Effects of graded amounts of duodenal infusions of methionine on the mammary uptake of major milk precursors in dairy cows.

Arteriovenous differences in plasma and rate of blood flow were measured across the udder of four lactating dairy cows to examine the effects of increased supplies of a limiting AA on nutrient balance to the mammary gland. Treatment consisted of graded amounts of duodenal infusions of DL-Met (0, 8, 16, and 32 g/d) over 4-d periods according to a 4 x 4 Latin square design. Cows were fed a diet formulated to meet protein and energy requirements and based on 70:30 corn silage:concentrate ratio and dehydrated alfalfa (1.5 kg/d of DM). The basal diet met 70% of Met requirements. Yields of milk, fat, and protein and milk fat content were unaffected by infusions of Met, but milk protein content tended to increase by 5% to a maximum at the third treatment. Surprisingly, the rate of mammary blood flow declined in response to infusions of Met. The mammary uptake remained constant, nevertheless, for oxygen, energy-yielding nutrients, and nonessential AA. In contrast, the quantity of essential AA taken up by the udder decreased. As a result, milk protein yield seemed sustained by an increased efficiency of the mammary gland. In addition, in response to the Met infusions, the arterial supply of Met nearly increased 3-fold, and the mammary uptake of Met was similar across all treatments. The uptake of Met by the udder seems to be more related to the quantity of Met required for milk protein synthesis than to its concentration in the artery.