Body composition and net and dietary inorganic macroelements (Ca, P, Mg, K and Na) requirements of Holstein calves fed diets with different levels of forage

The objective of this work was to evaluate the effects of different forage levels in the diet on the net and dietary calcium (Ca), phosphorus (P), magnesium (Mg), potassium (K) and sodium (Na) requirements. Forty-Four pure Holstein bulls calves, 60 days old with initial average live weight (LW) of 78 kg were used. Fight reference animals were slaughtered at the beginning of the experiment, to estimate the initial empty body weight (EBW) and initial body concentrations of Ca, P, Mg, K and Na. The remaining 36 animals were allotted in the treatments in a complete randomized experimental design, in four groups of nine animals, in function of forage levels in the diet: 10, 25, 40 and 55%, on the DM basis, using chopped coast-cross (Cynodon dactylon) grass, ground corn and soybean meal, which constitute diets with approximately 16% CP. Five animals from each treatment were slaughtered with 190 +/- 10 kg LW and the other 4 with 300 +/- 10 kg LW. Regression equation of the logarithm of inorganic macro minerals body contents in function of the EBW logarithm, were fitted for each forage level. The average net requirements of Ca and Mg increased, while the requirements of P, K and Na decreased with ani mal EBW. The net requirements of Ca, P, Mg, K and Na for 1 kg EBW gain for an animal of 300kg LW, using the overall equation, were, respectively 16.96, 9.59, .37, 1.88 and 1.20 g, while the dietary requirements fur 1 kg LW gain were 26.30, 17.40, .70, 2.01 and 1.55g, respectively. The body inorganic macroelements concentrations were affected by forage levels and by the increase of the EBW of the animals.