Reproductive Physiology of Sheep
Ewes are seasonally polyestrous, cycling every 16–17 days during the breeding season. The major environmental factor controlling the estrous cycle is the photoperiod. Decreasing photoperiod after the summer solstice causes secretion of melatonin, which triggers the hypothalamus to produce gonadotropin-releasing hormone. Geographic location and environmental temperatures also modify the length of anestrus, as does the breed of sheep. Fine-wool breeds (eg, Rambouillet, Merino), tropical breeds, and Dorsets have a shorter anestrous period than other breeds such as the Suffolk, Hampshire, Border Leicester, and Columbia. Regardless of this breed-related variation in the length of the breeding season, all breeds are most fertile in the autumn, and anestrus is an unlikely problem associated with regular annual mating.
The duration of estrus (~30 hr) is influenced by the breed and age of the ewe, the onset of puberty, the presence of the male, and the season. Estrous periods that occur in the autumn are longer and more intense, and maiden ewes have a shorter and less intense estrus than mature ewes. The optimal time to mate ewes (naturally or artificially) is in the first half of the estrous period, or 12–18 hr after the onset of estrus. Ewes show no overt signs of estrus, and heat detection requires the presence of a ram, a teaser ram (made infertile by either vasectomy or epididymectomy), or a testosterone- or estrogen-treated wether.
The age of puberty of ewe lambs varies greatly and is influenced by breed, nutrition, presence of the ram, and season of birth. Well-grown ewe lambs with a body condition score of 3–3.5, particularly of the meat breeds, can be mated at 7–8 mo of age and 70% of mature body wt. Two exceptions exist: lambs born in the autumn to a spring breeding will reach puberty in the spring and be less fertile to an induced estrus, and some breeds of sheep are slow maturing (eg, range breeds) and may not be fertile at 7–8 mo of age. However, breeding ewe lambs early is encouraged; ewes that breed as lambs are able to produce more lamb crops in their lifetime than those bred as 2-yr-olds.
Ewe lambs should be separated from ram lambs by 5 mo of age to avoid unwanted pregnancies, and from all market lambs to avoid overconditioning from a diet too high in energy. Overconditioned ewe lambs are less fertile and produce less milk than those fed to achieve a body condition score of 3.
Follicle development and ovulation rates are major determinants of fertility. Ovulation rate is a polygenic trait showing marked breed difference; heritability estimates are moderate (0.3%–0.5%). Although by selecting replacement animals born as twins or triplets will slowly increase prolificacy within a flock, using more maternal breeds is recommended because their offspring will also have desirable traits of better mothering and milk production.
Nutritional supplementation starting a few weeks before mating (“flushing”) may result in higher ovulation rates if the ewes are not in good body condition (score <3). Ewes already in good fit will have a similar ovulation rate and will not respond further to flushing. The diet should be balanced for protein (not >14% crude protein) and have a good availability of energy. High levels of soluble protein can cause early embryonic death through increased urea nitrogen levels in the blood. Overfeeding of energy to ewes in good body condition can cause decreased fertility.
Estrus and ovulation can be induced in anestrous ewes by the introduction of rams (ram effect) or by treatment with exogenous progestagen or equine chorionic gonadotropin (eCG), or through the effects of melatonin either by manipulating the photoperiod in housed sheep or through the use of exogenous melatonin in the feed or as an implant. (Also see Hormonal Control of Estrus.) Factors affecting fertility after estrus induction include breed, season, lactation, nutritional status, and postpartum period, as well as the ewe's dry/suckling status, ram to ewe ratio, time of ram introduction, mating by natural or artificial insemination, and number of inseminations (one or two).
The sudden introduction of novel rams or teasers ("ram effect") to anestrous ewes can induce the onset of ovarian cyclicity during the transition season, most often 4–6 wk before the onset of the ovulatory season in seasonal breeds; however, it works poorly to synchronize estrus once the ewes have started to cycle. Responding ewes commonly ovulate within 48 hr of ram introduction but usually are not receptive, ie, the estrus is silent. In ewes with a silent estrus, ovulation is followed by the formation of either a normal or a short-lived (5–6 days) corpus luteum (CL). After regression of a normal CL, most ewes display estrus (~19 days after ram introduction). After regression of a short-lived CL, ewes ovulate without displaying estrus and commonly form a normal CL. Regression of this CL results in estrus (~25 days after ram introduction). This gives two peaks of breeding activity and very good synchronization of estrus without the use of exogenous hormones.
Acyclic, seasonally anestrous ewes can be induced into estrus and ovulation with exogenous melatonin. Ewes are given melatonin 6 wk before joining and are isolated from rams during that period. The joining period should cover two complete estrous cycles (35 days). Exogenous melatonin is more successful toward the end of the seasonal anestrous period, ie, transition period. This product is not commercially available in many countries. A similar effect can be obtained by manipulating the photoperiod under management programs in which the ewes and rams are in confinement housing. Usually starting in late autumn up to the winter solstice, ewes are exposed to artificial light, usually 16 hr/day for 8–12 wk. At the end of this period (eg, mid-winter), the length of light exposure is reduced to 8 hr/day. This may require darkening windows to reduce exogenous light sources and completing barn chores during only those 8 hr to avoid using lights during the dark period. After 6–8 wk, ewes will start to cycle. If the rams are housed under similar conditions, scrotal circumference and breeding capacity will increase.
Estrus induction in acyclic ewes requires exposure to exogenous progestagen for a minimum of 5 days; most programs use 7 days. The progestagen used may be either natural progesterone delivered intravaginally in a controlled intravaginal drug-releasing device (CIDR), or melengestrol acetate (MGA) fed at 12-hr intervals at a dose of 0.125 mg/head. Ovulation and subsequent fertility is usually poor unless eCG is given at removal of the progestagen; usually 500 IU is administered. If ewes are cycling, then either the progestagen exposure needs to be sufficiently long to outlast any CLs present on the ovaries. This is usually 11–14 days. Use of eCG at progestagen removal time is optional (200–400 IU) and is generally not necessary during the ovulatory season if artificial insemination is not being used. Shorter exposures in cycling ewes can be used if prostaglandin (PG) F2α or its analogues is administered either at, or 12–24 hr before, pessary or CIDR removal. Estrous cyclic ewes can also be synchronized by two injections of PGF2α or its analogues 8–11 days apart, although fertility is poorer than with progestagen programs. Estrus commonly is seen 1–3 days after progestagen removal, with a shorter interval during the autumn or with the use of CIDRs, or within 3 days of the second PGF2α injection.
Before a program is set up, the veterinarian should be sure sufficient numbers of rams are available for the program and that the rams are fertile. During the normal ovulatory season and without synchronization, a ram to ewe ratio of 1:40 is usually sufficient, although if the rams are young, or the environment is rough, then the number of ewes per ram should be lowered. This is also true if estrus is synchronized, in which case the ram to ewe ratio should be ~1:10 to 15. When breeding out of season, ram fertility is usually lower, and if estrus is synchronized the ram to ewe ratio should be ~1:5 to 7.