Editor’s note: Last in an ultrasound series
Most veterinarians do not make a big effort to identify twins via palpation because it is time consuming and not very accurate. In an older study published in Theriogenology (Callahan and Horstman), cows were palpated at 35–60 days in pregnancy by two experienced clinicians with the specific aim of identifying twins. Even though they knew the purpose of the study was to find twins, their accuracy was still only 65%.
“Ultrasound can be over 95% accurate for twin diagnosis,” says Jill Colloton, DVM, Bovine Services, LLC, Edgar, Wis. “Ultrasound can identify twins as soon as pregnancy can be identified, but it becomes easier after about 35 days.”
Colloton, who offers practitioners bovine reproductive ultrasound training, says with ultrasound it’s usually very easy to identify twins in the same uterine horn because often they will be seen in the same frame. Twins in opposite uterine horns require a more thorough, systematic exam.
Monozygous twins are rare in cattle, so the number and location of corpora lutea (CL) can be used to “cheat,” she explains. “On my first exam at 27-34 days I will note the number and location of CLs on pregnant cows. If I can find twins quickly, I will identify them at that time. Otherwise, I’ll wait until the recheck exam at day 60-67.”
Knowing the location of the CLs helps narrow down the search. It is helpful to record if the fetuses are unilateral or bilateral. Colloton says twins in opposite uterine horns don’t have a much higher rate of loss than single pregnancies (in one study about 8% from initial exam on day 36–42 to recheck at day-90, Lopez-Gatius, Theriogenology, 2005). Twins in the same uterine horn have a much higher rate of loss (32% from first exam on day 36–42 to recheck at 90 days, same study). “It is wise to add an extra recheck exam for unilateral twins sometime prior to dry-off,” Colloton suggests.
Why is twin identification important?
Colloton does not recommend aborting twins in adult cows because lactating dairy cows are hard enough to get pregnant in the first place. Instead, she recommends close attention to body condition score. Cows carrying twins can be left in the high group longer to improve their body condition before drying off.
“Conventional wisdom holds that cows carrying twins should be dried off earlier because they tend to calve 10–14 days early,” she explains. “However, current research on shorter dry periods may indicate there is benefit to leaving twinning cows on a lactating ration longer to improve body score, even if their dry period is shorter.” Of course, cows carrying twins should receive extra attention at calving to avoid dystocia and to improve the survival of the calves.
Twins are rare in heifers because twinning is correlated to milk production. However, when twins are diagnosed in a heifer, abortion may be a good option if the heifer is not too old. “If twins can be diagnosed before 90 days, prostaglandin will usually abort the fetus quickly and the heifer can be re-bred two weeks later,” says Colloton. “I leave this decision up to the client.”
Fetal sexing is not yet an industry standard, but there are beef and dairy producers who are using fetal gender to increase the profit of their operations. Many registered herds are interested in fetal sexing for obvious reasons: knowing they can fill a bull contract, maximizing the use of recipients and being able to sell pregnant animals carrying heifers for more money. But fetal sexing by ultrasound is not just for registered herds anymore.
“A few years ago, one of my large commercial dairy owners came to me and asked me if I could fetal sex,” says Craig DeMuth, DVM, Truxton, N.Y. “I told him I could, but I questioned why in his operation he would want this information. He looked me in the eye and said ‘If you are accurate with your information, I will figure out how to use it’. I realized that if veterinarians cannot provide these services, their clients will find someone who can.”
Colloton also had commercial clients start to demand the service. “The reason they wanted it was because their biggest use of the data is cull decisions on marginal or sick cows,” she explains. “They also use it for Johne’s control, herd inventory planning and for deciding who gets the calving pen.”
How it’s done
For most people, fetal sexing is the most difficult reproductive ultrasound procedure to learn. However, learning to fetal sex is not rocket science, according to DeMuth, and he has some suggestions to shorten the learning curve. The first is to find a teacher and a short course that can teach you the basics of fetal sexing; a course of this content will dramatically shorten learning time.
“Do not try and become self taught,” DeMuth advises. “Do not guess. And do not take lessons from someone who is not capable of teaching you. I would not take golf lessons from someone with a swing worse than mine.”
Colloton says it takes 300-500 cows before most people feel confident of their fetal-sexing skills. There are two critical components to accurate fetal sexing: having a quick eye and confidence in what you’re seeing, and ability to properly position the probe. Most veterinarians can do the latter but have more trouble with the former.
After taking a short course, there are training CD’s that will help you become proficient at fetal sexing. Brad Stroud, DVM, has a DVD, Bovine Fetal Sexing Unedited, that is a terrific training tool for the eye, says Colloton. It consists of 52 real-time, real-life fetal sexing cases in quiz form. It’s available through Colloton’s website at www.bovineservices.com.
For greatest accuracy in fetal sexing DeMuth has a time window of 60–80 days in gestation when the fetus is well developed and easy to reach. After that, the fetus may be over the brim of the pelvis and difficult to scan. The lower limit for fetal sexing accuracy is generally pregnant 54 days. Prior to this the genital tubercle may be visualized, but it has not fully migrated to its final position below the tail or behind the umbilicus.
In general, fetal sexing can be attempted 60–100 days in dairy cows and 60–120 days in beef cows. “For the fastest and most accurate exam, I would recommend a window of 60–80 days,” says DeMuth. “If you have clients interested in your service, inform them that fetal sexing is most accurate at this time. Do not torment yourself by starting with a bunch of ‘two-ton Tillie’s’ carrying 4-month-old calves.”
Colloton notes that it’s not any easier to scan later pregnancies in heifers than in cows. “Although the fetus may not be as deep in heifers, the rectal wall is thinner so the probe position can’t be as easily manipulated.”