Early pregnancy loss
In a well-managed herd with no health or nutritional problems, cows should breed quickly, Tibary says. Many producers allow only two cycles for heifers and three for cows; all should be bred and settle in that length of time.
“Early loss generally doesn’t affect length of time in which the cow returns to estrus. She comes back in heat on schedule, just as though she wasn’t bred,” he explains.
When a cow is cycling and bred to a fertile bull, the chances of fertilization are well above 90%. “Most lost embryos degenerate and die before the eighth day – before or just as they come down through the fallopian tube into the uterus. There’s also substantial loss observed before day 14, which won’t affect the cow’s return to heat in her next cycle,” Tibary says.
The causes of early loss include alterations in the egg or 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.
Some studies have shown that the quality of the cycle just previous to breeding affects the quality of the egg, as well as the embryo’s chances to survive to term, Tibary adds. Postpartum cows that haven’t yet cycled normally before they’re put with a bull – and bred on their first heat after calving – may have less chance for normal pregnancy.
Some cows have a short cycle in their first heat after calving. They may breed, suffer embryonic loss, return to heat 8-10 days later, and settle when bred on that heat.
“This is due to the quality of the corpus luteum that develops on the ovary. These cows may not have enough progesterone to maintain pregnancy. They come back in heat and cycle more normally after that,” he explains.
This is one reason not to breed heifers before they’re sufficiently developed and have normal cyclicity. “They’ll be more fertile, and more able to maintain pregnancy, once they’ve established normal cycles,” he says.
Tibary says synchronization schemes for artificial insemination (AI) should be carefully followed to quantify early pregnancy losses.
“Observe the rate of return to estrus; do an early pregnancy diagnosis, then do a confirmation check later. More than 65% of all losses happen before the embryo attaches to the uterine lining. We just see cows coming back into heat, so we don’t know whether it was the fault of the cow or the bull,” he says.
A smaller proportion – 5-8% of losses – of pregnancies that continue beyond 21 days are lost in the first 42 days – as the conceptus transitions from embryo to fetus, Tibary says. These are called late embryo/early fetal losses.
Rapid weight loss in the cow, or other stress such as transportation in early pregnancy, can lead to losses. There are also risks with toxic plants. Endophyte fungus in tall fescue, for instance, is often responsible for early pregnancy loss, in addition to hindering the animal’s ability to thermo-regulate. In some situations, feed may contain mycotoxins, mold, or estrogen-like substances that may interfere with pregnancy.
“The biggest factors include a sudden change in nutrition, and environmental stresses such as 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 explains. A long truck haul, gathering cattle swiftly out of a range pasture to get them away from an approaching fire, or cattle being continually harassed by wolves, are examples of stress that may lead to pregnancy loss.