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Methionine is one of the first limiting AA in dairy cows. The use of rumen-protected Met to correct deficient diets is limited by the lack of a product that could be incorporated into a pelleted concentrate. The main objective of this trial was to test, at practical doses (approximately 10g of absorbable Met), the efficacy of 2 forms of pelletable Met hydroxy analogs, d,l-2-hydroxy-4-(methylthio)-butanoic acid (HMB) and the isopropyl ester of HMB (HMBi), to provide Met to cows, especially for milk protein synthesis, compared with a negative control and to Smartamine M (SmM). These treatments were tested according to a 4×4 Latin square in 16 Holstein cows. Plasma Met concentrations were increased by 110 and 65% that of the control value after HMBi and SmM treatments, respectively. Milk protein yield increased by 32 and 41 g/d for HMBi and SmM, respectively. d,l-2-Hydroxy-4-(methylthio)-butanoic acid supplementation did not improve Met availability to the cows for milk protein synthesis. The HMBi treatment induced an increase in 15:0 in milk at the expense of a general reduction in even-numbered short-and medium-chain fatty acids. Moreover, HMBi and SmM supplements led to an increase in the saturation level of C 18 fatty acids consistent with the improvement of Met supply. It was concluded that HMBi is a new "rumen-protected" form of Met that can be supplied to cows integrated into pellets.

Vitamin A is an essential fat-soluble vitamin involved in critical biological functions. Because most mammals cannot carry out its de novo synthesis, vitamin A must be provided by the diet as retinol or as provitamin A, which may, after cleavage, yield retinal, and then retinol. Vitamin A is necessary for embryonic development and should be provided to the fetus through the placenta and after delivery, through milk, for the growth of the newborn. Past studies on the status of β-carotene and vitamin A in bovines have focused on the achievement of suitable plasma and milk levels during the peripartum period to maintain cow's health, to prevent reproductive disorders, and to ensure an adequate vitamin supply to calves via colostrum. Recently, the role of carotenoids and fat-soluble vitamins in the nutritional and sensorial properties of dairy products and their potential use as biomarkers for the traceability of cows’ feeding management have been established. To better characterize how feeding could influence the appearance and nutritional properties of milk and dairy products, studies that follow variations of carotenoids, vitamin A, and color in cow's plasma and milk during lactation have been carried out. Furthermore, processing of dairy products has been improved to protect these micronutrients which are essential for health. All these aspects are summarized in this article.

International audience; Milk is essential in human nutrition at birth but also after weaning due to milk and dairy products consumption. Bovine milk makes a significant contribution to the reference intake for several micronutrients: vitamins A (15-20%), B-2 (60-80%) and B-12 (90%), but all the other vitamins are also present. Clinical deficiencies in vitamins are more controlled today and some of them have almost disappeared. However, pregnant women and breast-fed babies remain at risk. A positive relation between suboptimal vitamin intakes and the prevalence of chronic diseases in the population has been observed. The increase in milk nutritional quality could help to optimize the dietary intakes of vitamins but complementary studies are needed to control precisely the vitamin composition in milk, especially for vitamins D, K and group B.

The present experiment was undertaken to determine the effects of dietary supplements of folic acid and vitamin B 12 given from 3 wk before to 8 wk after calving on lactational performance and metabolism of 24 multiparous Holstein cows assigned to 6 blocks of 4 cows each according to their previous milk production. Supplementary folic acid at 0 or 2.6 g/d and vitamin B 12 at 0 or 0.5 g/d were used in a 2×2 factorial arrangement. Supplementary folic acid increased milk production from 38.0±0.9 to 41.4±1.0 kg/d and milk crude protein yield from 1.17±0.02 to 1.25±0.03 kg/d. It also increased plasma Gly, Ser, Thr, and total sulfur AA, decreased Asp, and tended to increase plasma Met. Supplementary B 12 decreased milk urea N, plasma Ile, and Leu and tended to decrease Val but increased homocysteine, Cys, and total sulfur AA. Liver concentration of phospholipids was higher in cows fed supplementary B 12 . Plasma and liver concentrations of folates and B 12 were increased by their respective supplements, but the increase in plasma folates and plasma and liver B 12 was smaller for cows fed the 2 vitamins together. In cows fed folic acid supplements, supplementary B 12 increased plasma glucose and alanine, tended to decrease plasma biotin, and decreased K m of the methylmalonyl-coenzyme A mutase in hepatic tissues following addition of deoxyadenosylcobalamin, whereas it had no effect when cows were not fed folic acid supplements. There was no treatment effect on plasma nonesterified fatty acids as well as specific activity and gene expression of Met synthase and methylmalonyl-coenzyme A mutase in the liver. Ingestion of folic acid supplements by cows fed no supplementary B 12 increased total lipid and triacylglycerols in liver, whereas these supplements had no effect in cows supplemented with B 12 . The increases in milk and milk protein yields due to folic acid supplements did not seem to be dependent on the vitamin B 12 supply. However, when vitamin B 12 was given in combination with folic acid, utilization of the 2 vitamins seems to be increased, probably more so in extrahepatic tissues. Metabolic efficiency seems also to be improved as suggested by similar lactational performance and dry matter intake for cows fed supplementary folic acid but increased plasma glucose and decreased hepatic lipids in cows fed folic acid and vitamin B 12 together.

Adequate Met supply is especially important in the dairy cow for milk protein synthesis. Because of insufficient Met contents in the most frequently used feed-stuffs, Met becomes limiting in the diet of the dairy cow. To restore the amino acid balance of the diet and consequently to optimize lactation performance, Met must be supplied in a protected form because of its high degradability as a free amino acid by rumen microorganisms. A new chemical derivative of Met, the isopropyl ester of the 2-hydroxy-4-(methylthio)-butanoic acid (HMBi) was tested for its metabolic fate by following the evolution of plasma concentrations of its metabolites (2-hydroxy-4-(methylthio)-butanoic acid (HMB), Met, isopropyl alcohol, and acetone) after spot-dose supplementation (50 g Met equivalent) to 15 cows. Results indicated that HMBi would be quickly absorbed and hydrolyzed into HMB and isopropyl alcohol, and then converted to Met and acetone, respectively. In our experimental conditions, the Met availability for cows was estimated to be 48.34 +/- 2.05% using a calibration curve established by modeling the area under the curve response to increasing doses of Met supplied as Smartamine M, whose bioavailability (80%) is considered the reference value. Plasma kinetics and bioavailability of Met were compared between HMBi and Smartamine M in the same cows. Comparison of the kinetics suggests that HMBi would be absorbed through the rumen wall providing good protection against rumen microorganisms. It can thus be concluded that HMBi is a new source of Met for ruminants with an acceptable bioavailability.