Respiratory disease is the single biggest killer of cattle in the United States, responsible for nearly one in four non-predator deaths. After that, the list fans out across digestive emergencies, toxic plants, infectious diseases, metabolic failures, and environmental extremes. USDA data puts respiratory problems at 23.9% of all non-predator cattle deaths, followed by unknown causes (14%) and old age (11.8%). For calves, respiratory disease is even more dominant at 26.9%.
Respiratory Disease
The umbrella term for most respiratory deaths in cattle is bovine respiratory disease complex, often called “shipping fever.” It typically starts when stress suppresses a calf’s immune system. Weaning, long truck rides, and mixing with unfamiliar animals are the classic triggers. Viruses move in first and damage the airways, then bacteria colonize the weakened tissue and cause severe pneumonia.
The bacterial infections are what usually kill the animal. The lungs fill with fluid and fibrous tissue, making it progressively harder to breathe. Vaccination programs that target the major viruses and bacteria involved have significantly reduced death rates in herds that use them, but shipping fever remains the leading cause of feedlot losses across North America.
Toxic Plants
Poisonous plants kill cattle remarkably fast, sometimes within minutes. The most dangerous plant in North America is water hemlock. Its toxin triggers violent seizures, and death can occur as quickly as 15 minutes after a lethal dose. Yew is similarly ruthless: the alkaloid it contains stops the heart, and survival after ingestion is rare. As little as half a percent of an animal’s body weight in yew foliage is enough to be fatal.
Several plants kill through cyanide poisoning. Chokecherry leaves and arrowgrass both release hydrogen cyanide during digestion. A 600-pound cow can be killed by as little as a quarter pound of stunted arrowgrass. Chokecherry poisoning comes on fast, sometimes fatally within minutes. Larkspur, common on western rangelands, contains alkaloids that paralyze muscles, leading to respiratory failure. Poison hemlock works similarly, killing through respiratory paralysis in two to three hours.
Some plants work more slowly. Bracken fern has a cumulative toxin that damages bone marrow and causes fatal internal bleeding over time. Milkvetch species containing nitro compounds can kill a 1,000-pound cow with just two pounds of green plant material, typically within three to four hours.
Digestive Emergencies
Bloat is one of the most recognizable and dramatic ways cattle die. In a healthy cow, the rumen constantly produces gas during fermentation, and the animal belches it out. Bloat happens when something traps that gas inside. In “frothy bloat,” fine foam forms on top of the rumen contents, preventing normal gas release. The rumen inflates like a balloon, pressing against the diaphragm and lungs. Once rumen pressure reaches about 70 mmHg, the animal can no longer breathe effectively. Excess carbon dioxide also gets absorbed into the bloodstream from the distended rumen, compounding the respiratory distress. Without intervention, the cow suffocates.
Frothy bloat is particularly common in feedlot cattle eating high-concentrate diets and in pastured cattle grazing lush legumes like alfalfa or clover. It can kill within hours if not relieved.
Calf Scours
Diarrhea, known as scours, is one of the top killers of young calves. Different pathogens hit at different ages. In the first four days of life, a type of E. coli that attacks the gut lining causes profuse, watery diarrhea that can rapidly lead to fatal dehydration and shock. From about one to four weeks of age, a parasite called Cryptosporidium becomes the primary concern. Coronaviruses are a major cause of scours in calves four to 30 days old.
The dehydration itself is what kills most calves. As they lose fluid, their blood volume drops, electrolyte levels shift dangerously, and cardiac arrhythmias can develop. Calves that didn’t receive adequate colostrum (and the antibodies it carries) in their first hours of life are far more vulnerable. Overall, diarrhea deaths should stay around 1% in well-managed beef herds, but in dairy calves the figure runs closer to 5% from birth to weaning.
Clostridial Diseases
Blackleg is one of the most notorious sudden killers. Caused by a soil-dwelling bacterium, it typically strikes young, fast-growing beef cattle in excellent condition. The bacteria are swallowed while grazing, pass through the gut wall into the bloodstream, and lodge silently in muscle tissue. They can remain dormant until something, possibly a bruise or hard exercise, activates them.
Once active, the bacteria destroy muscle tissue and produce gas, causing dark, spongy, crackling swellings under the skin. The affected muscle turns black and gives off a distinct sweet odor. Onset is sudden. Ranchers often find animals dead with no prior warning signs. When symptoms are observed, the disease progresses from swelling to prostration, tremors, and death within 12 to 48 hours. Vaccination is highly effective and routine in most beef operations, which has made blackleg far less common than it once was.
Milk Fever in Dairy Cows
Milk fever is a metabolic crisis that strikes dairy cows right around calving, when the sudden demand for calcium to produce colostrum and milk drains calcium from the blood faster than the body can replace it. Blood calcium drops below critical levels, and because calcium is essential for both nerve signaling and muscle contraction, the cow’s body begins shutting down.
Early signs include muscle tremors, head shaking, tongue protrusion, and teeth grinding. As it progresses, the cow goes down onto her chest, then rolls onto her side. Heart rate drops, consciousness fades, and coma sets in. The low calcium also paralyzes the digestive tract and prevents the teat sphincters from closing properly, setting up secondary problems. Without treatment, 60 to 70% of cows with clinical milk fever die. Treatment with intravenous calcium can reverse the condition rapidly, but the window is narrow.
Anthrax
Anthrax remains a threat in certain regions worldwide. Caused by a spore-forming bacterium that can survive in soil for decades, it typically surfaces after heavy rains or flooding expose buried spores. Cattle ingest the spores while grazing, and the resulting infection moves quickly. In its most common form in cattle (the acute systemic version), death can occur before any obvious illness is noticed. Carcasses of anthrax-killed animals often have dark, unclotted blood oozing from the nose, eyes, and other natural openings.
Anthrax is a reportable disease in most countries, meaning any suspected case triggers mandatory notification of animal health authorities. Carcasses should not be opened, because exposure to air allows the bacteria to form spores that contaminate the soil for years. Vaccination is the primary prevention tool in areas where anthrax is known to occur.
Heat Stress
Cattle are far more sensitive to heat than most people realize. They generate enormous internal heat through rumen fermentation and have limited ability to cool themselves. Researchers use a combined measure of temperature and humidity (the Temperature-Humidity Index, or THI) to gauge risk. Cattle begin experiencing mild stress when the THI reaches 68, moderate stress at 72, and severe stress at 80 or above.
During heat events, cattle pant heavily, stop eating, and seek shade. Their core temperature rises, and if it stays elevated long enough, organ damage and circulatory failure follow. Heat kills can come in waves: cattle that survived a brutal afternoon may die overnight if temperatures don’t drop enough for them to dissipate the heat they’ve stored. Feedlot cattle, especially those carrying heavy body condition, and black-hided breeds are at highest risk. Major heat events in the U.S. have killed thousands of cattle in a single week.