Dairy genetics: Trying for more female calves

SALEM, Ohio – Milk makes dairy farmers money. And female cows make milk.

If there was just some way to fiddle with the genetics, maybe farmers could have more milk and more female calves.

Heifers are more valuable, and profitable, to dairy farmers than bulls. Sure, farmers can raise the bull calves and sell them at auction. But without heifers, milking herd numbers drop and farmers have to settle for low-producing cows.

This may not be the case for long.

Preliminary findings from a Michigan State dairy reproduction study say it may be possible to manipulate the gender of calves.

Female follow-up. During a 1998 study at Michigan State, dairy reproduction expert Richard Pursley altered artificial insemination timing to find the best conception rates.

He found the highest rate of conception came when cows were inseminated 12 hours before ovulation.

Another observation, however, was more unexpected: More female calves were born when the sperm sat in the reproductive tract for 12 hours.

Pursley decided to follow it up.

Insemination. Pursley and fellow Michigan State researchers finished a similar study in January with 1,600 cows.

One technician inseminated all the cows using deep horn uterine insemination and body bred insemination at 36 hours and 12 hours before ovulation.

The result? More female calves were born when insemination was 36 hours before ovulation.

The longer the sperm sat in the cows’ reproductive tracts, the more likely they were to have heifer calves, Pursley found.

Conception cost. But those females calves came with a price.

The conception rate dropped.

Conception was 39 percent when cows were bred 12 hours before ovulation. It dropped to 29 percent when the cows were bred 36 hours before ovulation.

The average conception rate is 30 percent to 35 percent, Pursley said.

He hypothesized there is a separation in numbers of X- and Y-bearing sperm over longer periods of time. The more time that goes by, the more likely the Y-bearing sperm will die, he said.

If a X-bearing sperm fertilizes the egg, the calf will be female. If a Y-bearing sperm fertilizes the egg, the calf will be male. This means if the Y-bearing sperm die, it is more likely a X-bearing sperm will fertilize the egg and a female calf will be born.

It may be a tradeoff, he said. The conception rate decreases, but more female calves are born.

Pursley stresses outside effects or farm differences may have influenced the results. He said it is too early to give definitive answers.

Farm results. The experiment was conducted at three farms.

On the first farm, 70 percent of the calves born to the cows bred 36 hours before ovulation were female. Of the calves born to the cows bred 12 hours before ovulation, approximately 54 percent were females.

The second farm had similar results.

Numbers from the third farm weren’t as drastic.

In the 36-hours-before-ovulation group, 49 percent were female. In the second group, when cows were bred 12 hours before ovulation, 40 percent were female.

Normally, Pursley said the average gender ratio is 46 percent female to 54 percent male.

Ovsynch. To get these results, Pursley said farmers must use Ovsynch.

Ovsynch is a series of injections of prostaglandin and gonadotrophin-releasing hormone (GnRH), which stimulate cows to ovulate. It eliminates the need for farmers to detect which cows are in heat.

In normal Ovsynch protocol, cows are inseminated approximately 16 hours after the final GnRH shot.

In Pursley’s study, however, the cows were bred at the same time as the last injection – which is the same as 36 hours before ovulation.

On the flip side. Pursley isn’t the only one who says more research is needed. So does Virginia Tech’s Ray Nebel, another dairy reproduction specialist.

That’s because Pursley’s and Nebel’s findings differ.

Nebel is in the middle of a study where cows were bred at the same time as the last GnRH shot, eight hours later and 24 hours later.

Ninety of the 320 calves are already born, and the numbers of heifers and bull calves are almost identical, Nebel said.

“If it’s as easy as changing when the semen is put in the cow, we would’ve figured this out a long time ago,” he said.

(Reporter Kristy Hebert welcomes reader feedback by phone at 1-800-837-3419, ext. 23, or by e-mail at khebert@farmanddairy.com.)