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.
Davis
S R
SR
Dairy Sci. Dept., University of Florida, Gainsville 32611.
Collier
R J
RJ
McNamara
J P
JP
Head
H H
HH
Sussman
W
W
D000818:Animals; D002302:Cardiac Output / Q000187:drug effects*; D002417:Cattle / Q000502:physiology*; D005260:Female; D013006:Growth Hormone / Q000494:pharmacology*; D007774:Lactation / Q000187:drug effects*; D008321:Mammary Glands, Animal / Q000098:blood supply* / Q000187:drug effects; D008892:Milk / Q000378:metabolism; D011247:Pregnancy; D012039:Regional Blood Flow; D013974:Thyroxine / Q000494:pharmacology*
D013006:Growth Hormone; D013974:Thyroxine