Utilisation des protéines du lait et du lactosérum par le jeune veau préruminant. Influence de l'âge et de la dénaturation des protéines du lactosérum

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Abstract
Three milk replacers (control, RD [denatured whey] and RN [native whey]) were prepared. In the control milk replacer, proteins were totally provided by skim milk powder supplemented with methionine; in RD and RN, 58% of proteins were supplied by a concentrate of RD or RN, the rest by skim milk powder. Both concentrates were pasteurized (72.degree. C for a few seconds) then concentrated and spray-dried. Denaturation of RD was obtained by a supplementary heating (75.degree. C, 30 min) after concentration, which increased the percentage of denaturated proteins from 8-34. Each diet was offered to 5 Holstein x Friesian male calves from the 1st day of age. Digestibility was measured from 3-8 days of age (P1) from 14-19 days of age (P2) and from 28-33 days of age (P3). The apparent digestibility of the control diet increased regularly from P1-P3; thus for protein it rose from 0.871-0.935 and 0.966. That of the other 2 diets only increased between P2 and P3 changing or protein from 0.888-0.896 and 0.950 (RD) and from 0.859-0.854 and 0.923 (RN). The individual variations were larger during P1 and P2 than during P3. The increase observed between P2 and P3 was significant for every diet. Protein digestibility was significantly higher with the control diet than with RN diet during P3. Analysis of amino acid composition of the feces showed that increase with age in the apparent digestibility of protein was probably due to an improvement in the digestion of dietary and endogenous proteins. This accounts for the higher true digestibilities observed in some calves during P1. The true digestibility of the proteins of RD and especially RN diets were lower than that of the milk proteins. The blood levels of free amino acids, especially the lysine content were little affected by denaturation. Digestive utilization of milk and whey proteins increased from birth to the 5th wk of age. Whey proteins were less used than milk proteins during this period, but their partial denaturation by heat-treatment contributed to improving the true digestibility. Moderate heat treatments may increase the nutritional value of some dairy products.