After a cow is bred, she should calve about 283 days later. But sometimes the pregnancy is terminated early and when you go hunting for a reason you’ll discover they are many causes for a lost pregnancy.
Most of the time when there’s a poor pregnancy rate in a herd we suspect infectious causes like trichomoniasis, campylobacteriosis (“vibrio”), lepto, IBR or BVD, but there may be non-infectious causes. Pregnancy may be lost within the first week after conception — or at any stage of gestation.
Late pregnancy loss is visible; you may find the aborted fetus or a cow with placental membranes hanging from the vulva, but with early pregnancy loss you may have no indication, says Dr. Barry Blakley, a toxicologist at Western College of Veterinary Medicine, University of Saskatchewan. And if cows are pregnancy-tested early, a few that were determined pregnant will lose their fetus before calving time.
Blakley says there are four basic causes of loss: genetics, infections, toxins and nutritional deficiencies.
Losses due to genetic causes are nature’s way of weeding out defective offspring that cannot survive. These include defects in DNA that terminate the conceptus very early on, as well as some abortions and stillbirths.
Stress is another factor that is often overlooked. There are many kinds of stress, including nutritional deficiencies, environmental stress (heat, cold, bad weather), systemic disease, etc., that make conditions less than optimum for continuing the pregnancy. Reproduction is a luxury; if the cow is in poor body condition, ill or otherwise stressed, she will either not become pregnant, or may lose the calf, says Blakley. She must first be healthy herself to have a healthy pregnancy.
Dr. Ahmed Tibary, professor of theriogenology, department of veterinary clinical sciences, Washington State University, says pregnancy loss should be considered a possibility whenever there’s a longer-than-average calving season, or a higher-than-usual number of late calvers. Cows that settled late may have bred early but lost their pregnancies and rebred on a later cycle.
Early pregnancy loss
Causes of early loss include alterations in the egg itself, and in the fertilizing semen. “Researchers are looking at possible defects in semen and the effect of genetics on quality of the embryo. In some situations the bull-cow combination (genetically) may lead to increased early embryonic loss,” he says.
“About five to eight per cent of pregnancy losses in beef cattle (of pregnancies that continue beyond 21 days) are lost in the first 42 days. Rapid weight loss in the cow, or other stress such as transportation in early pregnancy can lead to losses. The biggest factors include sudden change in nutrition, environmental stresses like severe cold, or heat. Stressful conditions may cause hormonal disturbances. Moving cattle, particularly in the first 42 days of pregnancy, can be a factor, especially if it involves a lot of stress,” Tibary says. A long truck haul, gathering cattle swiftly to get them away from an approaching wildfire, or cattle being continually harassed by wolves are examples of stress that may lead to pregnancy loss.
Toxic causes of pregnancy loss
Some toxicants affect the fetus — including lupine, hemlock, locoweed, pine needles, ergot alkaloids, certain moulds, etc. For instance, pine needle abortion causes loss in late pregnancy when cows consume Ponderosa pine needles. The greatest risk is when cows consume pine needles for three days or more, and have a high level of toxicant — which constricts blood vessels in the placenta, hindering supply of oxygen and nutrients to the fetus, killing it. The toxicant also increases uterine contractions that expel the fetus.
“Mycotoxins are common causes of pregnancy loss,” says Blakley. “These are metabolites produced by fungi. Zearalenone is one that produces estrogenic effects and infertility,” he says.
“T-2 toxin, dioxynivalenol (also known as vomitoxin or DON), etc., affect rapidly dividing cells — the embryo or fetus — causing embryonic death or abortion. There are at least half a dozen mycotoxins that affect rapidly dividing cells or affect hormones in the animal. They may cause abortions or deformities, or failure of the conceptus to implant in the uterus,” explains Blakley.
Another toxic concern is ergot. “This mycotoxin is a mixture of several alkaloids that can have adverse effects on the animal. In Canada we are seeing more of this problem with changing growth conditions, moisture, temperature and weather,” he says.
