Effects of phosphorus supplementation in high-grain diets on blood, chemical and physical parameters of bones of feedlot Nellore bulls

The study was conducted to evaluate the effects of phosphorus supplementation (PS) on blood, chemical and physical parameters from bones of feedlot Nellore bulls. Forty-two animals were distributed in a completely randomised design consisting of three treatments and 14 replicates. The treatments consisted of no supplementary phosphorus (P; T1; 2.4 g P/kg DM), a commercial mineral supplement formulated according to recommendations from the main Brazilian industries of mineral supplements for feedlot beef cattle (T2; 4.2 g P/kg DM) and dicalcium phosphate (T3; 5.0 g P/kg DM) in high-grain diets. The animals submitted or not to PS had similar calcium (Ca) and P values, Ca : P ratio, specific gravity, cortical thickness and bone mineral density in the 12th rib. However, a decrease in magnesium (Mg) concentration was observed in the 12th rib in T1 and T3 treatments throughout the study (P < 0.05). The animals in T1 had lower cortical thickness of the metacarpus (P < 0.05), without compromising its resistance and stiffness (P > 0.05). To evaluate the P status in the animals, we performed the needle test, which allowed an assessment of differences in bone strength on the basis of the resistance to needle penetration on the transverse process (TP) of the lumbar vertebrae. Throughout the trial period, no animals were positive for the needle test conducted in the transverse processes of the lumbar vertebrae. The PS significantly changed parathyroid hormone and alkaline phosphatase concentrations (P < 0.05) and the animals in T1 had lower concentrations of these hormones. However, there was no effect of PS on the concentration of Ca and P in the serum of animals (P > 0.05). The inclusion of a mineral mix supplement in T2 resulted in higher Mg concentrations in the serum of animals (P > 0.05). P supplementation for Nellore bulls, fed high proportions of concentrate during the feedlot period, resulted in no benefits in neither chemical nor physical properties of their bones and the level in the control diet appeared adequate in these circumstances.