Hormonal Control of Estrus in Cattle
In cows, ovulation may be synchronized with a progestagen and estrogen combination treatment, a two-dose prostaglandin regimen, or a GnRH and prostaglandin combination. Administration of PGF2α (25 mg, IM) or prostaglandin analogue (cloprostenol at 500 mcg, IM) to cows with a corpus luteum from 5 days after ovulation results in estrus in ~2–5 days. Two prostaglandin injections given 12–14 days apart synchronize estrus and ovulation in most cows. Time to estrus is more variable than with progesterone suppression, so insemination should be based on detection of estrus. Ovulation may also be synchronized by administration of GnRH, 100 mcg, IM (day 0), followed by prostaglandin treatment on day 7 and a second GnRH treatment on day 9. Cows should be inseminated 0–20 hr after the second GnRH treatment. This GnRH and PGF2α protocol is termed “ovsynch.” There are many variations on this protocol, using additional steroids, prostaglandin, or GnRH treatments, that may increase the degree of synchrony or pregnancy rates after artificial insemination.
A progesterone-releasing intravaginal device (PRID) consists of micronized progesterone distributed homogeneously in an inert silicone rubber coated onto a cylindrical stainless spiral coil. The PRID is maintained in the vagina for 7 days, and a luteolytic dose of PGF2α is administered 1 day before or at "pull-out" day. There are no milk or meat withdrawal requirements. In noncycling animals, an injection of equine chorionic gonadotropin (eCG) is administered when the PRID is removed.
A controlled intravaginal drug-release (CIDR) device may also be used for estrus synchronization. A CIDR is an intravaginal device constructed of a progesterone-impregnated medical silicone elastomere molded into a T-shape. It is labeled for estrus synchronization in beef and dairy cattle. Cows are administered GnRH, and concomitantly a CIDR is inserted and maintained for 7 days. At the time of removal, cows receive an injection of PGF2α. Cows can be inseminated, with or without another GnRH injection, 48–72 hr after PGF2α injection. The most effective synchronization treatment, an IM injection of a combination of 5 mg estradiol valerate and 3 mg norgestomet, with an ear implant of 3 mg norgestomet left in for 9 days, is no longer commercially available in the USA because of the ban on the use of estrogens in food animals. Note that all estrus synchronization protocols are less effective in Bos indicus cattle and B indicus crossbreds than in Bos taurus cattle.
Melengestrol acetate (MGA) is a steroidal progestagen used as a feed additive to promote growth and suppress estrus in heifers. MGA is used at a dosage of 0.5 mg/head/day for 14 days for estrus synchronization. Fertility after this treatment is low, and females should not be bred. This protocol could be improved by administering PGF2α 17 days after the last feed containing MGA. The fertility of this estrus is reestablished.
Ovulation may be induced in cows with mature follicles (10–15 mm diameter) by treatment with GnRH at 100–250 mcg, IM; luteinizing hormone (LH) at 25 mg, IM; or hCG at 5,000–10,000 IU, IM. Because the endogenous LH peak develops at the onset of estrus, this administration will not speed the time of ovulation in estrous cows but may be used to ensure luteinization in cows with histories of cystic ovarian disease or to induce ovulation in anestrous postpartum cows.
Superovulation may be achieved in cows by treatment with eCG (not currently commercially available in the USA) in mid-diestrus followed by prostaglandin-induced luteolysis 2–3 days later, or by treatment with FSH (potencies differ, refer to label instructions), typically IM, bid for 4–5 days, at a decreasing or constant dose with administration of prostaglandin (25–35 mg, IM) usually on day 3 or 4 of treatment. FSH treatment is discontinued at the onset of estrus.