“Some of these problems are increasing because of the way farmers manage their crops. No-till farming is becoming more widespread, and if farmers don’t till the fields, the fungus stays on top and spreads. If surface material is tilled into the ground, then the bugs and microbes eat the fungi and get rid of it,” says Blakley.
“Harmful fungi and mycotoxins can also develop in moist areas like ditches and then spread into the grain fields and affect various grasses. If farmers don’t till their fields properly or don’t till them at all, the fields are more heavily contaminated and then the crops are more contaminated,” he says.
“Ergot doesn’t affect the fetus in terms of deformities, but can affect blood supply to the uterus (which could lead to death of the fetus) and also causes band contractions on the uterus, which might expel the fetus.”
“Consumption of certain metals like lead can cause problems including abortions. In the area where we are, there are many oil industry chemicals, and some of these can cause abortion due to organ damage. Certain hormones build up or don’t build up, because of this damage, or may be altered by impaired metabolism, or it may allow for buildup of other toxic chemicals which would otherwise be eliminated by the body.” If the liver isn’t functioning properly it can’t break down and eliminate toxins.
“And in some cases these chemicals actually cause the liver to break them down more quickly and this causes lower levels of certain hormones like progesterone. Therefore the cow can’t maintain the pregnancy,” he says.
“Occasionally certain moulds enter the bloodstream and cause what we call a mycotic abortion. Cattle may also inhale spores from mould and get a fungal pneumonia — which stresses the animal,” he says.
Some plants contain nitrates under certain conditions, and nitrate poisoning can cause fetal death. “Another concern is fertilizers that contain nitrates. Cattle may eat the fertilizer or drink contaminated water. There may not be enough to kill the cow, but enough to affect the fetus. The fetus is much more susceptible to nitrate poisoning than the cow,” Blakley explains.
“Some plants accumulate cyanide which can cause fetal deformities and abortion —though often it kills the cow. Wilted/frosted cherry or chokecherry leaves cause a lot of problems. In Canada we also have a problem with arrowgrass,” he says.
Problems can also be caused by plants that accumulate selenium (since too much selenium is toxic), or plants that contain harmful alkaloids. “The alkaloids cause liver damage, and then the cow can’t maintain the pregnancy,” he says.
“A major concern in certain areas of the world is poor-quality water. It may have high levels of sulphate, which interferes with absorption of copper, etc.” This creates a deficiency in the animal even if soils and feeds contain adequate amounts of copper.
“Certain parts of North America are deficient in copper and/or selenium,” says Blakley. “If the animals aren’t getting enough of those important trace minerals they will have reproductive problems. I’ve seen some herds where the reproduction was poor, and after supplementation with copper the reproduction rate returned to normal,” he says.
“Our No. 1 problem in Canada in winter is associated with cattle eating poor-quality feed. The vitamins that were at high levels in forage during summer — particularly vitamins A and E — drop dramatically after feed is stored for a long time during winter. These vitamins are necessary for placental and fetal development. We encounter many vitamin A and E deficiencies in stored feeds. In milder climates this isn’t a problem because there’s green feed nearly year-round. But in our region, starting in October there is almost no green feed available and it may snow under; cattle must eat cut forage,” says Blakley.
“Properly feeding your animals is the best way to prevent pregnancy loss in most situations. Certain genetic problems might be an issue in purebred herds, but under normal conditions with crossbred cattle these would not occur because the defective genes would not be expressed,” he says. Most of these defects have to be inherited from each parent.
Loss due to twins
When a cow conceives twins, there may be twin reduction in early gestation. If a cow carries a twin in each horn of the uterus, the risk for loss is less than when twins are carried in the same horn. Sometimes the cow loses both, and sometimes just one. The producer may find only one calf, but if it’s a heifer freemartin, you know there had to be a twin bull calf present earlier.