Estrous cycle regulation

Anestrous

Seasonal anestrous

Timing: Occurs during late spring or early summer when days are longer.

Cause: Sheep are short-day breeders and longer daylight suppresses reproductive hormones, leading to a lack of estrous cycles.

Significance: This is the primary anestrous period, helping synchronize lambing in favorable spring conditions when food is more abundant.

2. PostPartum anestrous

Timing: After lambing, ewes undergo anestrous periods that can last from a few weeks to several months.

Cause: Lactation and the recovery of the reproductive system delay the return to estrus.

Significance: This period allows ewes to recover from pregnancy and lambing before becoming fertile again.

Seasonal Regulation

Mother sheep and lamb photo WP03586 (warrenphotographic.co.uk)

Short Day Breeders

Sheep are short-day breeders meaning they begin to cycle in the fall and enter a period of anestrous in the spring and summer months. While not fully understood, it is known that photoperiod has a large effect on seasonal breeders. It is believed that the retina is stimulated by increasing photoperiods which transmits a signal to the hypothalamus and then to the adrenal glands to inhibit the production and release of norepinephrine. A decrease in norepinephrine causes a decrease in the release of melatonin from the pineal gland which causes RFRP neurons to be activated and secrete the neurotransmitter RFRP-3. This increase in RFRP-3 during the spring and summer months inhibits kisspeptin neurons from stimulating the release of GnRH and therefore prevents the sheep from cycling during this time. During the fall, the decreasing photoperiods cause melatonin secretion to increase which prevents RFRP-3 from inhibiting kisspeptin neurons. During this time, GnRH secretion will increase, and cycling will begin.

Estrous Synchronization

CIDR Inserts for Sheep - Premier1Supplies

Estrous synchronization is not an uncommon practice for sheep producers as it decreases labor costs, shortens the lambing period, and allows for easier management of breeding operations. In season, producers can use progestin or prostaglandin to prevent ovulation and estrus sync a group. Out of season, prostaglandin does not work for estrous synchronization and instead, producers must use prostaglandin and supplement it with injections of equine chorionic gonadotropin. Progestin is most commonly given through a Controlled Internal Drug Release (CIDR) device that is inserted into the vagina and left for a period of generally 5-7 days before it is pulled out. After the CIDR is pulled out, progesterone levels will begin to drop and ideally, the sheep should begin cycling.

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