There are several factors that play a role in cow fertility and some of them are interrelated. Whether or not a heifer will breed quickly (reaching puberty at a young age) or a cow will breed back in a timely fashion after calving and become pregnant will depend on a combination of genetics, nutrition, health, etc. In cows, fertility may also be affected by reproductive disease.
Cow-calf producers who make a living from their cows know that fertility is by far the most important economic trait. Yet fertility is usually not at the top of the list when they are selecting replacement heifers.
Studies have shown that reproductive traits are twice as important as growth traits, which are twice as important as carcass traits. Yet most beef producers have been selecting almost exclusively for growth and carcass traits for the past 50 years, at the expense of reproduction. Academia has told us heritability of fertility is very low. Perhaps if you were able to isolate fertility from everything else, that assumption could appear to be true. In the real world, however, nothing is isolated.
“Inherent fertility is extremely important but there are other factors that obviously come into play, such as nutrition,” says Dr. Steve Hendrick of the Coaldale Veterinary Clinic in Coaldale, Alta. “If cows or heifers are too thin, they won’t breed. Any female less than 2.5 on the 5-point scale is less likely to breed. There is higher risk for cows to be in anestrus (not cycling) when they are thin,” he says.
Part of the problem in many beef herds today is that stockmen have selected for more growth and less back fat, inadvertently selecting for lower levels of fertility. Their cows may wean off big calves, but they’ve created hard-keeping, high-maintenance cows that struggle to reproduce under what was once considered normal ranch conditions. The all too common “solution” to this problem has been to reduce stocking rates and/or increase supplemental feeding. Instead of producing cows that fit their environment, these producers have artificially changed the environment to fit their cows. Nutrition is important, but you also want to start with a fertile type of cow.
Hendrick says it’s always interesting to hear people discuss the question of 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. It’s all too easy when selecting heifers to choose the bigger animals, but they may be slower to mature.” The big heifers are often still growing when the smaller ones have already reached puberty.
“In my current job, I work with feedlot clients as well as cow-calf producers, and we get carcass data back on these animals. We’ve been estimating what their mature body weight would be. You can do that after slaughter, when you have carcass data. When I look at different breeds and cattle types, I am amazed at the wide variation in mature body weight. A lot of people think their cows are 1,200 pounds, but most cows today are much bigger. Many are actually 1,400 pounds or bigger,” he says.
The average cows in most breeds today are much larger than they used to be. “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 per cent of her mature body weight at breeding time, you don’t really know. You could look at the dam and maybe 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 explains.
There are frame score charts that 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 the mature size and body weight might be — at least in terms of whether the heifer will be a large frame or smaller frame,” says Hendrick.
“It’s also important to put selection pressure on your replacement heifers with a short breeding season — just one or two cycles. The ones that don’t breed early are not as fertile,” he explains. 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.
You may notice a subtle change in body type and size. Fertility is more a function of fleshing ability than anything else, and fleshing ability is a function of low maintenance requirements. Reproduction is a luxury; it can’t happen until maintenance requirements have been met and cows are storing energy reserves in the form of fat. Since fleshing ability and maintenance requirements are heritable, fertility is also heritable.
Cows that are easy-fleshing and easy-keeping tend to be more fertile than cows you have to pour a lot of feed into to get them bred. “Measuring individual animal intakes and feed efficiency is something we don’t give a lot of thought to in the cow herd,” says Hendrick, but there are always some cattle that hold their flesh better than others under the same conditions. Some may fall out of your program if they have to work for a living rather than being fed supplemental feeds. Cows need to be able to maintain themselves and still wean enough pounds of calf, and become pregnant again. If a cow holds her weight but doesn’t produce enough milk for her calf, she’s also not what you want. It’s often best to select heifers from the good old cows that stay in the herd, raise a good calf every year, and never missed a calf.
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 also need adequate nutrition to do this. “When you realize that gestation is roughly 285 days and you only have 365 days in a year — and you want them calving every 12 months — this only leaves 80 days to breed back,” says Hendrick.
