After a cow is bred, she should calve about 283 days later. However, sometimes pregnancy terminates early.
Ahmed Tibary, professor of theriogenology in the Department of Veterinary Clinical Sciences at Washington State University (WSU), says a longer-than-average calving season, or higher-than-usual number of late-calving cows may indicate some cows lost their pregnancies and rebred on a later cycle.
Between 40 to 56 percent of pregnancies are lost between fertilization and term, and 70 to 80 percent of these losses occur in the first month. “Usually we suspect infectious causes, but there may be non-infectious causes as well,” Tibary said.
With late pregnancy loss, you may find the aborted fetus or see the cow with placental membranes hanging from the vulva.
However, “Early pregnancy loss goes unnoticed and you don’t have a clue,” he said. Even if cows are preg-tested early or at mid-gestation, a few of those will lose the fetus between that time and when they would have calved.
“Heat stress effects can be long-lasting, because changes in the eggs can last a long time.”
Early loss usually happens so soon that the cow returns to estrus on schedule, just as though she wasn’t bred. Most embryos that are lost die before the eighth day, before they travel through the fallopian tube into the uterus.
“These embryos are already degenerating. There is also substantial loss before day 14. These losses will not affect the cow’s return to heat in her next cycle,” he said.
The egg may not have been good, and another factor could be the semen. “Researchers are looking at possible defects in semen and effect of genetics on quality of the embryo. In some situations the bull/cow combination [genetically] may lead to increased early embryonic loss,” Tibary said.
Cows that have not been cycling normally yet—and bred on their first heat after calving—may have less chance for normal pregnancy. If they don’t settle, they may return to heat about eight to 10 days later. “These cows may not have enough progesterone to maintain pregnancy; they come back into heat and their cycles are more normal after that,” he explains.
In heifers, this is why we shouldn’t try to breed them before they have sufficiently developed. “They will be more fertile, more able to maintain a pregnancy, once they have established normal cycles,” he said.
Pregnancy loss factors
Other factors may play a role in certain situations. Losses from heat stress may occur in the first week or so after breeding. Hot weather and humidity can also have a long-term effect on quality of the eggs, resulting in poor-quality embryos.
“Heat stress effects can be long-lasting, because changes in the eggs can last a long time. Even when temperature goes back to normal, the cow still has eggs in her ovaries that are poor quality, that won’t produce good embryos,” explained Tibary. It might take several cycles before she actually carries a pregnancy.
“Dr. Tom Spencer, professor in the Animal Science Department here at WSU, has done research on fetal/maternal interaction, and cows that are likely to lose pregnancies,” Tibary said. “They don’t have proper dialog between the embryo and endometrium of the uterus, after fertilization.” The uterus does not recognize the embryo.
“Other losses are related to heat synchronization. Some synchronization schemes need to be carefully followed to know whether some cows get pregnant, but lose it early,” said Tibary.
Most losses occur before the embryo attaches to the uterus, which means the heat cycle will not be altered. “We just see cows coming back into heat again, and don’t know whether it was the fault of the cow or the bull,” he said.
A smaller proportion (about 5 to 8 percent of all pregnancy losses in beef cattle) occur between 21 and 42 days of pregnancy, as the embryo is transitioning into a fetus.
Additional factors
Many factors have been linked to increased embryo loss, including nutrition, especially trace mineral deficiencies. “The trace minerals that affect reproduction [with deficiency possibly contributing to pregnancy loss] are selenium, copper and manganese,” Tibary said.
Poor preparation of heifers for breeding, or cows losing a lot of body condition just before or immediately after being bred can affect the hormonal profile.
Rapid weight loss, or some other extreme stress such as transport in early pregnancy, can lead to losses. There are also risks from toxic plants. Tall fescue is often responsible for early pregnancy loss, and fescue toxicity also hinders ability to thermo-regulate, and cattle suffer more heat stress—creating a double problem during breeding season. Sometimes feed is contaminated with mycotoxins, mold, or estrogen-like substances that may interfere with pregnancy.
[inline_image file=”4a4f0a1d41e5978c4e9cca43e35fd2a9.jpg” caption=”Non-infectious pregnancy loss in cows • table”]
When cows calve later or turn up open, was it due to not cycling soon enough (a nutritional problem) or did they get bred and lose the pregnancy? Was the loss due to infection, or to something like heat and humidity or some other stress?
Research is looking at genetic factors in early embryonic loss—from both the sire and dam side. “Errors in chromosomes on the sire side can lead to faulty embryos and higher incidence of early embryonic death. Sometimes one bull’s semen creates embryos that are not viable; the producer may have to change bulls to get better results,” Tibary said.
“On the female side, the uterus of some cows has less ability to interact properly with the embryo. Research is looking at heifers that have been identified as not being able to maintain pregnancy for this reason. We’re looking to see if there are certain genes that are not expressed in these cattle—which would not allow them to dialog with the embryo.” Heifers with this problem have high likelihood of failing to maintain a pregnancy their entire life. This is a good reason to cull any heifer that does not settle in her first breeding season.
“We generally discover a problem like this at pregnancy diagnosis. When a producer experiences a poor pregnancy or calving rate [or spread out calving season] I recommend monitoring the next breeding season to see if there is breeding activity and if females are returning to estrus after being bred,” he said.
“In one herd, we checked cows halfway through breeding season, to see how many actually bred in the first cycle. We give the herd a 21-day breeding period, then look at them about 30 days after that first cycle to see how many were bred—using ultrasound, or a blood test to check for Pregnancy Specific Protein B (PSPB),” said Tibary.
“Hopefully all the cows would have been covered if they are cycling normally. Then 28 days from that last day (39 days from when we put the bull in), we take blood samples and see how many are pregnant to that first cycle. This blood test works well on heifers, but cows must be more than 90 days post-calving at the time of testing. If they are tested too soon after calving, there may be some PSPB still in their bloodstream from the previous pregnancy.”
“Errors in chromosomes on the sire side can lead to faulty embryos and higher incidence of early embryonic death.”
To determine the cause of a problem, look at herd records and history, to figure out what has changed in nutrition or other factors; looking at body condition of cows, pasture conditions, examining the bulls, etc.
A few other causes of pregnancy loss involve human errors, such as use of certain drugs that might cause abortion. Treating a pregnant cow with dexamethasone or some other steroidal anti-inflammatory product may cause her to lose the pregnancy, especially during the last trimester. — Heather Smith Thomas, WLJ correspondent
