Subnormal Luteal Phase

During a subnormal luteal phase, defined by low secretion of progesterone, there is a sustained increase in frequency of pulses of LH from the anterior pituitary. Increased LH stimulates continued growth and persistence of the largest follicle, with greater secretion of estradiol-17p. Fertility is compromised by ovulation of a persistent dominant follicle.

When a persistent follicle ovulates, the oocyte is likely at a later stage of maturation. Although the oocyte is fertilizable, the resultant zygote often experiences retarded development and early embryonic death (between the 2- and 16-cell stages) in the cow. The effect might not occur in the ewe. In lactating dairy cows, low concentrations of progesterone (2.1 to 2.3 ng/ml) before estrus altered endometrial morphology during the subsequent cycle and increased secretion of the major metabolite of PGF2a, in response to oxytocin on day 15 of that cycle. These effects could decrease fertility even though the original oocyte was healthy.

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