By Donald Stotts

Freemartin heifers, those resulting from a twin birth where the other calf is a bull, have long been known to be infertile. So, if you get a set of twins this spring, remember to write down twin births of calves while they are still nursing the cow.

Estimates of the percentage of beef cattle births that produce twins vary. One of the more famous examples – reported in Hoard’s Dairyman in 1993 – puts the percentage at about 0.5% or one in every 227 births. Research indicates approximately half of the sets of twins should contain both a bull and a heifer calf.

Cow-calf producers should make certain they do not retain the heifer born twin to a bull as a replacement female, warns Glenn Selk, Oklahoma State University Cooperative Extension emeritus animal scientist.

“Freemartinism is recognized as one of the most severe forms of sexual abnormality among cattle,” he says. “This condition causes infertility in most of the female cattle born twin to a male. When a heifer twin shares the uterus with a bull fetus, they also share the placental membranes connecting the fetuses with the dam.”

A joining of the placental membranes occurs at about day 40 of pregnancy; thereafter, the fluids of the two fetuses are mixed. This causes exchange of blood and antigens carrying characteristics unique to each heifer and bull. When these antigens mix, they affect each other in a way that causes each to develop with some characteristics of the other sex.

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“Although the male twin in this case is rarely affected by reduced fertility, the female twin is completely infertile in more than 90% of cases,” Selk says.

The transfer of hormones or cells can cause the female twin’s reproductive tract to be severely underdeveloped, and sometimes even contain some elements of the bull calf’s reproductive tract. In short, a freemartin is genetically female but has many characteristics of a male.

Selk explains the ovaries of a freemartin do not develop correctly and typically remain small, sometimes very much so. In addition, the ovaries of a freemartin do not produce the hormones necessary to induce the behavioral signs of heat.

“The external vulvar region can range from a very normal-looking female to a female that appears to be male,” he said. “Typically the vulva is normal except that in some animals an enlarged clitoris and large tufts of vulvar hair exist.”

Freemartinism cannot be prevented. However, it can be diagnosed in a number of ways ranging from simple examination of the placental membranes to chromosomal evaluation.

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Beware the hidden freemartin

“Producers need to be aware that in some cases there are few if any symptoms of freemartinism because the male twin may have been aborted at an earlier stage of gestation,” Selk said. “These so-called hidden freemartins can be difficult to identify if purchased as replacement heifers.”

This is a major reason why OSU Division of Agricultural Sciences and Natural Resources animal scientists and OSU Cooperative Extension agricultural educators recommend producers cull any non-pregnant replacement heifers soon after their first breeding season.

Another reason is that female cattle nursing twin calves will require an estimated 13% more energy intake to maintain body condition.

“Be aware the additional suckling pressure on the cow will extend the post-calving anestrus period,” Selk says. “Therefore, cows nursing twins will take longer to rebreed for next year’s calf crop.”

In some cases, Selk says producers may want to consider early weaning of twin calves to allow the birth mother to cycle in time with the other cows in the herd. Otherwise, the producer may be headed for management headaches during the next breeding season.

Stotts is a communication specialist with the Oklahoma State University Division of Agricultural Sciences.