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Serum Hormone Concentration in Ruminants during Mammary Growth, Lactogenesis, and Lactation: A Review¹

Animal Reproduction Laboratory, Department of Dairy Science, Michigan State University, East Lansing 48824

ABSTRACT

Apparently the mammary gland grows in response to sustained high serum concentrations of estrogens and progesterone which are incurred during pregnancy in ruminants. Quantities of prolactin, growth hormone, and glucocorticoid in serum are probably sufficient to support mammary growth during pregnancy. In sheep and goats mammary growth may be stimulated by a lactogenic material of placental origin.

Increased mammary enzymatic activity during the weeks preceding parturition in cattle is temporally related to increased serum estrogen concentrations. Perhaps estrogens cause the marked increase in enzymatic activity that occurs at this time. The sharp decrease in progesterone that occurs 48 to 24 h prepar-tum in ruminants may allow final maturation of the mammary cell to one characteristic of lactation. If a progesterone repression of lactation occurs in ruminants, once the repression is overcome it is not reestablished to a noticeable degree by increased progesterone incurred during reoccurring estrous cycles or during concurrent pregnancy and lactation.

Increased serum corticoid and growth hormone at parturition may be involved in onset of milk secretion, but for now there is little evidence to support or refute this view. In contrast, the increase in prolactin at parturition apparently is involved in onset of milk secretion since inhibition of the prepartum prolactin surge will suppress onset of copious milk flow. In contrast, suppression of serum prolactin concentrations with ergocryp-tine during lactation does not decrease milk production in cattle. This does not mean that prolactin is not necessary for maintenance of lactation in ruminants but does suggest that serum prolactin levels are not indicative of only lactational events.

¹ Published with the approval of Director of the Agricultural Experiment Station as Journal Article Number 6487. Supported in part by NIH Grant HD 15899.

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