Question: I have a cow who had a set of twins, one heifer and one bull calf. The heifer calf is very nice and we do intend to keep her for further breeding. I have heard opinions that heifers coming from a mixed set of twins might not be fertile. What is your knowledge about this? Would you recommend to keep her or to ship her? — Isabelle Ruch, Prince George, B.C.
In response to this question I will discuss one of the most frequent congenital anomalies of cattle: the freemartin. A freemartin is a female born co-twin with a normal male. The freemartin appears like a normal heifer but approximately 92 per cent of such females have such extreme underdevelopment of the genital tract that the uterus and ovaries are absent, and the front portion of the anterior vagina is underdeveloped. The external vulva generally appears normal, but in some cases the clitoris is enlarged and resembles a small penis with a tuft of vulvar hair.
Why do freemartins exist? Normally when a cow comes into estrus (heat) a single follicle is released from one of the ovaries, and if successfully fertilized, the resulting embryo develops into a single fetus. There is a complex hormonal control mechanism involving estradiol released by the developing dominant follicle on the ovary, that generally prevents double ovulations. However, in some cows, double ovulation occurs, and if both oocytes are successfully fertilized, twins result. When twins results from double ovulations, 50 per cent will be a heifer/bull combination, 25 per cent will be a heifer/heifer combination, and 25 per cent will be a bull/bull combination.
It has been shown that certain genetic lines of cattle are more prone to double ovulations and there are some research herds that have selectively bred cows that have a 50 per cent twinning rate. It is also proposed that cows that are bred in early lactation (60 days post-calving) have a higher percentage of double ovulations and hence a higher twinning rate. This is almost certainly related to lower circulating estradiol levels. It should be pointed out that twins can also result from a single fertilized embryo splitting at approximately Day 8 to 14 and resulting in identical twins. One study demonstrated that only six per cent of twins in cattle are identical. Interesting, in my embryo transfer practice, we do occasionally have identical twins born from single embryos transferred into recipients. Identical twin heifers are never freemartins. Studies indicate that the abortion rate for twin pregnancies are above normal, and are especially high if both fetuses are in the same uterine horn.
Why do freemartins develop abnormally? Researchers have shown that the developing fetal membranes of twin fetuses meet in the uterus by about the 20th day of gestation. By day 28 the bloodstreams of the two placentas have united. This occurs well before sex differentiation of the fetuses has occurred, which takes place about 40-50 days of gestation. The genital structures, especially the testes, develop earlier in the male than the female. The interchange of embryonic cells and possibly also hormones secreted by the male reaches the female embryo and inhibits the development of the female ovary and genital tract.
How can you determine if a female twin to a bull is a freemartin? In most cases, the freemartin is diagnosed by a veterinarian on rectal palpation when she is 600 lb. or larger. Physically freemartins may more closely resemble a steer in appearance, and have a very small udder and teats. They do not come in estrus, and may have a fairly normal vulva except for the occasional presence of a prominent clitoris and a tuft of vulvar hair. Some make good “gomer bulls” for heat detection. A blood sample or hair sample can also be submitted from a female twin to a bull at a young age to identify if she is truly a freemartin. There is also a technique using a test tube inserted into the vulva of the heifer calf. The underdeveloped freemartin heiferâ€™s vagina can only be penetrated for three to four inches in a young calf, while a normal young heifer calf it can be passed in five to seven inches.
Although your chances are better than winning the Lotto 6/49, only about one in 12 females twin to a bull will be normal. Also the male co-twins to freemartins are usually considered to be normal and fertile, although a few may have underdeveloped testicles and some may have reduced fertility.
A freemartin is a classical example of a chimera. Chimeras are individuals which contain two or more cell types which originated in separate individuals. This occurs in utero due to the fusion of the two fetal circulations. Freemartinism is rare in other domestic species of animals but it has been reported in sheep, pigs and goats. Blood vessels of the fetal membranes in these species very rarely fuse. It is a good management practice to identify heifers twin to a bull at birth, usually on their ear tag, so that you donâ€™t retain or sell a freemartin as a potential replacement heifer. Their pregnancy rate is dismal and it is not good public relations with your buyer!