Cows can and do have twins, but the event is relatively uncommon compared to the high rate of single births. While biologically sound, the occurrence of twins presents distinct challenges for both the cow and the offspring. These multiple births introduce specific biological realities and management considerations not present in a typical single pregnancy. Understanding the dynamics of bovine twinning, particularly the associated risks, is important for anyone involved in cattle care.
Frequency and Types of Bovine Twinning
The frequency of twinning varies significantly depending on the breed, environment, and age of the cow. Beef cattle herds typically have a twinning rate of about 1% to 2% of all pregnancies. Dairy breeds, especially high-yielding Holsteins, can have rates ranging from 4% to nearly 7% in some herds. Older cows on their sixth or later calving also have a higher propensity for twins than those on their first or second lactation.
Most bovine twins (approximately 90% to 95%) are fraternal, known scientifically as dizygotic twins. These twins result from the cow ovulating and fertilizing two separate eggs during the same cycle. They are no more genetically similar than regular full siblings born in different years. Identical, or monozygotic, twins are much rarer, developing when a single fertilized egg splits early in development, accounting for less than 10% of all twin births.
The likelihood of a cow ovulating two eggs is influenced by genetics, nutritional status, and milk production. Cows with higher milk yields often have a faster metabolism, which can lead to reduced progesterone levels and an increased rate of double ovulation. A cow’s body condition and high-energy diets leading up to breeding can further increase the probability of multiple ovulations.
Understanding Freemartinism in Cattle
Freemartinism occurs when a cow gives birth to mixed-sex, fraternal twins (a male and a female). This phenomenon is a serious concern because the female calf, the freemartin, is almost always infertile. The female twin’s reproductive tract is severely underdeveloped due to hormonal exposure in the womb.
This condition is caused by a shared blood supply between the two fetuses, a process called vascular anastomosis, which occurs in the placenta around the 40th day of gestation. Because the male fetus develops sex-specific organs earlier, male cells and hormones (such as testosterone and anti-Müllerian hormone) are transferred to the female twin through the fused blood vessels. These masculinizing molecules disrupt the normal development of the female’s reproductive system.
The result is a genetically female animal that exhibits a partially masculinized reproductive tract. The freemartin heifer often has hypoplastic (underdeveloped) ovaries, a shortened vagina, and a uterus that may be reduced to a cord-like structure. While the male twin is usually unaffected, the female is sterile in about 90% to 97% of mixed-sex twin births.
Early identification of a freemartin is important for herd management soon after birth. A physical examination may reveal a shortened vaginal length or an enlarged clitoris. More definitive diagnosis involves genetic testing to detect the presence of male Y-chromosomes in the heifer’s circulating white blood cells, which is a clear sign of the shared blood exchange.
Care and Management of Twin Pregnancy and Calves
Managing a twin pregnancy requires careful adjustments due to the increased risks for the mother cow. Cows carrying twins have significantly higher chances of complications, including late-term abortion and a greater likelihood of a difficult birth (dystocia). The probability of a retained placenta, where the afterbirth is not expelled in a timely manner, is also much higher following a twin birth, which can lead to uterine infections.
The cow’s nutritional requirements during the final stages of a twin pregnancy are substantially elevated because she is supporting two growing fetuses. Producers often segregate cows identified as carrying twins and provide them with a more nutrient-dense diet to meet the greater energy demands. These cows are often dried off (taken off milking) earlier than usual to ensure they have a longer rest period and are in good nutritional standing before calving.
Twin calves are generally born with lower birth weights than singletons and are at a higher risk of mortality. They require more intensive monitoring immediately after birth, especially in cold or wet conditions. Ensuring that both twin calves receive an adequate amount of colostrum (the antibody-rich first milk) is particularly important for their immune development and overall health.
It is often challenging for a cow to adequately nourish and care for both calves, sometimes resulting in one calf being weaker or rejected. In such cases, intervention from the owner, such as supplemental feeding or cross-fostering one calf to a different mother, may be necessary to ensure the survival and health of both twins. Identifying the pregnancy early through ultrasound allows for proactive management, which can mitigate many associated problems.