Factors that affect fertility in cows | TheFencePost.com

Factors that affect fertility in cows

“It’s all too easy when selecting heifers to choose the bigger animals, but they may be slower to mature,” said Dr. Steve Hendrick, Coaldale Veterinary Clinic. "Big heifers are often still growing when the smaller ones have already reached puberty."
Photo by Heather Smith Thomas


“It’s important to put selection pressure on your replacement heifers with a short breeding season — just one or two cycles,” Hendrick said. “The ones that don’t breed early are not as fertile.” As some ranchers have discovered, it is easy to select for fertility by giving heifers a very short time with a bull, keeping only the ones that get pregnant — and keeping heifers from older dependable cows that have stayed in the herd a long time. Over time this creates a more fertile cow herd.

There are multiple factors that play a role in cow fertility. Whether a heifer will breed quickly (reaching puberty at a young age) or a cow breeds back quickly after calving depends on a combination of genetics, nutrition, general health, etc. and cows may also be affected by reproductive disease.


Cow-calf producers who make a living from their cows know fertility is the most important economic trait. Studies have shown that reproductive traits are twice as important as growth traits, which are more important than carcass traits. Before you can sell a calf, a cow must first get pregnant. Yet fertility is usually not at the top of the list when producers are selecting replacement heifers.

“Ironically, the beef industry has been selecting almost exclusively for growth and carcass traits, at the expense of reproduction,” said Kit Pharo, a seedstock breeder in Colorado. “For at least 50 years, academics have told us that heritability of fertility is very low — so low that we shouldn’t waste our time selecting for it. If you were able to isolate fertility from everything else, then that assumption might appear to be true. In the real world, however, nothing is isolated.”

Fertility is more a function of fleshing ability than anything else, and fleshing ability is a function of low maintenance requirements. Reproduction cannot take place until maintenance requirements have been met and cows are storing energy reserves as fat.” A really thin cow won’t be very fertile. Since fleshing ability and maintenance requirements are heritable, fertility is also very heritable.

As the majority of beef producers selected for more growth and less back fat, they inadvertently selected for lower fertility. “They created hard-keeping, high-maintenance cows that struggle to reproduce under what was once considered normal ranch conditions,” Pharo said. “The academic solution to this problem was to reduce stocking rates and/or increase supplemental feeding. Instead of producing cows that fit their environment, they artificially changed the environment to fit their cows.” The real solution is to change the focus of genetic selection — looking for more efficient, easy fleshing cows rather than the high-producing cows that often need additional inputs to breed on schedule.


Dr. Steve Hendrick, Coaldale Veterinary Clinic in Coaldale, Alberta, said the heifers and cows that breed quickly and calve early in the breeding season have more chance to rebreed, and tend to stay in the herd longer, but they need adequate nutrition to do it. “Gestation is roughly 285 days and you only have 365 days in a year, so this only leaves 80 days to breed back,” Hendrick said.

If cows are not in good body condition when they calve, it’s difficult for them to rebreed in 80 days. If you have a short breeding season they end up open. They need adequate nutrition to return to heat soon.

“Body condition score is often an indicator of energy and protein levels in the diet but we can’t forget minerals,” Hendrick said. “Some of the work I was involved with at the vet school in Saskatoon looked at mineral feeding. Minerals are important for cows cleaning, and having vigorous calves, but are also important for fertility.”

Certain pastures and certain regions with different soils seem to affect this. We see different minerals in the feed, and ultimately in the cows, and in how they perform reproductively. We see some differences in requirements in some cattle and even in different breeds. One mineral mix or product might work great for one producer but not so well for another. It’s best to work with your own vet and nutritionist to find the right feeding and mineral program for your herd.”

Producers may need to check their feeds, and sometimes their soils. “If I had to do one or the other, I would start with the feed,” Hendrick said. “If feeds are short on certain minerals then I’d check the soils those feeds were grown on.”

In some regions soil is short on copper, or selenium or some other important trace mineral.

“Some of the work we did was looking at different plant species in a pasture, and different types of tame pasture — whether grasses or legumes,” he said. “Mineral profiles in some of these plants will be totally different even when grown in the same soils.” Cattle are adaptable, and will seek out and select plants that help balance their diet, but if there is a serious deficiency it will show up — often in fertility and breed-back/pregnancy rate.

