Evolution des quantités d'aliment ingérées par les brebis à la fin de la gestation et au début de la lactation Incidences sur leurs performances. II. — Ration à base d'ensilage de maïs et de foin distribuée à volonté
Using a diet based on maize silage and hay, the factors of variation in the voluntary feed intake of ewes during late pregnancy (5 wk) and early lactation (5 wk) as well as its effects on health of the ewes and growth of the lambs, were studied. Limousine ewes (40) received the maize silage very finely ground (digestibility of the organic matter 70.8%) and meadow hay (digestibility of the organic matter 59%) ad lib and a constant amount of soybean meal (100 g/ewe per day during late pregnancy and 300 g during early lactation). They were housed in individual pens on straw litter in a closed shed. Feed intake and weight gain of all ewes were measured as well as the growth performances of their lambs. In addition, the milk production was recorded in some of the ewes and the plasma levels of non-esterified fatty acids (NEFA), glucose, urea and .beta.-hydroxybutyric acid were determined. During late pregnancy, the intake of silage decreased in large proportions from wk-4 and until lambing (between 16-20% of the intake at wk-4). The hay intake was low during this period (130-140 g of DM/ewe per day). During the first 5 wk of lactation, the silage intake of the ewes increased, first very rapidly (wk + 1 - + 3) and then more slowly (wk + 4 - + 5). But the intake of hay increased very little. The main factors of variation of the silage intake were the following: the intake of hay, the age of the ewes, the ewes live weight and the number of lambs born and suckled. Equations relating the silage intake and its factors of variation during late pregnancy and early lactation were calculated. During the last 5 wk of pregnancy, the weight gain was high (6.45 and 5.17 kg for single and twin bearing ewes, respectively). During early lactation (5 wk), the ewes also gained in weight (3.2 and 2.0 kg for ewes nursing 1 and 2 lambs, respectively) contrary to the observations made with the hay diets. The silage intake accounted in a great part for the individual weight gain differences, as shown by the relations presented in Table 3. The milk yield of the ewes and the growth of the lambs were lower or at the best equal to those obtained with ewes of the same breed fed with hay of medium quality and consuming less metabolizable energy both during pregnancy and during lactation. During late pregnancy, the calculated energy balance exceeded by 25-75% the estimated requirements (maintenance). During early lactation, the energy deficiency did not last for a long time (1-2 wk); the plasma levels of the analysed substances confirm these observations. The factors of variation of the forage intake are discussed in relation with the various physiological regulations probably involved. Analysis of animal performances in relation to energy balance shows that with a maize silage diet, as the fermentations in the rumen lead to a larger production of propionic and butyric acid, the energy is used more efficiently for fattening than for milk production. The grinding of the silage is most likely responsible for this phenomenon. More investigations are needed to study the supplementation of such diets.