Mighty good advice for your heifers


Source: Select Sires, by Mel DeJarnette, director of technical research, Select Sires Inc.

For any animal breeding operation with progressive genetic goals, the youngest animals in the herd should be the most valuable animals. This is especially true for the dairy or beef producer who is currently using artificial insemination.

Unfortunately, in many dairy operations, the heifers tend to be overlooked and undervalued. Because they’re not producing milk to contribute to the cash flow, they’re often looked at as simply an expense item on the balance sheet; a necessary evil of the operation.

Additionally, due to the limitations in space, heifers are often reared in facilities distant to the milking operation. Here they often receive less than optimal management attention. All too often the job of getting heifers bred is turned over to an unproven herd bull because no one has time to watch them for heat. This creates what I refer to as a “genetic stumbling block.”

I’m sure you’re familiar with the age old saying; “one step forward, then two steps back”. Using A.I. on your cows is taking a genetic step forward. However, allowing an unproven herd bull access to the most valuable animals you own is taking two genetic steps backwards.

Synchronizing your heifers with MGA and prostaglandins and breeding them to proven calving ease sires, can turn your genetic stumbling block into a genetic launching pad.

MGA stands for Melengestrol Acetate, a synthetic form of the naturally occurring hormone, progesterone. Prostaglandin is also a naturally occurring hormone produced by the uterus and commercially available under the trade names Lutalyse and Estrumate.

The MGA-PGF system (Figure 1) works as follows: MGA is fed to all animals at a rate of 0.5mg/head/day for 14 days. The progesterone activity of MGA will block the expression of estrus in the cycling animals during the feeding period. This hormonal stimulation will also induce or “jump start” many of the non- cycling animals. Within three to six days after the last day of MGA feeding, most animals will display a standing heat. DO NOT breed at this heat as conception rates are usually compromised. Wait 17 to 19 days later and inject heifers with a single dose of prostaglandin. Watch for heats for the next five to seven days and breed animals eight to 10 hours after detection of initial standing heat.

The MGA-PGF system has numerous benefits. Compared to using prostaglandin alone, the MGA system results in a higher estrous response rate and higher conception rates (figure 2). That’s right! Higher conception rates! Not only does MGA induce many anestrous heifers to cycle, but the progesterone stimulation during the 14 day feeding period appears to stimulate higher fertility in all animals at the prostaglandin induced heat. That’s something no herd bull can do for you.

Although not close enough to allow for fixed time A.I., the synchrony or “tightness” of estrous response is somewhat closer for the MGA-PGF system compared to using PGF alone. This allows you to focus heat detection labor into a shorter time period.

Traditionally, the recommended interval between MGA feeding and PGF injection has been 17 days. However, more recent research data suggests estrous response and synchrony may be slightly improved with a 19 day interval. This also lends flexibility to the protocol should labor or scheduling problems arise at the last minute.

For optimum success with the MGA-PGF system:

    • Feed heifers to reach at least 65% of their projected mature weight by 14 months of age.


    • If heifers are not accustomed to eating at the feed bunk, start feeding a grain supplement two weeks prior to MGA feeding.


    • MGA can be top dressed, but proper consumption rates are facilitated by thorough mixing of MGA into a grain supplement or a silage based TMR ration.


  • MGA usually comes in the form of a pelleted protein supplement. Read the label to determine the actual concentration of MGA/lb of pellet (usually 1.0 mg/lb) and mix pellets with grain or feed to ensure that each heifer is getting .5 mg of MGA/head/day.


The MGA-PGF system is ideal for many dairy operations because the heifers are already being fed in a feedlot situation. Implementation is basically hassle free.

Although the MGA-PGF system also works great on beef cows and heifers where feeding facilities are available, MGA is not approved for use in lactating dairy cows.

Most large feed suppliers will have access to MGA if they don’t already have it in stock. Should you have trouble locating a source, call your Select Sires sales representative or your member headquarters office and we can readily assist you in locating a supplier.

Although MGA actually stands for a reproductive hormone, I prefer to think of MGA as an abbreviation for “Mighty Good Advice”. In fact, I don’t think I can come up with any better advice for breeding age heifers than MGA-PGF.