Variations with energy nutrition in the concentrations of amino acids of the blood plasma in the dairy cow.

I . The plasma levels of individual amino acids were studied in dairy cows under different conditions of production and energy nutrition. 2. A preliminary experiment was conducted which established that there was no regular change in amino acid levels with time of sampling in animals offered food twice daily at milking. 3. For animals in the 8th month of pregnancy plasma concentrations of lysine, valine, serine and isoleucine were higher, and of threonine lower, in Jersey than in Friesian cows. Lactation was accompanied in most cows by a fall in the plasma concentrations of lysine, arginine, threonine, histidine, glutamic acid, leucine and alanine and a rise in the level of glycine. 4. In lactating cows an improvement in the plane of energy nutrition was associated on average with an increase in the plasma concentrations of ‘non-essential’ amino acids and a decrease in the concentrations of ‘ essential’ amino acids. 5. Intraruminal infusion of propionic acid in the lactating cow increased the concentrations of certain ‘ non-essential’ amino acids, glutamic acid in particular, and decreased those of most other ‘ essential ’ and ‘ non-essential’ amino acids. 6. These observations are discussed in terms of the possibility that an increased output of amino acids in milk proteins results in a depression in the concentrations of the ‘ essential’ and certain of the ‘ non-essential’ animo acids in the plasma. The hypothesis is put forward that the plasma supply of the other ‘ non-essential’ amino acids, glutamic acid and proline in particular, may limit synthesis of milk proteins.