Oklahoma State University recommends cow-calf producers write down calf numbers of twin births while the animals are still nursing the cow, and to never retain a heifer as a replacement female is she is born as a twin to a bull.
Estimates of the percentage of beef cattle births that produce twins vary. One of the more famous examples – reported in Hoard’s Dairyman, 1993 – puts the percentage at about 0.5 percent 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, OSU Cooperative Extension emeritus animal scientist and managing editor of the university’s popular Cow-Calf Corner newsletter.
“Freemartinism is recognized as one of the most severe forms of sexual abnormality among cattle,” he said. “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.
“Although the male twin in this case is rarely affected by reduced fertility, the female twin is completely infertile in more than 90 percent of cases,” Selk said.
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.
Laurie Ann Lyon in “The Causes and Effects of Freemartinism” indicated producers can predict the reproductive value of an applicable heifer calf at birth and save the feed and development costs if they are aware of the high probability of freemartinism.
“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 percent more energy intake to maintain body condition.
“Be aware the additional suckling pressure on the cow will extend the post-calving anestrus period,” Selk said. “Therefore, cows nursing twins will take longer to rebreed for next year’s calf crop.”
In some cases, Selk said 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.
Oklahoma is the nation’s fifth-leading producer of cattle and calves, according to USDA National Agricultural Statistics Service data.