Modulation de la qualité du lait par l'amidon de la ration

Abstract : The effect of variations in the source and amount of starch provided by the diet on the milk production of cows was studied du ring 5 trials with different starch levels and sources. These variations were realized using wheat or potatoes in total mixed diets based on grass silage or maize silage. ln sacco kinetics confirm that wheat starch disappears more quickly than potato starch in the rumen (instantaneous solubility of 65% and 40% respectively). The daily starch intake with maize silage based diets is over 6 kg per cow. ln view of the rapid degradation of maize silage starch in the rumen (86% of instantaneous solubility), these diets can be characterized by a rapid (wheat - maize silage) or mixed speed of starch degradation (potatoes - maize silage). The milk and protein yields increase with the starch supply even if the protein and fat contents decrease for supplies over 6-7 kg per day. This threshold separates the diets based on grass silage or maize silage. The dry matter intake, milk and protein yields do not vary between the diets containing one of the starch concentrates. So, the main result of this work is the decreased milk fat (content and yield) with wheat in comparison to potatoes in maize silage based diets (respectively - 2,6 to -3,3 g fat/kg milk and -71 to -122 g fat/day) i.e. in diets with a starch supply of over 7kg per day. The milk fat composition shows significantly fewer short and medium chain fatty acids and more long chain fatty acids (+3,6%) with wheat diets compared to potato diets. No differences in ruminai fermentation were shown between the two starch concentrates in these rations. The postprandial pH drop is stronger with wheat diets but no differences were observed for VFA concentration or composition or blood metabolites. As the milk fat precursors did not vary between the two diets, the lower milk fat content with wheat could be due to an inhibition of the activity of essential enzymes for milk fat synthesis. lndeed the milk fat of cows eating the wheat diet is significantly richer in C 18:1 trans (4,4 versus 2,7%) especially for the [delta]10 isomer (+ 1 ,46%) - a fatty acid known as being an efficient inhibitor of key-enzymes of de nova synthesis and desaturation.
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Stefan Jurjanz. Modulation de la qualité du lait par l'amidon de la ration. Sciences agricoles. Institut National Polytechnique de Lorraine, 2001. Français. ⟨NNT : 2001INPL590N⟩. ⟨tel-01749829⟩

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