean mass, and fat mass were measured by dual-energy X-ray absorptiometry on one bird per cage. Allometric regression predicted tissue growth in relation to BW. Growth performance, lean and fat masses were not modified by dietary treatments at 9 d; BW tended to increase (Lin, P = 0.086), and BMC tended to decrease (+39% in L vs H diet; P = 0.082). Birds fed both deficient diets (L and M) had reduced BMC (-20% and -14%, respectively; Lin, P = 0.003) compared with those fed the H diet at 35 d. Daily BMC deposition did not differ between birds fed L and M diets and tended to be higher for those fed the H diet (10 to 21 d, Quad, P = 0.081; 22 to 35 d, Lin, P = 0.037). Similarly, the allometric slope of whole-body BMC for birds fed the H diet did not deviate from 1, whereas in birds fed L and M diets, the slopes were 17% (L vs H) and 19% (M vs H) lower, respectively. Bone mineral content relative to protein deposition declined progressively with lower Ca and P diets. These findings suggest that bone mineralisation was more sensitive to diet than lean tissue growth in birds fed lower Ca and P diets, while both parameters increased proportionally to BW when birds were fed adequately. This implies that Ca and P deposition into bone is not strictly related to body protein.