. 1999. The effects of methionine supplementation upon milk composition and production of forage-fed dairy cows. Can. J. Anim. Sci. 79: [235][236][237][238][239][240][241]. Two experiments (mid-and late lactation) were conducted to test whether methionine (Met) limited milk production in Friesian cows fed sole diets of fresh perennial ryegrass-white clover pasture. In mid-lactation (exp. 1), 26 cows were assigned to one of three treatments: ruminally protected oral DL-Met (15 g d -1 ), continuous intrajugular infusion of L-Met (15 g d -1 ) and control. Twelve animals from exp. 1 were used during late lactation (exp. 2) and distributed in two experimental groups: ruminally protected oral DL-Met (15 g d -1 ) and control. Dry matter, metabolizable energy, crude protein and Met intakes, milk yield and composition and blood Met, cysteine and urea were measured. Oral and intravenous Met supplementation increased blood Met concentration by 50-90% compared with controls. Met supplementation did not alter the concentration of milk fat, protein or lactose in either experiment. Met supplementation had no significant effects on yields of fat, lactose, casein, whey proteins or non-protein nitrogen during mid-lactation. In late lactation, Met supplementation did not affect milk protein composition or yield of milk components, with exception of a decrease (P < 0.05) in the yield of β-casein. Intravenous Met supplementation increased (P < 0.05) the efficiency of conversion of pasture N to milk protein in mid-lactation. ) sur la composition et la production de lait ont été déter-minés chez les vaches de race Frisonne en mi-lactation (expérience 1) et fin de lactation (expérience 2). Les vaches ont été alimentées exclusivement avec du pâturage de raygrass et trèfle blanc fraîchement coupé dans les deux expériences. Dans l'expérience 1, 26 vaches en mi-lactation ont reçu un des trois traitements suivants: administration orale de la DL-méthionine protégée contre le dégradation ruminale, perfusion intraveineuse continue de la L-méthionine ou témoin. Dans l'expérience 2, 12 vaches de l'expérience 1 ont été utilisées en fin de lactation et distribuées en deux groupes expérimentaux: administration orale de la DL-méthionine protégée ou témoin. La complémentation orale et intraveineuse en méthionine a augmenté la concentration sanguine de méthionine. La complémentation orale et/ou intraveineuse en méthionine n'a pas modifié la concentration du gras, de la protéine et du lactose dans le lait en mi-et fin de lactation. La complémentation en méthionine n'a pas eu des effets significatifs sur la production de gras, de lactose, caséine, des protéines du lactosérum et de la fraction azotée non-protéique en mi-lactation. En fin de lactation, la complémentation orale en méthionine n'a pas affecté la production des constituants du lait ainsi que sa composition protéique à l'exception d'une diminution (P < 0.05) de la production de la β-caséine. La complémentation intraveineuse en méthionine a augmenté (P < 0.05) l'efficacité de la conversion de l...

The estimation of the mammary blood flow (MBF) is an important component of studies of the utilization of metabolites by the mammary gland of lactating animals. Although there are several flow meters available to measure MBF by direct contact with the vessels perfusing the mammary gland, their use is limited mainly by the surgical preparation required for their implantation. For that reason, the application of the Fick principle was assessed as a mean to estimate MBF as part of a study on amino acid utilization by the mammary gland in three pasture-fed dairy cows at either ad libitum or restricted dry matter intakes. Two methods based on the Fick principle were assessed, namely, arterio-venous differences of amino acids (methionine: Met or phenylalanine+tyrosine: Phe + Tyr) and tritiated water (TOH). The estimated MBF was not significantly different for the Met and Phe + Tyr methods (average of 8.1 and 8.8 L min–1, respectively). The TOH method yielded a significantly lower (P < 0.05) estimate of blood flow (average of 5.3 L min–1). Using the Met and Phe+Tyr methods, the MBF was lower during the period of dietary restriction compared with the ad libitum treatment (average of 9.4 and 7.5 L min–1, respectively). In contrast, the TOH method resulted in a numerically higher MBF for the restricted group (5.7 vs. 4.9 L min–1). The short sampling period a nd the loss of indicator in the TOH method appear to be a disadvantage for extrapolating the estimated values to balance studies with lactating cows involving longer periods of time. The estimated values obtained using the Met or Phe + Tyr appear to be re presentative of the MBF during the experimental period. Therefore, any of these methods (Met, Phe + Tyr) may be used alone or in combination as an alternative to flow meters in studies of mammary metabolism. Key words: Dairy cows, mammary blood flow, Fick principle