The present work was conducted in the Sheep Division of Animal Science Department at Universidade Federal de Lavras - M.G., aiming to determine the body composition of lambs and to estimate the nutritional requirements of the mineral macroelements Magnesium (Mg), Sodium (Na) and Potassium (K) for weight gain of growing lambs. It was used 48 not castrated male lambs, being 12 Santa Inês X Santa Inês (SI X SI), 12 Bergamácia X Santa Inês (BE X SI), 12 Ile de France X Santa Inês (IF X SI) and 12 Texel X Santa Inês (TE X SI). For determination of Mg, Na and K amounts kept in the animal body, three animals of each genetic group were slaughtered in the beginning of the experiment making as reference animals for the comparative slaughtered technique. The remaining animals of each genetic group were confined until the slaughter where they reached 25, 35 and 45 kg of live weight. The body composition in Mg, Na and K was estimated by the prediction equations obtained by the regression of the minerals amount logarithm in the empty body in function of the empty body weight logarithm (EBW). The net requirements of Mg, Na and K for the weight gain were estimated from the derivation of the prediction equations of the body composition. The body composition of Mg of genetic grups SI differed of the other geneticgrups and the values varied per kilo of empty body weight (EBW) for animals from 15 and 45 kg of live weight was: 0,606 and 0,514 g of Mg/kg of EBW for the Santa Inês pure animals and 0,524 and 0,475 g of Mg/kg of EBW for the other genetic groups. In the case of the Na and K there were differences in the estimates for the IF X SI genetic group and the values varied from 1,604 and 1,219 g of Na/kg of EBW and from 1,938 and 1,592 g of K/kg of EBW; for the other genetic groups the values varied from 1,665 and 1,270 g of Na/kg of EBW and 2,144 and 1,685 g of K/kg of EBW. The net requirement of Mg, estimate in this research were: 0,40 g and 0,35 g of Mg/kg of LW for Santa Inês pure animals and 0,38 g and 0,33 g of Mg/kg of LW for the other animals with 15 and 45 kg of live weight. The net requirement of K and Na for crossbred animals IF X SI differed from the requirements of the other genetic groups animals. The following values for animals of 15 and 45 kg of live weight were:1,30 g and 1,06 g of K/kg of LW, 0,99 g and 0,75 g of Na/kg of LW for IF X SI animals and 1,37 g and 1,07 g of K/kg of LW and from 1,03 g and 0,78 g of Na/kg of LW for the other animals.
body composition; magnesium; sheep; potassium; requirement; sodium