Introduction

Estrous cycles have two ovarian phases, luteal and follicular. The luteal phase is longer and is dominated by progesterone, secreted by the corpus luteum. After luteal regression, the follicular phase is relatively short, two to four days. Rapidly increasing secretion of estradiol, by the largest follicle(s), initiates the luteinizing hormone (LH) surge from the anterior pituitary. Ovulation occurs 24 to 27 hours after the LH surge. The hormonal relationships during the luteal and follicular phases in an example animal, the cow, will be presented in the discussion below. Most breeds of sheep and goats are seasonally polyestrus. Cycle length is marked by behavioral estrus, which occurs at average intervals of 21 to 22 days (range 17 to 24) in cows, 16 to 17 days in ewes, and 19 to 21 days in does (goat breeds vary).

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