Less-than-desirable pregnancy rates continue to impede reproductive performance on many dairy operations.

“Over time there’s been a decline in cow fertility,” says Mike Schutz, associate professor of animal sciences and extension dairy specialist at Purdue University. Certainly, many reasons exist for the increase in infertility on dairy farms. However, one of the reasons may be linked to over-emphasis on traits such as milk yield and dairy form — selection for thinner cows — during sire selection.

However, with the release of the February sire summary, you now have a new genetic evaluation — daughter pregnancy rate — to help you improve dwindling pregnancy rates on your dairy operations. Here’s what you need to know about this latest genetic tool and how to decide if you should use it in your breeding program.

A new tool
Last year, geneticists at the USDA’s Animal Improvement Programs Laboratory (AIPL) in Beltsville, Md., and their colleagues at several universities, examined several reproductive traits to determine if it’s possible to select for daughter fertility. The result of their work is the new daughter pregnancy rate evaluation.

Daughter pregnancy rate measures a bull’s effect on his daughters’ fertility at the time when his daughters are bred. The formula used to calculate the evaluation is based on pregnancy rate, or the percentage of cows that become pregnant within a 21-day period.

You won’t find a bull’s Predicted Transmitting Ability (PTA) for daughter pregnancy rate in the Net Merit Index just yet. Because of this, USDA-AIPL refers to the evaluation as preliminary. However, it plans to include sire PTAs for daughter pregnancy rate in future revisions to this index as soon as it determines the proper weight for each trait in the evaluation. Until then, sire PTAs for daughter pregnancy rate will be released separately, says Paul VanRaden, geneticist with USDA-AIPL.

Pay attention to reliability
Compared to other selection traits, daughter pregnancy rate has a low heritability — only 4 percent or so. In addition, daughter pregnancy rate evaluations are not available as early in a bull’s career as yield evaluations, says Duane Norman, geneticist with USDA-AIPL.

Because of these two factors, you should examine the reliability of each sire’s PTA for daughter pregnancy rate when using this evaluation, Norman stresses.

The reliability — how closely the estimated PTA represents the true genetic merit — of daughter pregnancy rate will probably be lower than some of the production traits for active AI bulls, Schutz says. After all, pregnancy rate is less heritable than production. These “bulls will probably only have a reliability around 60 percent at their initial release, similar to the productive life evaluation,” he adds.

For this reason, keep reliability in mind when analyzing sire summary data. For your convenience, the two charts at right show a comparison of the top-25 bulls ranked by daughter pregnancy rate and the top-25 bulls by Net Merit dollars.

Use it judiciously

Because fertility is such an important measure of profitability on dairy operations, you may be tempted to overuse daughter pregnancy rate.

Resist the temptation.

Because of its low heritability, placing too much emphasis on daughter pregnancy rate can be detrimental to the amount of progress you can make with other traits, like milk production.

Instead, “make your decision based on some type of overall selection index like net merit,” Schutz says. Then, use daughter pregnancy rate to decide how much to use bulls you’ve selected from that index.

Think of it as a tie-breaker, adds Jay Mattison, executive vice president of ReQuest Ltd., in Madison, Wis.

So, in other words, select the traits that you feel are important first — like production and type traits, for example — and then use a sire’s PTA for daughter pregnancy rate as a tie- breaker between bulls with similar rankings.

For example, compare the difference in PTA for daughter pregnancy rate of these two bulls with a similar net merit ranking:

Bull    NM$     PTA-daughter pregnancy rate
“A”      473    -3.0
“B”       442   +1.8
Difference       4.8

According to the information, daughters of Bull “A” are expected to have almost a 5 percent higher pregnancy rate than daughters of Bull “B.”

And, for each 1-percent increase in a sire’s PTA for daughter pregnancy rate, days open decreases by approximately four days. So, to convert to days open, multiply the difference in pregnancy rate — 4.8 in our example — by –4. By doing so, you see daughters of Bull “A” are expected to have 19 fewer days open, or almost a full cycle, than those of Bull “B.”

Although both bulls score well on net merit, these calculations show a difference between the two when it comes to their daughters’ fertility, Schutz says.

Doing what you can to overcome reproductive barriers on your operation is vital to the success of your business. If low pregnancy rates have created a reproductive obstacle course on your dairy, this new sire evaluation may be a tool you can use to tweak genetic selection and help your cows overcome the infertility barrier.

Why the need for a fertility evaluation?

The new daughter pregnancy rate evaluation developed by the USDA’s Animal Improvement Programs Laboratory can help you get a better handle on cow fertility.

Here’s why it was developed:

  • Fertility has declined over time. One reason is that fertility is negatively correlated with high milk production and dairy form, says Mike Schutz, associate professor of animal sciences and extension dairy specialist at Purdue University. In other words, as milk production increases, fertility declines. The same holds true for dairy form, or selection for thinner cows. “Thinner cows tend not to breed back as quickly,” Schutz adds.
  • A measure of cow fertility needed. An evaluation of bull fertility — estimated relative conception rate (ERCR) — already exists. “Now we have the cow’s own contribution to fertility,” says Paul VanRaden, geneticist with USDA-AIPL.
  • Economics. Fertility is an economically important trait.