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 fall, and anestrus is an unlikely problem associated with regular annual mating.
The duration of estrus (~30 hr) is also 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 fall are longer and more intense, and maiden ewes have a shorter and less intense estrus than mature ewes. In general, a ewe's reproductive performance is maximal at 4–5 yr. 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 (vasectomized or epididymectomized), 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, particularly of the meat breeds, can be mated at 7–8 mo of age and 70% of mature body wt; maximal conception rates are seen in ewes with a body condition score of 3.5. Ewes that breed as lambs are able to produce more lamb crops than those bred as 2-yr-olds.
Follicle development and ovulation rates are major determinants of fertility. Ovulation rate is a polygenic trait showing some breed difference; heritability estimates are low (0.3–0.5%). Single genes with a large effect have been determined in a number of breeds, most notably the Booroola Merino. This Fec-B gene (fecundity Booroola) has been introduced into other breeds, eg, Border Leicester.
Nutritional supplementation over a few weeks before mating (“flushing”) may result in higher ovulation rates if the ewes are not in a good body condition (score 3–3.5). 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. Overfeeding of energy to ewes in good body condition can cause decreased fertility.
Estrus, though not necessarily with a fertile ovulation, can be induced in acyclic and anestrous ewes by the introduction of rams (ram effect) or by treatment with progestagens, eCG, and exogenous melatonin. (Also see Hormonal Control of Estrus.)
The sudden introduction of rams or teasers (vasectomized or epididymectomized rams or testosterone-treated wethers) to ewes that have been isolated from rams, bucks, and their odor for >1 mo can induce the onset of ovarian cyclicity. The response to this “ram effect” is best during the transition season, most often 4–6 wk before the onset of the ovulatory season in seasonal breeds, but it will not work once the ewes have started to cycle. Merino ewes respond readily throughout the photoperiod anovulatory season. Responding ewes commonly ovulate within 48 hr of ram introduction and may or may not display estrus. 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).
Estrus can also be induced by treatment with a progestagen by either inserting an intravaginal device (eg, CIDR device containing natural progesterone or pessary containing a synthetic progestagen) or by supplying it through the feed (eg, MGA at 0.125 mg bid) for 7–14 days, followed by eCG at 500 IU at the time of removal of the progestagen. 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:ewe ratio, time of ram introduction, mating by natural or artificial insemination, and number of inseminations (1 or 2).
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 2 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 at the winter solstice, ewes are exposed to artificial light, usually 16–18 hr/day for 8 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. 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 can be synchronized in estrous cyclic and acyclic ewes by inserting progestagen-containing pessaries or CIDR devices into the vagina for 12–15 days, as well as through the feed (MGA). The longer period may yield tighter synchrony but lower conception rates. Synchronization and conception are improved by use of eCG (200–400 IU) at the time of pessary or CIDR removal, although this is generally not necessary during the ovulatory season if artificial insemination is not being used. They can be further improved with the use of prostaglandin (PG) F2α or its analogs (either at, or 12–24 hr before, pessary or CIDR removal), which regresses remnant corpus luteal tissue. SC elastomer “plugs” containing norgestomet (2 mg) can be inserted under the skin of the ear for 14 days for the same effect. Estrous cyclic ewes can also be synchronized by 2 injections of PGF2α or its analogs 8–14 days apart. Estrus commonly is seen 2–3 days after progestagen removal, with a shorter interval during the fall, or within 3 days of the second PG injection. Fertility is better using progestagen pessaries or CIDR than using PG alone, which is less reliable for fixed-time breeding programs.
Last full review/revision July 2011 by Paula I. Menzies, DVM, MPVM, DECS-RHM