Small ruminants are, in general, seasonally polyestrous breeders. Sexual activity is affected by daylight (sexual activity will start when the daylight is shorter). In tropical animals, the reproductive activity is unaffected by seasonal variations. During breeding season, in healthy nonpregnant females, ewes will have an estrous cycle every 16–17 days and does will exhibit estrous cycles every 20–21 days. Protocols of estrus synchronization or induction will depend on the time being used: either breeding season, transitional period, or nonbreeding season.
During the breeding season in cycling goats, luteolysis may be induced by administration of PGF2α (2.5–5 mg, IM) or cloprostenol (62.5–125 mcg, IM) as early as day 3 of the estrous cycle (estrus is day 0). In sheep, PGF2α (≥15 mg) or cloprostenol (125 mcg) is effective after day 5 of the cycle. Estrus may be synchronized by two doses of prostaglandin, 11–13 days apart in does or 7–9 days apart in ewes. Estrus may also be synchronized in cyclic does and ewes by administration of progestogens by intravaginal, implant, oral, or parenteral administration. Intravaginal sponges impregnated with progestogens (medroxyprogesterone acetate or fluorgestone acetate) have been the most widely used agents for control of ovulation but are not available for clinical use in the US. Progestogen treatment is administered for 10–14 days in sheep and for 14–21 days in goats. Shorter progestogen treatment length requires a luteolytic drug (ie, PGF2α) administered parenterally at the end of treatment.
In small ruminants, a controlled internal drug release (CIDR) intravaginal plastic device impregnated with progesterone (300 mg) can also be used. The CIDR is maintained in the vagina for 7 days, and a luteolytic dose of PGF2α is administered 1 day before or at device removal. Administration of GnRH at the time of CIDR insertion has also been shown to be effective. The estrus response is high within 48–96 hours. A portion of a bovine norgestomet ear implant (1.5 mg/goat) or injection of progesterone in oil (10 mg/day, IM) has also been effective. In sheep and goats, the implant is placed subcutaneously near the base of the ear or the base of the tail and then removed 7–9 days later. The norgestomet implant is not available in the US. Ewes should be joined with rams the day after cessation of treatment; does return to heat on the second or third day after treatment ends. Injection of equine chorionic gonadotropin (eCG; 100–500 IU) at the end of treatment increases synchronization of ovulation or ovulation rate. Alternatively, in does, progestogens may be given for 11 days with eCG and PGF2αadministered on day 9, and fixed-time insemination performed on days 12 and 13.
In regimens involving treatments other than prostaglandin alone, fertility may be reduced on the first estrus after treatment in ewes but not in does. During the transitional and nonbreeding season, melatonin implants (18 mg) induce fertile estrus 50–70 days after implant insertion. Better results are obtained when melatonin implants are inserted late in the nonbreeding season or during the transitional period. The implant is biodegradable and does not need to be removed. Estrus synchronization may be improved by using an estrus synchronization protocol at the end of implant action. During the nonbreeding season, a source of gonadotropin is required. The most common gonadotropin used is eCG, administered at the end of a progestogen treatment (or 24 hours before device removal). The dose of eCG depends on the season, breed, age, and postpartum interval, among other factors. A high eCG dose is characterized by superovulation, low fertility, shortened estrous cycles, pregnancy loss, and problems with multiple lambs or kids. The repetitive use of eCG has produced antibodies against eCG in certain sheep and goats, resulting in modification in the time of ovulation and consequent reduction of fertility when artificial insemination at a fixed time is done. A progestogen treatment of a combination of eCG and human chorionic gonadotropin (PG 600), marketed for used in pigs, administered at the end of the progestogen treatment, has been used successfully in small ruminants during the transitional period.