In cattle, a successful hormonal control of estrus requires healthy females of adequate age, weight, and nutrition status. Females that have previously calved normally need to be at least 40–45 days postpartum. The suckling effect of the calf has a potential negative effect on estrus synchronization in beef cattle. It is recommended that heifers have reached at least 60% of the expected mature weight for the breed. The minimum body condition score for a satisfactory hormonal control of estrus in beef cattle is 5 (scale 1–9) and for dairy cattle 2.5 (scale 1–5).
In cows, ovulation may be synchronized with a progestogen and estrogen combination treatment regimen, one- or two-dose PGF2α or a GnRH and PGF2αcombination. Administration of PGF2α (25 mg, IM) or prostaglandin analogue (cloprostenol at 500 mcg, IM) to cows with a functional corpus luteum starting 5 days after ovulation results in estrus ~2–6 days later. Two PGF2α injections given 12–14 days apart synchronize estrus and ovulation in most cows. Time to estrus is more variable than with progesterone suppression, so artificial insemination (AI) should be based on detection of estrus. Ovulation may also be synchronized by administration of GnRH, 100 mcg, IM (day 0), followed by PGF2α treatment on day 7 and a second GnRH treatment on day 9. Cows should be artificially inseminated between 0 and 20 hours after the second GnRH treatment without estrus detection. This GnRH and PGF2α protocol is termed “ovsynch.” There are many variations on this protocol, with addition of pretreatments, using steroids, PGF2α , or GnRH treatments that can be used with or without estrus detection. A combination of estrus detection and fixed-time AI may increase e pregnancy rates after AI compared with protocols that use only fixed-time AI.
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 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 hours 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 US.
Melengestrol acetate (MGA) is a steroidal progestogen used as a feed additive to accelerate weight gain and suppress estrus in heifers. MGA is used at a dosage of 0.5 mg/head/day for 14 days for estrus synchronization. In the US, this product is only approved for use in heifers. Fertility after this treatment is low, and females should not be bred. This protocol may be improved by also administering PGF2α19 days after the last feed containing MGA, followed by estrus detection. The fertility of this second estrus is reestablished.
Currently, there are multiple protocols of estrus synchronization and ovulation for beef and dairy cattle. When selecting a protocol, factors such as category of female (heifers versus cows), lactation, postpartum, suckling effect, estrus detection or not, movement of animals (stress), type of semen (sexed versus conventional), facilities, personnel available, experience, budget, and others need to be considered. There is not one effective protocol that will satisfy all operational conditions.
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 cows in estrus but may be used to ensure luteinization in cows with histories of cystic ovary disease or to induce ovulation in anestrous postpartum cows.
The estrus synchronization for Bos indicus cattle using similar protocols as for Bos taurus are less effective, resulting in lower pregnancy rates. The reasons are unclear, but multiple factors may be at play, such as follicle development, decreased estrus expression, dose of PGF2α for a complete luteolysis, days of estrous cycle in which the corpus luteum is fully sensitive to PGF2α, and others.
Cows may be superovulated by treatment with equine chorionic gonadotropin (not currently commercially available in the US) 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, twice daily 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.
