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1 Department of Animal Science, University of Arizona, Tucson 85721
2 Department of Animal and Veterinary Science, University of Idaho, Moscow 83844
3 Animal Agriculture Business, Monsanto Company, St. Louis, MO 63198
Corresponding author: R. J. Collier; e-mail: rcollier{at}ag.arizona.edu.
A dry period, typically 40 to 60 d, between lactations is believed to be required to maximize milk yield in the subsequent lactation. Several hypotheses have been proposed to explain the requirement for the dry period, including (1) replenishment of body reserves, (2) regeneration of mammary tissue, and (3) optimization of benefits from endocrine events near the time of parturition. Continuously milked cows or glands have depressed milk yields but no differences in mammary DNA content or cell number. Nutritional status and endocrine hormones are not factors in reduced milk yield in continuously milked glands. Data from continuous lactation studies suggest that depressed milk yields are due to reduced functionality of mammary parenchyma. There is a need to reevaluate effects of continuous lactation on milk yield in today’s high-producing dairy cow because studies on this topic were done using cows achieving peak milk production of 18 to 30 kg/d compared with 45 to 50 kg/d in today’s dairy cows. Another factor that has not been considered in conjunction with current milk production levels is the use of bovine somatotropin (bST). Supplementation with bST increases milk yield, improves lactation persistency, and may improve milk yield in continuously milked cows. Future research goals are to examine the effects of continuous lactation in high-producing cows and to determine the effects of bST on milk yield and mammary functionality in continuously milked cows.
Key Words: dry period length • management • mammary development • bovine somatotropin
Abbreviation key: CM = continuous milking, MEC = mammary epithelial cells, TGF-ß1 = transforming growth factor ß1
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