Published on Thu, 03/17/2022 - 5:24pm
By Heather Smith Thomas.
Twins can be a challenge during birth and a common cause of dystocia, especially if they are both trying to enter the birth canal at once. If they come one after the other they can be born easier, but occasionally one twin may be backward and won’t survive unless you are there to help.
Another problem with twins is when one is a bull and the other a heifer. Often the heifer is a freemartin—a female with improperly developed reproductive tract, incapable of becoming pregnant and having a calf. When selecting heifer calves to go back into the dairy herd, it’s always wise to make sure none of them are freemartins.
Dr. Scott Poock (Associate Extension Professor, University of Missouri), says freemartins are more common in dairy cattle than beef cattle, partly because dairy cows tend to have more twins. “Some people think that high-producing dairy cows would have fewer twins because they are supposedly less fertile (putting so much energy into producing milk). But we’ve found that they actually have more incidence of twins. The high-producing cows are metabolically different, with higher metabolism rates. They therefore metabolize more of the reproductive hormones, which leads to a higher number of multiple ovulations,” he explains.
Thus a high-producing dairy cow is more likely to have two dominant follicles. If there are two eggs there is even more chance of having a male and female set of twins. Identical twins occur when one egg splits and becomes two embryos.
Twins often run in families as a genetic tendency, as well. Certain cows and family lines may have multiple incidence of twins. “So it can be partly due to genetics and also to production levels, with multiple eggs released—and more chance of male/female combinations,” says Poock.
In this situation, the males tend to be normal, but the female embryo is impacted by a male hormone called anti-mullerian hormone (AMH). “This hormone in the male helps in development of the male reproductive tract, but the female is influenced adversely by it and her reproductive tract is not fully developed.”
Dr. Colin Palmer (University of Saskatchewan) says a freemartin heifer is a result of shared exposure—hormones from the male and female calf--in the uterus. “This occurs very early in the pregnancy, around 30 days of gestation. At that time the two placentas that surround and support the male and female calf can become closely connected or fused which allows an exchange of cells and hormones. About that same time some of the organs are starting to develop, including the determination of sex. This becomes complete at about 6 weeks of pregnancy, and after that we call it a fetus rather than an embryo,” he says.
“The fusion of placentas and sharing of blood supplies allows some exchange of cells and hormones and this is why there can be variability in how the sex organ develop. There are basically two reproductive systems developing in the embryo/fetus – the male and the female system. For the most part, development of the male reproductive system dominates during early development.
Through the exchange of cells as well as hormones like testosterone, the male chromosome complement (XY) also becomes present in the female calf (XX) causing the development of a freemartin heifer with a mixed XX/XY chromosome also known as a chimera,” Palmer explains.
The abnormality in the chromosomes, the mix of male and female chromosomes, is one reason a freemartin heifer often does not look feminine. As she gets past weaning age and starts to develop, she generally looks more masculine—more thick and solid, like a young bull or steer.
“These heifers sometimes have masculine behavior as well, and more likely to mount other cattle. They don’t have a penis, however, and can’t breed, but in their tiny ovaries they might have what almost looks like testicular tissue,” says Palmer. They are probably producing more male hormones than a normal heifer, and the extra testosterone leads to the bullish behavior.
The reproductive tract in these heifers is not normal, even externally. “There is generally shorter distance between the anus and the vulva. In a normal yearling heifer there’s usually about 3 inches between the top of the vulva and the anus, and in a freemartin these are closer together. The vulva is very tiny and the bottom part is often tipped up, with a large tuft of hair at that location,” says Palmer.
Poock says that most of the time when he palpate freemartins, they have small, rudimentary reproductive organs and it’s often very difficult to feel any ovarian or uterine structures. “If I do find anything, it is generally much smaller than normal. Often, especially in Holsteins, these heifers look more like steers, and their pelvises tend to be narrow. The male pelvis is not as big as the female pelvis, and this is also true of a freemartin. If a heifer looks masculine, you might suspect she is a freemartin.”
In a high-producing dairy herd you might get 6 to 8% twin pregnancies whereas in beef herds it may be below 1%. “In my experience with dairy herds, only about 5% or less of the female twins born with a bull twin were normal. In my 34 years of being a veterinarian which include 19 years of practice in Wisconsin I only remember 2 that were normal, out of many twin heifers,” Poock says.
“One that was normal was from a black Holstein cow owned by a client. The cow had twins—a black bull calf and a red heifer calf. The heifer was a beautiful dark red, and the dairyman’s wife loved it. The dairyman decided to keep it around as a pet for his wife but assumed the heifer would not be fertile. At a year of age, however, the heifer started coming in heat. She was bred, and calved normally. Eventually, most of the red animals in their herd descended from that cow. She ended up being a good cow but only because the wife liked the heifer and she didn’t get sold.” Sometimes it’s worth having a heifer checked, but most people just assume it’s a freemartin and sell it.
“On some dairies I worked with, the replacement heifers would all be lined up in the headlocks and I’d be going down the line palpating and checking them for pregnancy. If I came to a freemartin my first thought when I stuck my hand in to palpate would be that this is the bull! The pelvis is narrow and there may be no ovaries or uterus. But then I look closer at the back end and realize it is a female, but a freemartin,” Poock says.
In species that have twins routinely, like sheep and goats, freemartins are rare. Those species normally have twins and even triplets, and the female twins are normal. “It would be more common for them to have an actual hermaphrodite (animal with both male and female reproductive organs) than a freemartin,” he says.
“In the bovine freemartin the uterine horns may be tiny or practically non-existent, rarely bigger than a pencil. This animal could never reproduce,” says Poock.
Occasionally a heifer calf might be born as a single but she was actually twin to a bull calf that died early during gestation--but not before its hormones affected the female twin. “The damage is done so early that the heifer could be a freemartin even though you never saw evidence of another twin.”
You wouldn’t realize she’s a twin, unless twins were noticed earlier at preg-check time. “If preg-check is done with ultrasound, twins would be detected. In the dairy world we often do an early ultrasound, and then another one in another month or two to either determine fetal sex or to reconfirm that the pregnancy is still there. You might find twins at 32 days and then do the next ultrasound at 70 days and find that the cow only has one fetus. It might be a heifer, but you don’t know if the one she lost was a bull and might have damaged the heifer,” he explains.
“Anyone who buys young dairy heifers needs to be aware of the possibility that some might be freemartins,” says Poock. It pays to check, and know if a heifer is a freemartin before you spend time and money growing her up to be a replacement heifer.
Testing To See If She’s A Freemartin
There are ways to check a heifer to determine whether she is normal or not. “In earlier times, before DNA testing, we would wait until the heifer was a little older and bigger, and just use a tube to probe the vagina and see if it was normal or short in comparison with normal herdmates of a similar age,” Palmer says. Having a veterinarian palpate the animal to determine if the reproductive tract is normal is another way to determine if an animal is a freemartin.
Today it’s even easier to determine if the heifer is normal or not, using a DNA test. “The freemartin is a type of chimera—which in this case means she has both XX (female chromosome makeup) as well as XY chromosome makeup in her blood and gonadal cells. Chimeric conditions vary – not every cell type in the body will express the chimeric genotype. If you contact a lab that does the DNA testing and want them to check to see if a heifer is a freemartin, the lab will likely ask for a whole-blood sample,” says Palmer. A hair sample may not be adequate for this purpose.
If it’s a nice heifer, you might want to check to see if she is normal or not, and possibly keep her. “Up to 15% of co-twin heifers are normal and if she is normal, there is no reason not to keep her as a cow,” he says.