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Effects of Thyroxine and Growth Hormone Treatment of Dairy Cows on Milk Yield, Cardiac Output and Mammary Blood Flow¹

University of Florida⁵, Gainesville 32611

Abstract

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 treament. Milk protein content tended to dechne 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.

¹ Florida Agric. Exp. Sta. Publ. No. 6522. The support of the New Zealand-United States Technical Cooperation Program and the Upjohn Co., Kalamazoo, MI in funding this work is acknowledged gratefully. We also thank R. F. Olsen of the Upjohn Co. for the somatotropin assays.

² Ruakura Anim. Res. Sta., Ministry of Agric. and Fish., Private Bag, Hamilton, New Zealand.

³ To whom reprint requests should be directed. Present address: Monsanto Co., 700 Chesterfield Parkway, BB3F, St. Louis, MO 63189.

⁴ Dept. Anim. Sci., Washington State Univ., Pullman, WA 99164.

⁵ Dairy Sci. Dept.