Hormonal Control of the Estrous Cycle in Cattle

In cows, ovulation can be synchronized via any of three regimens: a combination of progestogen and estrogen, one or two doses of prostaglandin F2alpha (PGF2alpha), or a combination of GnRH and PGF2alpha.

Administration of PGF2alpha (25 mg, IM) or a prostaglandin analogue (cloprostenol at 500 mcg, IM) to cows with a functional corpus luteum starting 5 days after ovulation results in estrus approximately 2–6 days later (5). Two PGF2alpha injections administered 12–14 days apart synchronize estrus and ovulation in most cows. The time to estrus varies more with the PGF2alpha protocol than with progesterone suppression, so artificial insemination (AI) should be based on the detection of estrus.

Ovulation can also be synchronized via GnRH administration (100 mcg, IM) on day 0, followed by PGF2alpha treatment on day 7 and a second GnRH treatment on day 9 (5, 6). Cows should be artificially inseminated between 0 and 20 hours after the second GnRH treatment if there are no means of detecting estrus. This GnRH-PGF2alpha protocol is termed “Ovsynch.”

The Ovsynch protocol has many variations, with the addition of pretreatments that include steroid, PGF2alpha, or GnRH treatments with or without estrus detection. A combination of estrus detection and fixed-time AI can increase pregnancy rates after AI, compared with protocols that use only fixed-time AI to capture cows that might come into heat during administration.

A progesterone-releasing intravaginal device (PRID) that is used to synchronize estrus consists of micronized progesterone (1.55 g) distributed homogeneously in inert silicone rubber coated onto a cylindrical stainless spiral coil (7, 8, 9). The PRID is maintained in the vagina for 7 days, and a luteolytic dose of PGF2alpha is administered 1 day before pullout or on pullout day. There are no milk or meat withdrawal requirements.

A controlled internal drug release (CIDR) device inserted intravaginally can also be used for estrus synchronization. CIDRs are constructed of a progesterone-impregnated medical silicone elastomer molded into a T shape (1.38–1.9 g) (5). They are 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 PGF2alpha. Cows can be inseminated, with or without another GnRH injection, 48–72 hours after the PGF2alpha injection.

Another formerly used synchronization treatment—intramuscular injection of a combination of 5 mg estradiol valerate and 3 mg norgestomet once, with an ear implant of 3 mg norgestomet left in for 9 days (10)—is no longer commercially available in the US. No estradiol based products are legally approved for use in dairy cattle, veal calves, or breeding stock in the US, and extralabel drug use (ELDU) for reproductive management in food-producing animals with associated implants is not compliant with the Animal Medicinal Drug Use Clarification Act of 1994 (AMDUCA).

Melengestrol acetate (MGA) is a steroidal progestogen used as a feed additive to accelerate weight gain and suppress estrus in heifers. MGA is given at a dosage of 0.5 mg/animal every 24 hours for 14 days for estrus synchronization (6). In the US, this product is approved only for use in heifers.

Fertility after MGA treatment is low, and treated females should not be bred. This protocol can be improved by the administration of PGF2alpha (25 mg, IM) or cloprostenol (500 mcg, IM) 19 days after the last feed that contains MGA is given (5), followed by estrus detection. The fertility of this second estrus is reestablished.

Multiple protocols of estrus synchronization and ovulation are available for beef and dairy cattle. When selecting a protocol, factors such as category of female (heifers versus cows), lactation, postpartum status, suckling effect, estrus detection or not, movement of animals (stress), type of semen (sexed versus conventional), facilities, available personnel, experience, budget, and others need to be considered. No single effective protocol will satisfy all operational conditions.

Ovulation can be induced in cows with mature follicles (10–15 mm diameter) via treatment with GnRH at 100–250 mcg, IM, once; luteinizing hormone (LH) at 25 mg, IM, once; or human chorionic gonadotropin (hCG) at 5,000–10,000 IU, IM, once (5). Because the endogenous LH peak develops at the onset of estrus, LH administration does not speed the time of ovulation in cows in estrus; however, it can be used to ensure luteinization in cows with a history of cystic ovary disease or to induce ovulation in anestrous postpartum cows.

Estrus synchronization using similar protocols is less effective in Bos indicus (also called zebus, which have a fatty hump on their shoulders to help them thrive in hot climates) than in Bos taurus (common domesticated cattle), resulting in lower pregnancy rates. The reasons are unclear but could involve multiple factors, such as follicle development, decreased estrus expression, PGF2alpha dosage for complete luteolysis, days of the estrous cycle in which the corpus luteum is fully sensitive to PGF2alpha, and others.

Cows can be superovulated by treatment with equine chorionic gonadotropin (eCG, not currently commercially available in the US) in mid-diestrus, followed by prostaglandin-induced luteolysis 2–3 days later, or by treatment with follicle-stimulating hormone (FSH; potencies differ, so label instructions should be followed), typically IM, every 12 hours for 4–5 days (11, 12, 13, 14), at a decreasing or constant dose, with administration of prostaglandin (25–35 mg, IM, once) usually on day 3 or 4 of treatment. FSH treatment is discontinued at the onset of estrus.

• Kasimanickam R, Harting Q, Hanson R, Boler R. Estrus and ovulation synchronization strategies in beef cattle. Clin Theriogenology. 2025;17(special issue):9-23.

• The Cattle Estrous Cycle and FDA-Approved Animal Drugs to Control and Synchronize Estrus—A Resource for Producers. FDA.

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