Diarrhea in cattle has dozens of possible causes, but most cases trace back to one of four categories: infectious pathogens, dietary disruption, parasites, or toxic plants. The cause depends heavily on whether you’re dealing with a calf or an adult cow, and narrowing it down quickly matters because dehydration can become life-threatening within hours in young calves and within a day or two in adults.
Calf Scours: Age Tells You the Likely Cause
If your calf has diarrhea, its age is the single most useful clue. Different pathogens hit at predictable windows. E. coli (specifically the strain called ETEC) is overwhelmingly a problem in the first week of life. In one study of hospitalized calves, over 90% of E. coli cases occurred in calves 1 to 6 days old. This type of infection causes watery, secretory diarrhea and can lead to rapid dehydration because the intestinal lining floods fluid into the gut.
Rotavirus peaks between 4 and 14 days of age and damages the intestinal lining so the calf can’t absorb nutrients properly. Cryptosporidium, a protozoan parasite, is most common at 7 to 14 days and produces watery diarrhea that can drag on for several days. Coronavirus has a broader window, hitting calves anywhere from 4 to 30 days old, and tends to produce stool containing blood and mucus.
Many calves carry more than one of these pathogens at the same time, which makes the diarrhea worse and recovery slower. The practical takeaway: a calf scouring in its first few days likely has an E. coli problem, while a calf scouring at one to two weeks old is more likely dealing with rotavirus or Cryptosporidium.
Common Causes in Adult Cattle
Adult cows have a more developed rumen and immune system, so the list of likely causes shifts. The most common triggers are dietary issues, parasites, and a seasonal condition called winter dysentery.
Grain Overload and Diet Changes
Sudden changes in feed are one of the most frequent causes of diarrhea in adult cattle. When a cow eats too much grain or concentrates too quickly, the rumen’s microbial population shifts dramatically and pH drops. This is ruminal acidosis, and it sets off a cascade of problems. The rumen fills with excess fluid, the gut lining becomes inflamed, and osmotic diarrhea follows. In severe cases, the cow also develops dehydration, liver abscesses, laminitis (painful inflammation in the hooves), and even neurological symptoms.
Feedlot cattle pushed onto high-grain diets are especially vulnerable, but any cow that breaks into a grain bin or gets an abrupt ration change is at risk. Interruptions in normal feeding patterns, like a missed feeding followed by overfeeding, can trigger the same problem. The fix is gradual diet transitions over 10 to 14 days, giving rumen microbes time to adapt.
Coccidiosis
Coccidiosis, caused by Eimeria parasites, is often thought of as a calf disease, but it hits adult cattle too. The parasite is picked up through the fecal-oral route: contaminated feed, water, or soil. It destroys the intestinal lining, causing foul-smelling, bloody diarrhea, loss of appetite, weight loss, and staining around the tail and hindquarters with blood-tinged manure. Affected cattle often run fevers of 103 to 105°F, have sunken eyes from dehydration, and show a noticeable drop in milk production. In severe cases, you’ll see blood clots in the manure and visible straining.
If your veterinarian runs a fecal flotation test, moderate to high levels of Eimeria oocysts are considered clinically significant. For general intestinal parasites, calves with over 200 eggs per gram or cows with over 100 eggs per gram typically need treatment. Results showing only rare or few parasites usually aren’t the cause of the problem.
Winter Dysentery
If your adult cow develops sudden, dark, bloody diarrhea between November and March, winter dysentery is a strong possibility. It’s caused by bovine coronavirus and spreads rapidly through a herd. Beyond the diarrhea, affected cattle often have fever, loss of appetite, coughing, nasal discharge, and a sharp drop in milk production that can persist even after the diarrhea resolves. The virus survives well in cold weather (it’s actually heat-sensitive), which explains why outbreaks cluster in winter months. Wild ruminants like deer may serve as a reservoir, carrying the virus during warmer months and transmitting it to cattle when conditions favor the virus in winter.
Toxic Plants on Pasture
Grazing cattle occasionally eat plants that cause acute gastrointestinal distress. Several common species are known to produce diarrhea in cattle. Oleander causes bloody stool along with nausea and cardiac stress. Boxwood triggers gastroenteritis and bloody diarrhea. Iris (Dutch and German varieties) can cause violent bloody diarrhea, abdominal pain, and mouth sores, and can be fatal in calves. Castor bean, privet, wisteria, lobelia, and daffodil all cause some combination of gastrointestinal irritation, diarrhea, and vomiting in cattle.
Plant poisoning tends to come on suddenly and affect only the animals that ate the plant. If one or two cows in a group develop acute diarrhea while others are fine, walk the pasture and check for unfamiliar plants, especially after storms that may have knocked branches or plant material into grazing areas.
How to Assess Dehydration
Regardless of the cause, dehydration is the immediate danger. You can estimate how dehydrated your cow or calf is using two simple checks.
The skin tent test: pinch a fold of skin on the neck and count how long it takes to flatten. Under 2 seconds is normal. Two to 6 seconds means roughly 8% dehydration. Over 6 seconds means severe dehydration above 10%.
Eye recession: pull down the lower eyelid and look at the gap between the eyeball and the inner corner of the lid. Slightly sunken eyes are an early sign of illness and suggest around 6% dehydration. Deeply sunken eyes indicate 8 to 10% or worse.
You can also check the gums. Normal gums are pink and damp. White, dry gums point to 8 to 10% dehydration. Fluid loss beyond 8% typically requires intravenous fluids, and losses above 14% can be fatal.
Rehydrating a Scouring Calf
For calves that are mildly to moderately dehydrated (skin tent under 6 seconds), oral electrolyte solutions are the first line of defense. To figure out how much extra fluid the calf needs, multiply the calf’s weight in pounds by its estimated dehydration percentage. Then divide by 2 to get quarts. For example, a 100-pound calf at 6% dehydration needs about 3 extra quarts of electrolyte solution per day on top of its normal milk feeding. If that calf is being fed milk at 10% of body weight (5 quarts), total daily fluid intake should be around 8 quarts.
A good oral rehydration solution contains sodium, glucose in a roughly one-to-one ratio with sodium, an alkalinizing agent to counteract the acid buildup from diarrhea, and potassium. Commercial electrolyte products designed for calves meet these requirements. Feed electrolyte solutions separately from milk rather than mixing them, and spread the fluids across multiple feedings throughout the day.
Preventing Scours in Calves
The most effective prevention tool for calf scours is vaccinating the pregnant cow during late gestation. Scours vaccines are given during the third trimester so the cow builds antibodies against E. coli, rotavirus, and coronavirus while her colostrum is forming. The calf then receives those antibodies through the first milk after birth.
Heifers or cows receiving the vaccine for the first time need two doses spaced a few weeks apart before calving. After that initial series, a single annual booster in late gestation is all that’s needed for the rest of the cow’s reproductive life. If a cow hasn’t calved within two months of receiving her vaccine, she may benefit from an additional dose.
Beyond vaccination, colostrum management is critical. Calves need to drink adequate colostrum within the first few hours of life, before the gut loses its ability to absorb those protective antibodies. Clean, dry calving areas and separating young calves from older animals also reduce pathogen exposure during the most vulnerable first weeks.