Cattle producers using artificial insemination (AI) know estrous cycle synchronization (ECS) is the most efficient way to handle and inseminate cattle. But which protocol to use is often a difficult decision.

Considerations of handling stress (how many times the animal must pass through the chute), time and labor, cost, protocol effectiveness and AI pregnancy rates all must be considered in the decision. But for producers desiring a progestin-based ECS protocol, only two agents are currently commercially available.

Melengestrol acetate (MGA) is the only orally active progestin available to producers with limited labor and a desire to minimize animal handling prior to AI. The most cited problem of those synchronizing females with MGA is the 29-31 days required prior to insemination. This can potentially increase a herd's postpartum interval and requires foresight and planning.

Recently, we evaluated the use of short-term exposure to altrenogest (Regumate®) as an orally active progestin in an ECS protocol for beef heifers. A feed-grade product, Regumate is commercially available for ovulation control and pregnancy maintenance in mares, and recently was approved for swine under the name Matrix®. It requires only seven days to time of PGF2α administration, compared to 31 days using the standard MGA protocol.

In our study, a total of 144 crossbred (Simmental and Red Angus) heifers, 14-16 months old and 760-985 lbs., were randomly allotted to either a standard 31-day MGA ECS protocol (75 females), with MGA being fed for the first 14 days of the protocol; or to a short-term altrenogest ECS protocol (69 females), with altrenogest top-dressed at ~20 mg/head/day for seven days.

All animals received a 25-mg intramuscular dose of prostaglandin F2α (PGF2α) (Lutalyse®) 17 days after the end of oral MGA treatment, or at the end of the seven-day oral altrenogest treatment. Of MGA-treated heifers, 50% exhibited standing estrus and were inseminated once with a unit of frozen-thawed Red Angus semen, compared to 70% of altrenogest-treated heifers inseminated with semen from the same bulls.

More altrenogest-treated heifers exhibited standing estrus than MGA-treated heifers. This may have resulted from some MGA-treated heifers not consuming the recommended daily dose of MGA, however, as indicated by evidence of lowered palatability in young heifers when using this progestin agent. The interval from PGF2α treatment to onset of standing estrus was 55 hours for the altrenogest-synchronized heifers, similar to the 53 hours for MGA-synchronized heifers.

At 30 days following AI, pregnancy rate (via ultrasonography) for both treatments was 66%. Calving rate (heifers pregnant from AI that calved) was 87% for MGA-treated heifers and 85% for altrenogest-treated heifers. Heifer calves produced from altrenogest-treated heifers had normal growth rates, reached puberty at the same age as contemporary herdmates, and were fertile following AI.

The results demonstrate altrenogest can:

  • Effectively synchronize estrus in crossbred beef cattle.

  • Result in AI pregnancy similar to a standard MGA protocol.

  • Result in normal, healthy calves.

  • Result in heifer calves that develop and reach puberty similar to herdmates.

The potential use of altrenogest for ECS in beef cattle depends on regulatory approval, cost per animal treatment and the need by producers who lack facilities, time and/or labor to corral their cattle multiple times for ECS and AI.

Using the seven-day altrenogest protocol, a producer would top-dress pre-breeding rations for a week and pen the females only twice — once for a PGF2α injection and once for AI. The recommended MGA protocol is 31 days — 14 days of feeding MGA and a 15- to 17-day resting period, followed by a PGF2α injection to induce estrus.

The results show altrenogest can be a viable alternative for beef cattle ECS, and suggest timed-AI following the altrenogest protocol is a management option for producers.

C.E. Ferguson and R.A. Godke are faculty in the Department of Animal Sciences, Louisiana State University Agricultural Center, Baton Rouge.