MAMMARY DEVELOPMENT AND PITUITARY PROLACTIN LEVEL OF HEIFERS FROM BIRTH THROUGH PUBERTY AND DURING ESTROUS CYCLE

Nucleic acid, hydroxyproline (collagen), and lipid content of mammary glands of Holstein heifers from birth to 12 months of age and on Days 0 (estrus), 2, 4, 7, 11, 18, and 20 of the estrous cycle were determined and their relationships to pituitary prolactin were studied. Deoxyribonucleic acid content of the mammary gland increased 1.6 times faster (P < 0.01) than body weight between birth and two months of age. The comparable value increased (P < 0.01) to 3.5 between five and nine months and then decreased to 1.5 from nine to twelve months of age. During the estrous cycle mammary deoxyribonucleic acid increased 118% (P < 0.05) between Day 20 and the day of estrus but declined thereafter. All other mammary constituents measured followed similar patterns of change. Thus, mammary glands of heifers were stimulated to grow at an accelerated rate well in advance of first estrus and following the initiation of estrous cycles most growth occurred during estrus. Pituitary prolactin concentration increased 333% between birth and three months and was maximum at nine months, synchronizing with similar changes in mammary development. The greatest sustained levels of prolactin in the pituitary occurred within three days before ovulation, whereas minimal levels occurred around the time of ovulation. © 1969, American Dairy Science Association. All rights reserved.