Goat Reproductive Biology
Goats in the U.S are usually "seasonally polyestrous", meaning that they have estrus cycles during late summer, fall and winter. Some relevant characteristics of the estrous cycle are listed in Table 1. The estrous cycle length of goats is 20 to 21 days, with a range of 17 to 24 days. Does are in heat (estrus) for approximately 30 hours, and ovulate 33 hours after the onset of estrus. Most goat breeds are prolific, and mature females shed more than one egg when ovulating.
The reproductive tract of the mature doe consists of several segments. The ovaries are the primary sex organ and produce the ova (eggs) and secrete the female reproductive hormones (i.e. progesterone and estrogen). The oviducts transport the ova to the uterus and are the site of fertilization. The uterus is the site of embryonic implantation and consists of two uterine horns with a common uterine body. The fetus grows and develops in the uterus during gestation. The uterus is closed to the outside by the cervix, a muscular canal consisting of several cervical folds or rings. The exterior portion of the doe reproductive tract is the vagina which is the site of semen deposition during natural mating; it also supplies a fluid environment to support this process during the appropriate stage of the estrous cycle.
The events of the estrous cycle are largely controlled by the hormonal interactions of the ovaries with the secretory glands (pituitary, hypophysis) located at the base of the brain. In addition to internal stimuli, this system is also responsive to external stimulation such as changes in day length and the presence and absence of males. Follicles in the ovaries, containing primary oocytes (eggs), develop in successive waves and until some will rupture and release a secondary oocyte during ovulation. The released egg passes through the oviduct to join with spermatozoa, whereas the ruptured follicle transforms into a corpus luteum. The development of the follicle is under the control of gonadotropins (follicle stimulating hormone - FSH, and luteinizing hormone - LH) released by the pituitary gland. The gonadotropins, via a hormonal feedback loop, also control the release of estrogens by the ovary, which control the estrous behavior displayed by the doe (flagging, mounting, etc.). Following ovulation, the luteinized follicle (corpus luteum) secrets progesterone, which prepares the uterus for a possible pregnancy, and suppresses the secretion of gonadotropins to halt further follicular development. Failure to establish pregnancy will result in the release of prostaglandin from the non-pregnant uterus, which will regress the corpus luteum and allow a new cycle to proceed.
Pregnancy is established once fertilization of the ovum by the spermatozoon occurs. Fertilization occurs in the oviduct and requires the proper timing of insemination and ovulation, as spermatozoa remain viable for only 12 hours in the female reproductive tract, and the life span of the ovulated egg is limited to 12-24 hours. A healthy sperm will penetrate the zona pellucida surrounding the egg using enzymes contained in the cap of the sperm head. Fusion of the sperm cell with the egg will prevent penetration of other spermatozoa. Fertilized eggs move from the oviduct towards the uterus and initiate cell divisions within 24 hours. The developing embryo will continue to divide and remain free-floating until it attaches to the uterine wall 15 to 20 days after fertilization.
The uterus requires priming with progesterone for attachment of the embryo and membrane development to occur. In goats, in contrast to sheep, the corpus luteum is the primary source of progesterone throughout gestation. The developing fetus is contained in the placenta, a membrane that facilitates the exchange between the maternal and fetal circulation. While the placenta experiences the most rapid growth between 90 to 110 days, fetal growth increases exponentially during the last trimester of pregnancy. Gestation length in the goat is approximately 150 days, but is affected by breed and sex of kids, and tends to increase with age, and decrease with litter size.
Parturition is initiated by regression of the corpus luteum approximately 24 hours before delivery, associated with an increase in estrogen. In preparation for parturition the ligaments in the pelvic region relax, uterine contractions stimulated by oxytocin are initiated, and placental membranes rupture. Kids are usually born within 2-3 hours of initiation of labor. The rise in estrogen concentration at parturition is also involved in initiating maternal behavior and acceptance of the newborn. Maternal behavior is also fostered through stimulation of the cervical region as the kids pass through the birth canal.
