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Effect of Thyrotropin Releasing Hormone on Yield and Composition of Bovine Milk¹

Animal Reproduction Laboratory, Department of Dairy Science, Michigan State University, East Lansing 48823

ABSTRACT

Cows were paired in two groups by lactation number and milk production. Twenty cows in group 1 were injected intramuscularly twice daily (0700 and 1900) for 10 days with 50 µg thyrotropin releasing hormone in .5 ml sterile .85% NaCl and then injected with .5 ml .85% NaCl for days 11 to 20. The sequence of treatment was reversed for the 19 cows in group 2. Average milk yield was .66 kg/cow per day greater during the last 5 days of hormone administration relative to the comparable period of NaCl treatment. Average milk yields were increased .72 and .22 kg for groups 1 and 2 within 1 day after hormone injections were begun. Average milk production was slightly increased or maintained throughout hormone treatment relative to the period preceding, then decreased approximately 1.8 kg 1 to 3 days after withdrawal. Percent protein and fat were unchanged by treatment. We conclude that thyrotropin releasing hormone can increase milk production in dairy cows but does not affect milk composition. The modest increase in milk production caused by the hormone is encouraging to continued study of this tripeptide as a stimulus to milk production.

¹ Published with the approval of the Michigan Agricultural Experiment Station as Journal Article 6101.