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Extension Education | December 2023

Investigating the Cause of Cattle Abortions

When to involve your veterinarian and what happens next.


Matt Hille, DVM, MS, PhD Diagnostic Veterinary Pathologist and Nebraska Extension Specialist

David Steffen, DVM, PhD, DACVP Diagnostic Veterinary Pathologist

University of NE - Lincoln, UNL Beef



It’s fall, and for cow/calf producers throughout the region that often means it’s time to preg check. Confirming pregnancy in the herd is an important milestone in the overall cow/calf production system, but there’s still a lot that needs to go right before you’re admiring next year’s weaned calf crop. Unfortunately, reproductive losses can still happen between confirmation of pregnancy and calving. Beef producers and veterinarians often refer to any death loss before calving as an “abortion,” but in reality, true abortions only make up a portion of this loss. It is important to define some of the terms regarding reproductive losses:


Early embryonic death: Loss of pregnancy in the period from conception to maternal recognition of pregnancy (implantation). This occurs approximately 17 days after initial fertilization.

Late embryonic death: Fetal losses that occur from recognition of pregnancy (about 17 days post conception) until about 6 weeks post conception.

Abortion: Fetal losses from 6 weeks of gestation to calving.


Pre-breeding considerations to minimize reproductive losses

Reproductive losses that take place earlier on in gestation usually do not result in an observed lost embryo or fetus. These cows will most often present as open or late bred cows at preg check, depending upon if they returned to heat. Diagnosing these cases can be difficult (if not impossible) since the inciting factor happened weeks or even months before the issue is identified. Early reproductive management practices can be essential in minimizing these losses. It is important to assure cows are in adequate condition by the start of breeding season. Pregnancy is a physiologic luxury, and will only happen and/or be maintained when the nutritional and physiologic needs of the dam are met, and continue to be met throughout the pregnancy. There is a strong correlation of body condition with conception rates and being on the “gain” trend if entering breeding can mitigate these early losses, particularly in second calf heifers. Low body condition score and marginal nutrition is a common cause of infertility and early losses up to about 6 weeks. Routinely evaluating, recording, and addressing body condition scores in your cows before breeding and during early gestation is crucial. This type of methodical approach can provide useful data to troubleshoot reproductive losses when consulting with a veterinarian or herd nutritionist, should they occur.


Another important pre-breeding management tool is to ensure breeding soundness exams are performed on bulls each year. Approximately ~10% of adult beef bulls will fail a breeding soundness exam each year. The timing of these exams is important. Testing bulls too early in the year creates potential for infection, fever, or injury prior to turnout. It is recommended to re-test newly purchased bulls, even if tested prior to purchase for the same reason. Testing bulls immediately before turnout can be problematic as well if there is not adequate bull power available if there is an unexpected failure rate. This means when to test bulls will vary from ranch to ranch. The ideal scenario would be to test bulls as close to turnout as possible, but leave enough time to find replacement bulls if needed. This will also allow bulls to be re-tested if they failed due to a treatable issue. Performing breeding soundness exams in the breeding pasture, where bulls that pass the exam exit the chute and immediately join cows works great; until it doesn’t. I have personally had several instances where the producer had more bulls fail the exam than they anticipated, and they were left short on bull power with minimal options for immediate quality replacements. Some very basic discussions early on with your veterinarian surrounding breeding management can help minimize these issues.


When to be concerned about abortions

Normal rates for reproductive losses in cattle can vary substantially between operations based on a number of factors including farm specific management, time of year, breed composition, and geographic region. Therefore, the number of abortions that should be cause for concern will vary, and published thresholds of acceptable losses range widely from ~2% - 8%. You will need to develop a threshold for concern based on past experiences. Typically, we suggest a 2-2.5% rate of mid to late term abortions to be expected for the average herd in Nebraska. Regardless, knowing your own herd and your management strategies, should help define your action threshold. If in doubt, it is never a bad idea to discuss losses with your veterinarian early on and potentially even hold samples to allow for a retrospective look if more abortions occur. Your veterinarian works with numerous producers in the area, and may have insight into ongoing losses across many herds that may help explain your own. The cost-benefit ratio and a willingness to act upon any findings are important to consider before sending specimens to the lab for further diagnostics. Investigating reproductive losses when it is not warranted or without predetermined goals and action plans can be financially inefficient. Waiting too long to investigate loses opportunity for earlier interventions and potentially mitigating further losses. Thus, having a defined herd reproductive plan developed with your veterinarian can assure sample preservation and avoid the pitfalls of diagnostic inefficiency.


Recording data such as breeding dates, pregnancy dates, and birth dates of calves can be helpful in addressing reproductive failure. This data will allow retrospective calculations of pregnancy rates over 21 day intervals and can help identify periods in early gestation where bull factors might be impacting the calf crop. Healthy breeding bulls can cover for “bad” bulls for a short period, but can also start to fail from overuse and this type of data can help provide a clue. Clues toward an early bull failure may help guide future bull-cow ratios or the number of cleanup bulls to use in subsequent years. This can allow you to optimize the usage of bulls to moderate cost, but prevent catastrophic infertility and also return to your ideal calving window if it has been prolonged over time.


When a dead near term calf is found, it is important to distinguish whether the case represents abortion, a stillbirth, or perinatal death. A prolonged or difficult birth (dystocia) that goes unnoticed can resemble a late term abortion. Also, a live calf that dies before rising and nursing can appear as an abortion or a stillbirth. The list of differential causes in these cases can be quite different. Meconium or fecal staining on the skin (giving rise to a brown-yellow discoloration) is an indicator of fetal distress since calves will defecate in utero during a dystocia. If you see this, it is suggestive that the particular death was possibly due to a dystocia that wasn’t noticed. Swelling of the face, neck, and tongue of the dead calf are also indications of dystocia. In the absence of these signs, a post mortem exam (i.e. a necropsy) by your veterinarian can be beneficial.


