Five multiparous Holstein cows were used in a study with a 5 x 5 Latin square design to measure the effects of postruminal infusion of Met on lactational performance and plasma metabolites. The treatments were duodenal infusions of 1) 10 g/d of Lys (control), 2) 10 g/d of Lys plus 6 g/d of Met, 3) 10 g/d of Lys pus 12 g/d of Met, 4) 10 g/d of Lys plus 18 g/d of Met, and 5) 10 g/d of Lys plus 24 g/d of Met. The cows were fed a diet of 61% maize silage, 31% concentrate, and 5% dehydrated alfalfa. The DMI were similar among treatments. Milk yield, 4% FCM, and milk fat yield and content were not affected by infusions. In contrast, milk protein yield and content were increased linearly as Met infusion increased, which was true also for plasma Met and Cys concentrations. Using measurements of AA flow to the duodenum and assumed intestinal digestibilities of 0.8 for digesta and 1.0 for infused AA, estimated concentrations of Lys and Met in total AA absorbed in the small intestine were 7.3% for Lys and 1.52, 1.73, 1.94, 2.15, and 2.36% for Met for diets 1 through 5, respectively. The substantial linear increases in milk protein yield and content indicated that postruminal Met supply was not adequate over the entire range of Met infusions. In conclusion, the extent of Met limitation in this study could be defined only as that exceeding 2.4% of total AA absorbed in the small intestine.
We conducted two trials to study the effect of graded amounts of glucose infused into the duodenum on milk yield and composition as well as on plasma metabolites, using diets based on grass silage. In trial 1, four fistulated Holstein cows were arranged in a 4 x 4 Latin square design and received 0, 750, 1500, and 2250 g of glucose/d in the duodenum. In trial 2, five fistulated Holstein cows were arranged in a 5 x 5 Latin square design and received 0, 250, 500, 1000, and 2000 g/d of glucose. In both trials, cows were fed a basal diet of 38% grass silage, 10% dehydrated alfalfa, 49% energy concentrate, and 3% oil meal. The treatments (feed plus infusion) were isoenergetic and isonitrogenous. Increased amounts of glucose increased milk yield up to 2.4 and 1.6 kg/d in trials 1 and 2, respectively. Lactose content was not affected, while fat yield and content decreased linearly. The decrease in milk fat resulted from a reduced yield of C18, probably caused by a lower mobilization of fat. The glucose treatments significantly affected the profiles of medium-chain fatty acids, by enhancing the elongation process (up to C14). Glucose infusions induced an asymptotic response of protein yield (linear increase up to 1000 g of glucose, after ceiling). It appears that with poor postruminal starch diets, such as grass silage-based diets containing 35 to 40% of concentrate, the glucose supply to the mammary gland may be limiting for milk synthesis.
Lactating cows were fed fresh ryegrass. The long-chain fatty acid balance in the rumen was negative inprimarygrowthandleafyregrowths,andpositive (+ 11.2*26.2Eo) in stemmy regrowth (P<0.05). Their (Fig. l). In all periods, fatty acid flows into the duodenum were below corresponding dietary intakes (-15.7 to -32.6Va of intake) exceot in the stemmy regrowrh (P5: r I l.2Vo, p6: + 20.2Vo; P<0.05) (Fig. 2). The losses of fatty -F--Tnc--rre-rF For personal use only.
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