“If cows are not in good body condition when they calve, however, it’s difficult for them to rebreed in 80 days. It may stretch to 100 days or longer.” 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 also can’t forget minerals. Some of the work I was involved with at the vet school in Saskatoon was looking at mineral feeding. Most people think in terms of minerals being important for cows cleaning, and having vigorous calves, and minerals do play a role in those things, but also are important for fertility,” he says.
“Certain pastures and certain areas of the country 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 might not work for everyone. 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. If feeds are short on certain minerals then I would check the soils those feeds were grown on,” says Hendrick. In some regions soil is short on copper, selenium, or some other important trace mineral. You may still need to check your soils because they can vary from place to place or from one pasture area to another.
“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. The mineral profiles in some of these plants will be totally different even when grown in the same soils,” he says.
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. “This is often the place it will show up, because cattle try to maintain themselves or grow, and this takes priority over pregnancy. If we shortchange them, this is where it will show,” he says.
Traits like birth weight (for calving ease) and growth (bigger calves to sell) are important. A small calf may be a disappointment, and a dead calf doesn’t gain, but if you never have that calf in the first place — because the cow was not fertile — you are completely out of luck. That’s the most important starting point: you need cows to be fertile enough to breed.
“Another factor we can’t forget is the bull. We often correlate the bull’s scrotal circumference with fertility of his daughters. There’s been a push to select for larger scrotal size, and I think some emphasis on this is good, at least meeting the minimum for the age of the bull. Also the bull needs an acceptable number of normal sperm, and good libido.” A bull that is borderline in fertility may sire daughters that are less fertile than they should be.
Disease can also be a factor in cow fertility. Reproductive diseases like vibrio or trichomoniasis may adversely affect pregnancy rates. Diseases like IBR, BVD, lepto, etc., may cause abortion. “These are problems we may not see until after the breeding season, when cows come up open, or lose their calves during pregnancy. Some of these diseases are real sleepers because you see the cows being bred and think everything is fine, but with early embryonic loss the cows may cycle back again,” says Hendrick.
It is important to work with your local veterinarian to come up with a vaccination program or testing program that makes sense for your own herd. “There’s not a vaccine available for trich, so it’s important to keep this disease out of your herd in the first place or do some testing and culling to get rid of it,” he says.
Other factors that may adversely affect fertility are dystocia or injury/infection in the cow’s reproductive tract.
“If you had to assist a cow or heifer at calving, or deliver the calf by C-section, some of these females will be at higher risk for complications afterward. If you are getting very many calves that are way too big, it might be necessary to try to select for lower birth weights. We do whatever we can to minimize calving problems. The less we have to intervene at calving, the less risk for causing infections in the cow or heifer afterward.” A cow with a uterine infection may or may not clear the infection in time to rebreed on schedule.
The ability to remain in the herd having a calf on time year after year is what we hope for in a cow.
“There’s been a lot of genetic work trying to predict these kinds of traits. Genetics is a big factor, but management also plays a big role. We can’t totally blame it on genetics, but some cows just aren’t as fertile,” he says.
It helps if you can select for genetics that produce lower birth weight and still have good growth after birth. “Fertility and performance don’t have to be completely antagonistic and we can try to come up with a happy medium rather than saying some traits are maternal and some are terminal. With conscientious selection, the herd can do a good job of both.” There are some cattle that are doing this. Short gestation (which translates into lower birth weight) also gives the cow more time to rebreed. If the calf has good growth after birth, you can get the best of both traits. There’s a happy medium, and that can help us in our quest for more fertile cows.
“There are some new tools now available that can help us identify some of these animals that do the best job. Some of the traditional tools are also useful, such as getting your herd preg-checked so you know what the herd fertility is like, and the percent that calve in the first and second cycle of the breeding season, rather than calving late. This can help you know if you are going in the right direction,” says Hendrick.