Effects of thyroxine and growth hormone treatment of dairy cows on milk yield, cardiac output and mammary blood flow.

Four cows received thyroxine injections (T4; 20 mg/d) and three cows received growth hormone injections (GH; 44 mg/d) for 4 d during successive 16-d experimental periods. Measurement was made of milk yield, protein yield, mammary tyrosine and phenylalanine uptake, blood plasma hormone concentrations, mammary blood flow and cardiac output. Milk yield increased by 25% with T4 and 21% with GH treatment. Milk protein content tended to decline during T4 treatment and increase following GH treatment. Cardiac output increased by 8.9 liter/min (20%) and 4.6 liter/min (10%) with T4 and GH injection. Mammary blood flow (half-udder) increased from 3.6 to 4.9 liter/min (35%) and from 3.3 to 4.4 liter/min (33%) with T4 and GH treatment, respectively. These increases calculated on a whole-udder basis, accounted for 28% (T4) and 48% (GH) of the increases in cardiac output. The proportion of cardiac output perfusing the (whole) udder increased to 19.1% (T4) and 18.7% (GH), increases of 17 and 30%, respectively. Heart rate increased with T4 (but not GH treatment) from 80 to 115/min. Ratio of blood flow to milk yield was not changed by either treatment. The proportion of cardiac output perfusing the udder likely plays a major role in facilitating the partitioning of nutrients for milk synthesis.