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Blood Metabolites in Cows Abomasally Infused with Casein or Glucose¹

Departments of Dairy Science and Animal Husbandry, Michigan State University, East Lansing 48824

ABSTRACT

In four trials lactating cows were infused abomasally with glucose, saline, or varying casein (with added methionine). Plasma glucose was increased by infusions of glucose and of casein in all but one trial where infusion depressed feed intakes. Glucose and casein infusions lowered milk fat content. Casein infusions increased blood urea nitrogen suggesting greater deamination and less efficient dietary protein conversion to milk protein as nitrogen intakes were raised whereas glucose infusions lowered blood urea nitrogen compared to controls. Alpha amino nitrogen in plasma also was raised by casein infusion and lowered by glucose, a pattern similar to that for plasma amino acids. Casein infusions consistently increased the ratio of essential to nonessential amino acids in plasma and were associated with significant increases in milk protein production. Mammary transfer efficiencies calculated from the data and corrected for published arteriovenous differences suggested that phenylalanine and lysine were the essential amino acids in most critical supply for milk production regardless of treatment. Without supplementation, methionine appeared deficient relative to need, but infusion of 15 to 30 g per day made it plentiful. Threonine, leucine, and histidine frequently ranked among the four least abundant essential amino acids.

¹ Published with the approval of the Director of the Agricultural Experiment Station as Journal Article no. 6707.

² Present address: Institute of Animal Nutrition, Agricultural University, 1432 N.L.H., Aas, Norway.

³ Present address: Department of Animal Science, Texas A & M University, College Station, Texas 77843.