Goat Reproductive Physiology
Due to the temperature conditions in the United States, most goats are seasonal breeders, with more active breeding happening during the seasons with shorter day lengths and a period of little to no breeding during long day lengths. In females, reproduction is controlled by the estrous cycle. This represents the time from one standing heat (estrus) to the next. This cycle is usually 21 days, with the actual time for standing heat being one to three days. This cycle continues throughout the life of the female and is interrupted only by season, pregnancy and lactation (milking).
The events of the estrous cycle are controlled by the relationships of the hypothalamic releasing hormones, gonadotropins, and ovarian hormones. Gonadotropin releasing hormone (GnRH) comes from the hypothalamus of the brain and causes the pituitary gland to release follicle stimulating hormone (FSH) and luteinizing hormone (LH). The hormone FSH stimulates the production of estradiol, or estrogen, and inhibin and also promotes follicular growth. Estrogen is responsible for the demonstration of estrus behavior, duct development in the mammary glands and the development of secondary sexual characteristics. Inhibin acts as a negative feedback to inhibit the release of FSH from the anterior pituitary. Luteinizing hormone stimulates ovulation and also promotes the formation and function of the corpus luteum (CL). The CL is the structure that is formed on the mammalian ovary following ovulation. It is responsible for the secretion of progesterone following ovulation. A high concentration of this hormone inhibits the release of GnRH, FSH and LH. It also functions in preparing the uterus for a possible pregnancy and suppressing estrus behavior. Failure to establish pregnancy results in the secretion of the hormone prostaglandin 2-alpha from the uterus that causes regression of the corpus luteum and allows a new estrous cycle to begin. If pregnancy is established, then progesterone will continue to be secreted. Its secretion will maintain pregnancy by decreasing uterine contractions, increasing gland development in the endometrium and promoting the development of the mammary gland. It is the interaction of these hormones and their individual functions that makes it possible to regulate and manipulate the reproductive function of the females. This interaction is the basis of hormonal treatments used in the control of goat reproduction.
As previously mentioned, goats experience seasonal anestrus due to the effect of temperature and day length on their reproductive cycle. This condition might prevent the female from conceiving during months when survival of the developing fetus would be low. Pre-attached embryo survival is reduced when the humidity and temperatures are high during the months of summer. Temperature and photoperiod are two main factors that affect the commencement of the breeding season. GnRH secretion is integral to seasonal anestrus. Before the breeding season begins, the hypothalamus must be able to secrete GnRH to elicit the release of FSH and LH in sufficient amounts to maintain follicular development and initiate ovulation in females as well as stimulate testosterone production and sertoli cell function in males. Melatonin secretion is required to stimulate GnRH secretion in order to promote cycling. This hormone is synthesized and secreted during the night hours when it is converted from serotonin through circuitous neural paths. Therefore, light entering the eyes inhibits pineal gland conversion of serotonin to melatonin. The pineal gland acts as a regulator of reproductive activity since it can either stimulate or inhibit gonadal function. During short photoperiods, such as in the fall, the long duration of high melatonin secretion switches on short-day breeders, such as sheep and goats, and switches off long- day breeders likd the hamster. This explains why increased light would cause anestrus in goats, since it inhibits the conversion of serotonin. Goats are therefore considered to be short-day breeders because they begin to cycle during the shorter days of fall. The normal breeding season of goats is during September, October and November, varying for different breeds and areas of the country.
It is therefore necessary for treatment strategies aimed at breeding goats outside of their normal breeding season to somehow override the light and pineal control of the hypothalamic-pituitary-gonadal axis in order to induce estrus and ovulation. Previous research has been successful in inducing estrus and subsequent ovulations by not only manipulating the light-dark cycle, but also by the administration of exogenous hormones such as melatonin and gonadotropin-releasing hormone.
Reference: P.L. Senger. Pathways to Pregnancy and Parturition