INFLUENCE OF FEEDING LEVEL ON LIVE WEIGHT-GAIN AND CARCASS COMPOSITION IN YOUNG BULLS OF DIFFERENT BREEDS

Salers, Charolais .times. Salers and Charolais bulls (51) received 2 diets of different composition and 2 levels of metabolizable energy between the age of 9 and 15 mo. The diets contained either a large proportion of forages (80% dehydrated lucerne and 20% dehydrated beet pulps) or a large proportion of cereals (70% barley, 14% peanut meal and 16% dehydrated lucerne). The feeding levels were defined as follows: the low level corresponded to consumption of the forage diet offered ad lib or to that of the concentrate diet restricted to obtain daily gains identical to those of the animals on the forage diet. The high levels corresponded to the concentrate diet offered ad lib. For the same amount of metabolizable energy and the same carcass weight, the nature of the diet did not change the physical composition of the carcasses whatever the breed. The increase of energy intake affected variably the empty live weight and carcass composition according to the genotype of the animals and their maturity. In the Salers breed this resulted in a slight increase in the empty live weight (2.5%) and carcass weight (1.8%), for constant fattening length, and in a large modification of the carcass composition, for constant weight (321 kg), a 37% increase in the weight of fat. The carcass composition of Charolais (much later maturing animals) was not modified as much (8.6% increase in the weight of fat), but the empty live weight and carcass weight were more markedly enhanced (9%) than those of the previous animals. The reaction of the crossbred Charolais .times. Salers bulls was intermediate: like Charolais, their empty live weight and their carcass weight were highly increased (11.6 and 10%, respectively) and like Salers, the composition of their carcasses was modified (for the same carcass weight, 38% increase in the weight of fat). This reaction of the young bulls to variation in the feeding level depended on the maturity of the animals and on their muscle growth potential.