Hendrick said it’s interesting to hear people discuss proper age and size of heifers at breeding, as the two most determining factors on pregnancy rates. “I agree with age as a criteria, but size is a tricky topic,” Hendrick said. “It’s all too easy when selecting heifers to choose the bigger animals, but they may be slower to mature.” Big heifers are often still growing when the smaller ones have already reached puberty.

“When I look at different breeds and cattle types, I am amazed at the wide variation in mature body weight,” he said. “Many people think their cows are 1,200 pounds, but most cows today are much bigger.

“This makes it hard to judge a heifer and determine whether she will mature to be 1,300 pounds or 1,500 pounds. When estimating whether a heifer is between 55% to 65% of her mature body weight at breeding time, you don’t really know. You could look at the dam and have an idea based on her size, but that’s only half the equation. We don’t always know what genes for size the heifer got from her sire,” he said.

Frame score charts can help a person estimate mature frame size, if you know the age of the heifer and can get her weight and hip height. “These might help you guess what her mature size and body weight might be — at least in terms of whether the heifer will be a large frame or smaller frame,” Hendrick said.


Dr. Eduardo R. Cobo, assistant professor, Production Animal Health at the University of Calgary said the two main reproductive diseases that reduce pregnancy rates in cows are trichomoniasis and campylobacteriosis (formerly called vibriosis). “These diseases are often under-diagnosed. Producers may think that vaccination against these disease will prevent the problem, but vaccination is not always efficient for protecting cows against trichomoniasis, and hasn’t been fully tested for campylobateriosis. If a herd has fertility problems and uses natural service for breeding, producers should consider that they might have one or both of these diseases,” he said.

“My former supervisor, Dr. (Robert) BonDurant at University of California-Davis, found that up to 15% of the beef herds in California had at least one bull infected with trich. These diseases are silent and can sneak into a herd,” he said. They may go detected until there are too many cows coming up open or late.

“Trichomonads can be diagnosed with a PCR test or culture,” Cobo said. “The campylobacter is a tricky organism to culture. Most of the time a veterinarian will not take samples to perform a test for campylobacter for diagnosis because it may be impractical. We don’t have a good diagnostic test.”

Trichomoniasis testing has advanced more. “Commercial labs can usually grow trichomonas in a culture, and PCR is also being used,” he said. “Many labs can do PCR tests or cultures or both. We have more tools for diagnosing this disease, or for evaluating whether a vaccine works.”

Both of these diseases kill the embryo or the fetus, usually in the first three months of gestation. The cow is bred and becomes pregnant, but the pregnancy doesn’t last. “It’s more like an early embryo mortality than an abortion; you don’t see the fetus,” he said. This is different than most of the other reproductive diseases like brucellosis, leptospirosis, IBR or BVD.When those diseases kill the fetus it might be several months old and you see the abortion.”

With trich or vibrio, most of the time the loss is so early that the embryo is simply absorbed. You see the cow return to heat and think she didn’t settle. “The producer may think it’s an infertile bull,” Cobo said. “The bull is the one infecting the cow, but the problem is not fertility of the bull.

“The next year you don’t have the pregnancy rate you’d expect. It may drop to 50%, depending on how many animals are infected and age of the bulls. The bull is the main factor because he has the ability to infect many cows, spreading it from an infected cow to all the others he breeds.”

The cows can get infected by an infected bull, but then have an immune response. “After a time the immunity may overcome the infection and the cow will maintain a pregnancy,” he said. “We don’t know how long this might take, and it may vary from cow to cow. The cows that get over the infection will still be susceptible to becoming infected again.

“Studies in the 1990s showed that even though some cows look like they get over the infection, they still have a very low level of trichomonas that is hard to diagnose. Some cows can carry the infection from one year to another, and infect a bull that breeds them the next year. Even though the bull is the main carrier, cows can sometimes act as carriers as well.

“The important thing for producers to understand is that these two diseases can be a silent cause of early pregnancy loss, so there will be more open cows. They may have been pregnant for a few days or weeks, but end up open.”

Bulls should be checked every year before the breeding season for trichomoniasis, and only “clean” bulls should be used for breeding. ❖

— Smith Thomas is a cattle rancher, horseman, freelance writer and book author, ranching with her husband near Salmon, Idaho. She can be reached at hsmiththomas@centurytel.net.