The male reproductive tract can also be divided into several segments. The primary sex organs are the testes, which produces the male gametes (spermatozoa) and sex hormones (i.e. testosterone). Sperm production (spermatogenesis) takes place inside the germinal epithelium, whereas Leydig cells in the interstitial tissue are responsible for hormone production. The testes are located in the scrotum, which aids in temperature regulation, and maintain the testes at 3-5°C below body temperature for optimal function. Spermatozoa produced by the testis enter the epididymis, also located in the scrotum, which serves as the site of sperm maturation (sperm acquire motility and fertilizing capacity), and storage prior to ejaculation. The vas deferens connects the epididymis to the ampulla and accessory sex glands. These glands are located in the pelvic region and provide the spermatozoa with the fluids that make up the ejaculate. The penis is the final component of the male reproductive tract and used to deposit the semen into the female. In the buck, erection is achieved through the extension of the sigmoid flexure that allows an extension of up to 12 inches and by filling of the cavernous tissues with blood. In the non-erect state the penis is contained in the sheath.
Just like in the ovaries, the events in the testis are controlled by the gonadotropins LH and FSH. In contrast to the female, where all eggs that will be developing are present at birth, spermatozoa are produced through continuous divisions throughout the reproductive life of the male. In the final step of spermatogenesis, cells develop the characteristics of the functional spermatozoa (head, mid-piece and tail). Spermatozoa are then released from the germinal epithelium and pass to the epdididymis, where fluid is removed and the sperm suspension is concentrated. Here sperm are stored in a dormant state prior to ejaculation, or are voided in the urine in sexually inactive male. There is considerable variation in ejaculate volume and sperm concentration, dependent on season, age, and sexual activity. A normal range for volume and concentration is 0.5 to 1.5 ml, and 1.5 - 5.0 billion sperm/ml (Table 1).
Puberty is generally defined as the point of sexual development at which the animal becomes capable of reproduction (first ovulation in the female and first spermatozoa in the ejaculate of the male), but often animals are not fully sexually mature at this stage. In both the male and female goat, puberty may be reached without having achieved adequate physical growth to support reproduction. In the doe, this may be expressed as first ovulation not coinciding with first estrus, while in the male ejaculate quality and quantity are insufficient to achieve extensive breeding success. In immature bucks, the penis has adhesions that prevent the penis from being fully extended. At puberty, these adhesions dissolve and the penis can be fully extended.
Sexual development in the goat, as in other mammals, is a process of gradual maturation, a result of the interaction between the brain and gonads, initiated during embryonic development. Sexual development is influenced by both genetic and environmental factors. In does and bucks the age at puberty ranges from 120 to 230 days, dependent on nutrition, location, breed and season of birth. Nutrition is among the most significant factors influencing reproductive development and the onset of puberty. Inadequate nutrition delays occurrence of the first estrus. Increasing the level of nutrition generally advances the onset of puberty, but overfeeding prior to puberty will decrease subsequent fertility and impair mammary gland development. Season of birth also has a significant impact on the timing of puberty in both doelings and bucklings, with sensitivity to changes in day length already being in effect during the prenatal stages of development. Puberty in spring-born kids has to be achieved in the same year's fall breeding season or will be delayed until the following year's breeding season. There are some indications that the introduction of bucks may encourage doelings to reach puberty early.
The environmental cue most dominantly affecting seasonal breeding in small ruminants is the annual change in day length (short day breeders). As the day length decreases in late summer and fall, does initiate estrus activity and males become more sexually active. However, following extended exposure to decreasing day length, animals become accustomed to the short day length and will again stop cycling.
Differences exist in the onset and length of the breeding season among the various breeds of goats, and even between individual animals within a breed. Geographic location, particularly degree of latitude, has a significant impact on timing and length of the breeding season. At locations close to the equator, tropical breeds of goats often are aseasonal breeders, and breed throughout the year. However, factors such as rainfall/nutrition, and lactational status can also effect breeding season. Other stressors such as transportation or illness may cause a temporary stoppage of estrus activity. In seasonally breeding does, the breeding season is framed by transitional periods during which gonadotropin levels are increasing but not to levels that will trigger estrus and ovulation. Often onset of estrus activity can be hastened through appropriate management techniques (i.e. introduction of males) during this transitional period. In the male, seasonal breeding is associated with changes in testis size and libido, and the development of a distinct buck odor.