Puberty and Estrus in Goats
Goats are spontaneously ovulating, seasonally polyestrous animals with peak sexual activity occurring in the fall when day length is decreasing. Factors that affect onset and length of the breeding season include geographic location (latitude and climate, specifically), breed, social structure, and photoperiod. In temperate regions, the natural breeding season is mostly restricted to the fall and winter to allow for kidding in the spring and summer, when nutritional conditions are adequate. Under tropical and subtropical conditions, where temperature and photoperiod are less variable, certain breeds can have an extended breeding period if appropriate resources are available to allow for kidding year-round. The average duration of the goat estrous cycle is 21 days but can vary with different breeds or environment. A relatively high frequency of short cycles is characteristic of goats and tend to occur in young does, at the onset of the breeding season, and with prostaglandin induction of ovulation. Longer cycles may be seen later in the season.
The average duration of standing estrus is 36 hr but can range from 24–48 hr depending on age, breed, season, and presence of a male. Breed-specific estrus duration has been reported for Mossi (20 hr), Angora (22 hr), Creole (27 hr), French Alpine (31 hr), Boer (37 hr), and Matou (58 hr) breeds. Estrus detection is based on behavioral signs, bleating, flagging of the tail, reddened vulva, vaginal discharge (which causes the tail hairs to stick together), and occasional “riding” by other does, although this last sign is far less common than in cattle. Goats can show overt signs of estrus while pregnant, and although natural service will not interfere with pregnancy, these does should not be artificially inseminated. Ovulation can occur anytime from 9–72 hr after the onset of estrus, typically toward the end of standing estrus. The ovulation rate varies based on breed, season, and nutrition. Angora goats typically have a single ovulation but may have two if sufficient nutrition is available. The average ovulation rate has been reported to be 1.7 eggs per doe in Boer goats and 1.5 in Maure goats. "Focused feeding," in which a nutritional boost is supplied in a short period of time, has enhanced reproductive efficiency in ruminants without affecting body condition. Also known as both "acute" or "immediate nutrient" effect, this practice leads to a positive energy balance, which increases leptin and insulin concentrations, enhances glucose uptake, and is positively associated with increased folliculogenesis and increased ovulation rate. Supplementation with β-carotene, a vitamin A and retinoid precursor, at 50 mg/goat/day has been shown to produce this effect in does.
The onset of puberty typically occurs at 6–8 mo of age but varies depending on the season of birth, breed, nutritional status, and presence of a male. Pygmy goats and does of larger breeds may reach puberty as early as 3 mo old; however, breeding should be delayed until the animal has reached at least 60% of its mature body weight to allow for higher conception rates and safer parturition. Larger goat breeds (eg, Nubian, LaMancha, Boer, and Saanen) can be safely bred at ~70 lb (32 kg). Angora kids should weigh a minimum of 27 kg and are frequently not bred until they are 1½–2½ yr old. Puberty in well-grown bucks can be seen as early as 4 mo.