Taux butyreux et composition de la matière grasse du lait chez les petits ruminants : Effets de l'apport de matières grasses ou d'aliment concentré

Abstract

Fat content yield and composition of milk in small ruminants: effects of concentrate level and addition of fat. Effects of concentrate level and addition of fat were quantified from a data base recording milk fat content and yield, and fatty acid composition of milk in dairy sheep and goats. Increasing concentrate level decreases milk fat content, more intensively in total mixed rations than in rations where concentrate and forage were fed separately. In these last mentioned rations, fat yield response to concentrate was curvilinear showing a forage: concentrate ratio 1:1 to maximise fat yield. In these conditions, percentages of short and medium chain fatty acid were not affected, whereas percentages of long chain saturated and mono unsaturated (cis) fatty acids were dramatically reduced in favour of trans mono-unsaturated fatty acids and linoleic acid. Addition of non protected fat increased milk fat content and yield in a similar way for seeds and animal fat. With non protected fish or vegetable oils, these responses were highly variable, depending on the decrease in the synthesis of short and medium chain fatty acids and on the increase in long chain fatty acids. Concentrations of linoleic, linolenic and CLA in milk may only be slightly increased with seeds or rapeseed oil. Non protected fish oil weakly increased long chain poly-unsaturated fatty acids in milk. The increase in milk fat content with the addition of calcium salts of fatty acids (mainly from palm oil) was more marked for ewes than for goats; fat yield was increased similarly in the two species with addition of up to 150 g/dof calcium salts. Calcium soaps reduced percentages of medium chain fatty acids, but not that of short chain fatty acids, nor that of palmitic acid in milk; milk concentration of stearic and more markedly oleic acid were increased with these calcium salts of fatty acids. Encapsulated seed or fish oils limited the decrease in short and medium chain fatty acids observed when the same non protected fat oils are used. They also reduced stearic and oleic acid contents in favour of trans fatty acids. In these conditions, transfer efficiency of linoleic and linolenic acids in milk increased according to the content of these acids in the seeds. Encapsulation is a moderately efficient way to increase the transfer of poly-unsaturated fatty acids of fish oil, or feed CLA, to milk. Moreover, the paper shows the lack of experimental data on nutritional factors allowing to control the composition of milk fat in sheep and goats, especially concerning fatty acids having a potential effect on human health.