Melengestrol acetate (MGA) is the common denominator in three widely-used systems for synchronizing and artificially inseminating (AI) cows and heifers.

MGA, fed at a rate of 0.5 mg/animal/day in a single daily feeding with grain or protein and often top-dressed on other feeds, suppresses estrus and prevents ovulation in cows and heifers.

“While the duration of feeding MGA may vary, it's critical the daily intake be consistent,” says Dave Patterson, University of Missouri animal scientist. “Animals failing to consume the required MGA amount on a daily basis may prematurely return to estrus during the feeding period.”

Cows and heifers normally exhibit estrus beginning 48 hours after MGA withdrawal, and continue estrus for 6-7 days.

“It's generally recommended females exhibiting estrus during this period not be inseminated or exposed to natural service because of the reduced fertility females experience at the first heat after MGA withdrawal,” Patterson says.

  • Method 1: MGA + Prostaglandin

    This method combines MGA with prostaglandin (PG), which is available in five different products — Lutalyse, ProstaMate, InSynch or Estrumate, and estroPLAN.

    In this program, MGA is fed for 14 days, and PG is administered 19 days after the last day of MGA feeding. Producers should heat detect and AI cows exhibiting estrus from the time PG is administered for up to six days after PG.

  • Method 2: MGA Select

    This program involves feeding MGA for 14 days, followed by a GnRH injection on Day 26, and a PG injection on Day 33. The addition of GnRH to the 14- to 19-day MGA-PG protocol improves synchrony of estrus and maintains high fertility in postpartum beef cows.

    Producers should observe for heat for up to six days after PG administration. Cows may be fixed-time AI'd using this protocol, with AI performed 72 hours after PG. Administer GnRH at the time of AI to all cows inseminated by appointment.

  • Method 3: 7-11 Synch

    This system was designed for two key functions: to shorten the MGA feeding period without compromising fertility, and improve synchrony of estrus by synchronizing development and ovulation of follicles from the first wave of development.

Cattle are fed MGA for seven days, with PG administered on the last day. Four days after MGA withdrawal, cattle are given GnRH, followed by PG seven days later.

Producers should heat detect and AI for up to six additional days from PG administration.

Other considerations

“An additional consideration for Methods 1, 2 and 3 pertains to cows or heifers failing to exhibit estrus after the last PG injection,” Patterson says. “Such cows or heifers can be re-injected with PG 11-14 days after the last PG injection. Then observe them for behavioral estrus signs for an additional 6-7 days.”

The procedure can help maximize efforts to AI as many females as possible in the first two weeks of the breeding period, he says.

Cows inseminated during the first synchronized period shouldn't be re-injected with PG. In addition, the decision to use Methods 2 or 3 in heifers should be based on careful consideration of her age, weight and pubertal status.

This article is provided by the National Association of Animal Breeders. Learn more at www.NAAB-CSS.org, or call 573/445-4406.