Hormonal Control of Estrus in Goats and Sheep
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. In sheep, PGF2α (≥15 mg) or cloprostenol (125 mcg) is effective after day 5 of the cycle (estrus is day 0). 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 or anestrous does and ewes by administration of progestagens; impregnated intravaginal sponges (medroxyprogesterone or fluorgestone) have been the most widely used agents for control of ovulation but are not currently available for clinical use in the USA. 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 administered 1 day before or at device removal. The estrus response is high within 72–84 hr. 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 ear implant is placed subcutaneously near the base of the ear or the base of the tail and then removed 7–9 days later. Progestagen treatment is administered for 10–14 days in sheep and for 14–21 days in goats. 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, progestagens may be given for 11 days with eCG and prostaglandin administered 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 nonbreeding season, melatonin implants (18 mg) induce fertile estrus 50–70 days after implant insertion. Better results are obtained when melatonin implants are inserted early 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 also required. The most common gonadotropin used is eCG, administered at the end of a progestagen treatment (or 24 hr 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.