One important component of a necropsy is to determine if the calf died in utero or shortly after birth. Inspecting the lungs can give important clues as a live calf will usually take a breath and inflate the lungs. These inflated lungs will often appear lighter pink compared to the darker red seen in collapsed or non-inflated lungs. Partial or fully inflated lungs indicate either an issue with birthing or a failure to thrive after birth and a necropsy can help to correctly characterize these calves. Perinatal losses (death shortly after birth) can be due to dystocia and result in an oxygen deprived calf being born alive. They can also be due to inadequate nutrition during the last trimester of pregnancy. The losses can be the result of vitamin and/or mineral imbalances, particularly if the majority of gestation takes place in a dry lot with stored feed or byproducts being fed. It is critical to properly account for loss of nutrients in stored feeds in these non-grazing situations. A necropsy will often allow proper characterization of the death, but will rarely have gross abnormalities that provide definitive diagnosis. If indicated, your veterinarian may recommend further diagnostics in an attempt to narrow down the possibilities or determine a cause for the losses.


The diagnostic laboratory is useful to evaluate infectious causes of abortion, inherited anomalies, and some nutritional aspects that can impact birth rates and calf survivability. The causes within these groups are numerous, and detailing each specifically is outside the scope of this discussion. But, by either submitting the entire aborted fetus or submitting samples from specific tissues to the diagnostic lab, veterinary diagnosticians can look to confirm or rule out some of the more common causes. Diagnosticians at the laboratory will review the case, order appropriate tests, and aid in the interpretation of the results. Including a complete history improves the chances of obtaining a definitive diagnosis and can keep costs down. Most diagnosticians are happy to discuss the case prior to investigation to help guide sample collection. The more common tests performed include microscopic examination of the tissues, bacterial culture, and toxicological or nutritional analysis of fetal tissues. PCR tests are also common and allow for highly sensitive detection of small amounts of nucleic acid from viruses or bacteria known to cause bovine abortions. The results are interpreted in context with the goal to assist your veterinarian in treatment or preventative management in the herd.


Unfortunately, the majority of abortion workups in veterinary diagnostic labs result in a diagnosis of “idiopathic abortion”, which is a fancy way of saying we don’t know what caused the abortion. The poor success rate of obtaining a definitive diagnosis for abortions is likely due to a number of factors. One of these factors is that some of these cases are not true abortions. There are undoubtedly cases pursued as abortions that are dystocias, failure to thrive, or due to nutritional or stress factors not evident in routine lab examinations. Working cows that are very near full term can induce physiologic stress causing endogenous steroids to be released that result in pregnancy termination. The same type of stress can happen in extraordinary weather events like snow storms. These types of abortions leave no detectable evidence in the tissues of the fetus and end up being classified as idiopathic if examined in the lab. The cost of an abortion workup varies by laboratory, but it is not uncommon to end up with several hundred dollars in testing fees. This highlights the importance of working with your veterinarian to establish thresholds for intervention, a diagnostic sampling plan, and to establish goals for using the diagnostic data obtained.


Receiving a diagnosis of an “idiopathic abortion” does not necessarily mean the investigation was not useful. As veterinarians and veterinary diagnosticians, our goal is to promote herd health and help inform management decisions. Some causes of abortions in cattle can cause abortions rates of 50% or more, and ruling out these causes with negative test results can provide value. That is, sometimes knowing what did not cause the abortion is as important as knowing what did cause the abortion.


Tips for producers dealing with suspected aborted calves

» Remove the dead calf from the pasture or pen to prevent predation.

» Keep the carcass cool until it can be examined by your veterinarian or shipped to the diagnostic lab.

» It may be desirable to freeze back select samples from early losses to be available if the threshold for action is reached.

» When possible, obtain the expelled placenta in addition to the fetal carcass. Placenta is a valuable sample if additional diagnostics are pursued and increases the likelihood of a diagnosis in abortions caused by infectious agents.

» Ingestion of pine needles in excess can cause abortions in cattle. Historically this has been a difficult diagnosis to make beyond speculating based on the history of access and potential ingestion. Recently, we have been able to test aborted fetuses and confirmed this as the cause in a number of abortion cases in areas of Nebraska where cows have access to pine needles. If you suspect this may be the case in an abortion, let your veterinarian know. The sample of choice to test for pine needle abortion is thoracic fluid from the aborted fetus.


Take home messages

Determining the cause of embryonic losses early in gestation is extremely difficult since the losses often happen well before we’re aware there is an issue. Developing an overall herd health plan with your veterinarian can improve the recognition of preventable reproductive losses and enable preventative management strategies. Special attention should be made regarding the nutritional status of the herd, reproductive health of the bulls, record keeping, and a proactive vaccination schedule to combat infectious causes.


Average annual abortion rates can vary widely from ranch to ranch. Use your own previous years’ experiences with your herd to help you decide when abortions reach an abnormal level. If in doubt, call and discuss the situation with your veterinarian to determine if investigating the losses is warranted.


It is not practical to investigate every suspected abortion due to the associated costs. Confirming suspected cases as true abortions, as opposed to death loss due to a difficult birth or failure of a live calf to thrive is the first step in determining whether further investigation is warranted.


Diagnostic workups for abortions historically have low rates of success in determining a definitive cause. However, ruling out causes of abortion via negative test results can provide valuable diagnostic data. There is always value in knowing what did not cause increased abortions in your herd.


Courtesy University of Nebraska - Lincoln, UNL Beef.


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