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Ovarian Steriod Hormones


Ovarian
Steriods

Cows walking to the barn.

Ovarian Steroids

Ovariectomy has no effect on postpartum mammary growth or lactation, suggesting that ovarian steriods are not necessary for maintenance of lactation. On the other hand, increased blood concentrations of estrogen may affect milk production.

Although relatively few species are concurrently lactating and pregnant, concurrent pregnancy does influence persistency of milk yield in the declining phase of lactation. This is particularly evident in dairy cattle. Inhibitory effects of pregnancy on lactating cows do not become apparent until about mid pregnancy (Wilcox et al. 1959; Bachman et al. 1988). An inhibitory effect of pregnancy on lactation has been noted in a number of other species, as well (Wilde and Knight 1989; Tucker 1994). The mechanism of this effect is not fully understood. Progesterone seems to have no effect on milk yield in the lactating cow because (1) progesterone has a higher affinity for milk fat than for glucocorticoid receptors, and (2) there are no progesterone receptors in the mammary gland during lactation. However, the timing of inhibition of milk yield in cattle coincides approximately with the period of increasing placentally-derived plasma estrogen (Robertson and King 1979).

Administration of pharmacological doses of estrogen will decrease milk yield. In fact, administration of estrogen to cows in late lactation can enhance the rate at which the mammary gland undergoes involution at drying off. A role for estrogen in mammary gland involution in dairy cattle has been indicated (Athie et al. 1996). Estogen was administered to late lactation dairy cattle for 4 days prior to dry-off, or milk stasis, resulting in a more rapid rate of involution in the estrogen treated cows. However, cows used for that study were already producing low yields of milk (means of ~11 kg daily with 3X/day milking) and already had elevated lactoferrin levels in milk (an indicator of mammary gland involution) prior to drying off. Lactoferrin is usually relatively low in concentration in lactating cattle that do not have mastitis. The administration of estrogen significantly reduced milk yield even further prior to drying off (halting of any further milk removal). In effect, this treatment gave the estrogen treated cows a 4 day start on the involution process, accounting for some of the 6 day difference noted in involution rate. Nevertheless, that study and other studies on the effects of estrogen on milk composition during lactation (Bachman 1982) strongly suggest that estrogen, and perhaps pregnancy, may have an effect on mammary gland involution in cattle.


 
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