The effects of individual amino acid infusions, formulated from each cow’s blood amino
acid profile, on milk protein production were studied in twelve lactating Holstein cows
fed a total mixed ration with 15.4% crude protein. Treatments were jugular infusions of
either 0.9% saline (SAL), limiting amino acids according to the NRC (NRCT), or limiting
amino acids according to the blood profile (BT) in a Latin square design for three, 10 d
periods where milk and blood samples were collected. NRCT and BT reduced milk
protein content. Essential and branch chain amino acid concentrations increased with
NRCT and BT. The expected increase in milk protein yields and content in response to
amino acid infusion did not occur. Rather, amino acid infusion harmed production,
presumably because infusion induced an amino acid imbalance, increasing the
catabolism of the truly limiting amino acid and reducing milk protein synthesis.