It has long been known that getting your heifers bred early in the breeding season sets them up for a lifetime of productivity. And aficionados of artificial insemination (AI) have long had the tools to synchronize the estrus cycle and accomplish that goal.

However, most cow-calf producers, for a variety of very good reasons, believe AI simply doesn't fit their program. But there's a component of the estrus synchronization mantra that can work in commercial operations and pay dividends for the productive life of your first-calf heifers. And the best thing, says David Patterson, University of Missouri Extension beef reproduction specialist, is that it's fairly simple.

Here's the protocol: feed half a milligram (mg) of MGA (melengestrol acetate) to your heifers for 14 days. Wait 10 days after you stop feeding the MGA and turn the bulls out.

Simple, right? Well, yes and no. There are some considerations, Patterson says, but overall, he thinks the protocol is an excellent way for commercial cow-calf producers to get their breeding program shaped up.

Settle 'em early

Getting your heifers settled in the first 21 days of the breeding season not only increases the efficiency of your breeding program, but of your entire herd.

“Heifers that conceive earliest in their first breeding season will remain in the herd much longer than heifers that conceive either in the mid or late part of the breeding season,” Patterson says. The reason is that by calving early, they've got a longer period to recover going into their second breeding season. “Heifers that conceive as yearlings during the mid to late portion of their breeding season are at a much higher risk of falling out of the herd either the second year or subsequent years because they start out behind.”

The economics are clear. Not only are your costs higher per animal the first few years of a heifer's life as she earns her place in the cowherd, but calves born earlier in the calving season are older at weaning and thus, weigh more.

How it works

MGA is a progestin that is mixed in the feed and suppresses estrus. “The key is adequate intake on a daily basis,” Patterson says. He recommends feeding MGA once in the morning and that cattle consume 3-5 lbs./day of the carrier feed. That's so they don't fill up on bulky feed, leave the feeding area and return later. “If that's the case, you're probably going to have cattle that aren't getting the required 0.5 mg/day on a daily basis.”

That's why he recommends against using the carrier feed as a top dress over silage or hay. “It will filter down through the silage and they won't get that required half milligram.”

Many commercial feed companies have products available that contain MGA, he says. In the West, where range cubes are common, feed companies have “cake” formulations that contain MGA. “That's how a lot of folks are getting MGA to heifers on pasture,” Patterson says, when feed troughs aren't available. If you're introducing a new feed to cattle as a way to provide MGA, Patterson suggests a warm-up period of a week or so where you feed the ration without the MGA so the heifers get used to the new feed.

Early heat

When MGA is withheld following the 14-day feeding period, heifers will often exhibit estrus about two days later, continuing for 6-7 days. This is a sub-fertile estrus, he says. Waiting 10 days allows ample time for the heifers to experience the first sub-fertile heat, then begin cycling again.

And the practice has positive benefit for all heifers, whether they're cycling or not. “What will occur is a high percentage of the heifers that will not have reached puberty by the time they went on MGA will be jump-started,” Patterson says, and will begin cycling following withdrawal. They'll experience the short-cycle, sub-fertile heat, then begin cycling normally. “Realistically, they could come back into heat somewhere around day 24 (from the first day of being fed MGA). That will be a normal heat.”

When heifers do begin their fertile cycle, estrus is spread out over seven to 10 days. “And that's advantageous from the bull's standpoint.” However, you likely will need a few more bulls than the standard bull-to-cow ratios suggest.

“Especially if you're using yearling bulls, the maximum number of heifers you probably want to expose to a single bull would be in the range of 10-15,” Patterson says. For more experienced bulls, he says 1:15 or 1:20 is recommended, with a max of 1:25.

“Yearling bulls can, quite honestly, be a problem in this situation just from the fact that they really haven't figured things out yet in some cases,” Patterson says. “Often they'll pair up with one or two heifers and ignore the others.”

Heifers only

Because MGA isn't labeled for use in mature cows, the protocol should only be used on virgin heifers. However, Patterson says synchronizing cows every year has a cumulative effect. “Essentially what occurs is those cows will back themselves up and you'll have more cows calving earlier and earlier each year. The reproductive management implications relative to a cow herd can be significant just because of that alone.”

There are several protocols for synchronizing cows, but Patterson says a CIDR, which is a vaginal insert containing progesterone, is a workable alternative. While not as simple as MGA because it requires several trips through the chute to apply and then remove, it might be a consideration for some producers.

Synchronization should not be viewed just as a tool to facilitate AI, Patterson says. “In my opinion, a stronger reason to use it is to enhance what you're doing in the reproductive management of your herd. And we all know, from a cow-calf standpoint, you can't sell anything from a cow that's not pregnant. But the second part of that is (the importance of) when she becomes pregnant. It helps that whole series of events.”

Breeding Tip

MGA protocol

  1. Feed half a milligram of MGA (melengestrol acetate) to your heifers for 14 days.

  2. Wait 10 days after you stop feeding the MGA and turn the bulls out.

Feeding Tip

MGA consumption

  1. Feed MGA once in the morning with a carrier feed.

  2. Cattle should consume 3-5 lbs. of carrier feed.

  3. Don't use carrier feed as a top-dress over silage or hay.

  4. Warm cattle up to the carrier feed a week before feeding MGA.