Body composition and net and dietary requirements of macrominerals (Ca, P, Mg, Na and K) of genetically selected Caracu (CaS) and Nellore (NeS) for yearling weight (378 days of age) as well as of ordinary Nellore (control; NeC) were investigated in this trial. Fifty-six young bulls averaging 18 months of age were used. Twelve animals were slaughtered after an adaptation period and were used as reference. The 44 remaining animals, 16 NeS, 16 CaS and 12 NeC, with initial average body weight (BW) of 392, 421 and 337 kg, respectively, were distributed to a completely randomized design in a 3 x 2 factorial arrangement (genetic group [NeS, CaS and NeC] and feeding regime [restricted and ad libitum]). Animals were fed a diet with a 50:50 forage (corn silage):concentrate ratio and were slaughtered after reach 4 mm of subcutaneous fat thickness measured by ultrasound. After slaughter, body parts of all animals were weighed and sampled. Samples were then freeze-dried, treated with ether to remove fat, ground and contents of macrominerals determined. Estimation of body retained macrominerals were done by regressing the logarithm of Ca, P, Mg, Na and K body content on the logarithm of empty body weight (EBW). Net macrominerals requirements for 1 kg of EBW gain were obtained by derivation of the prediction equations. There was a significant effect of genetic group on the body content of the studied macrominerals. Body contents and requirements of all macrominerals followed EBW and increased as EBW of animals increased.
beef cattle; calcium; phosphorus; potassium; sodium
