[Feed conversion and the energy requirement of pregnant heifers]

Eight Holstein heifers were subjected to experiments running over the entire pregnancy period to investigate the conversion of rations mainly made up of maize silage (85% of the ration's dry matter) by the experimental animals. Both digestible and metabolizable energy (ME) and the energy requirement were studied. The live weight (LW) of the heifers was found to rise linearly during the entire pregnancy period being 422 kg at service and reaching 565 kg at calving. Feed intake, referred to 1 kg live weight0.75, was almost constant until the 7th month of gestation and declined gradually until parturition. The degree of digestibility and metabolizability of the energy contained in the silage-bases rations prove equal over the whole gestation period. The energy required for maintenance and gravidity (y) was found to rise exponentially as pregnancy advance (t). This relationship is reflected by the equation: ME, kcal/kg LW0.75y=135+0.2590.0206t range of significance=+/-0.07 During the last two months of gestation, a negative energy balance was found. This negative energy balance corresponds with a negative fat balance and a reduction, of the respiratory quotient, a fact suggesting to the mobilisation of body fat reserves by the mother to cover the increased energy requirement during pregnancy. The protein balance was found continuously positive during the entire gestation period. The conversion of metabolizable energy for fetus development was found to be 8.13%.