Milk fever is a sudden drop in blood calcium that strikes dairy cows around the time of calving. Despite its name, it has nothing to do with infection or a high temperature. The condition occurs because a cow’s body cannot mobilize calcium fast enough to meet the enormous demand of producing colostrum and milk. Without treatment, milk fever progresses from restlessness to collapse and can be fatal within hours. It affects roughly 5 to 10 percent of dairy cows in most herds, though rates can exceed 25 percent on some farms.
Why Calcium Crashes at Calving
A cow in early lactation may need three to four times more calcium than she did while pregnant. Colostrum is especially calcium-rich, so the first milking after birth creates a massive withdrawal from the bloodstream. Normally, the body compensates by pulling calcium from bones and absorbing more from the gut, a process regulated by parathyroid hormone and vitamin D. But these systems take 24 to 48 hours to ramp up fully, and in some cows they simply can’t keep pace.
The result is a dangerous gap between supply and demand. Calcium is essential for muscle contraction and nerve signaling throughout the body. When blood levels fall too low, muscles weaken, the gut slows down, and the heart struggles to maintain its rhythm. That cascade of failures is what produces the visible symptoms of milk fever.
How It Looks in Three Stages
Milk fever typically unfolds in a recognizable progression. In Stage 1, the cow is still on her feet but visibly off. She may seem anxious or hypersensitive, with fine muscle tremors along her flanks and shoulders, twitching ears, head bobbing, and restless shuffling of her hind feet. Some cows bellow repeatedly. This stage is easy to miss because the signs are subtle and the cow is still walking.
Stage 2 is the classic presentation most farmers recognize. The cow goes down and cannot stand. She often lies with her head turned back toward her flank in a distinctive S-curve posture. Her muzzle is dry, her ears and extremities feel cold, and her pupils may be dilated. Heart rate increases while gut sounds disappear because the digestive tract has essentially stalled.
Stage 3 is an emergency. The cow lies flat on her side, unresponsive, with a barely detectable heartbeat and bloating from a paralyzed rumen. Without intravenous calcium at this point, death can follow within hours from heart failure or complications of prolonged recumbency, such as muscle damage from the cow’s own body weight.
Which Cows Are Most at Risk
Age is the single biggest risk factor. The likelihood of milk fever increases by about 9 percent with each successive lactation. Cows five years and older are especially vulnerable because their ability to mobilize calcium from bone declines over time. Among cows in their third or later lactation, a striking 73 percent show at least subclinical hypocalcemia, meaning their blood calcium dips below normal even if they never go down.
Breed matters too. Jersey cows are significantly more susceptible than Holsteins because they have fewer vitamin D receptors in their intestines. Since vitamin D is the key driver of calcium absorption from feed, Jerseys are working with a smaller absorption capacity right when they need it most. High-producing cows of any breed face greater risk simply because they produce more milk and therefore lose more calcium.
The Hidden Problem: Subclinical Hypocalcemia
For every cow that collapses with obvious milk fever, several more experience a milder, invisible version. Subclinical hypocalcemia produces no outward signs, but the cow’s blood calcium is still low enough to impair muscle and immune function. Research on dairy herds found that subclinical hypocalcemia was present in 25 percent of first-lactation heifers, rising to 54 percent of cows in their fifth lactation.
These cows don’t look sick, but the consequences add up. Low calcium weakens the smooth muscle of the uterus and the teat sphincter, which can slow calving, delay uterine recovery, and increase the risk of mastitis. It also impairs the rumen, reducing feed intake at exactly the time a fresh cow needs to eat aggressively to support milk production. The economic damage from subclinical cases, spread across an entire herd, often exceeds the cost of the dramatic collapses that grab a farmer’s attention.
Treatment and Recovery
A cow with clinical milk fever needs calcium delivered directly into her bloodstream, and quickly. Intravenous calcium solutions are administered slowly by a veterinarian or experienced farmer, with careful attention to heart rate, because pushing calcium in too fast can cause cardiac arrest. Most cows in Stage 2 respond dramatically: within minutes of treatment, muscle tone returns, the gut starts moving again, and the cow attempts to stand.
About 25 to 30 percent of treated cows relapse within 24 to 48 hours as calcium demand continues to outstrip supply. Oral calcium supplements given after the initial IV treatment help sustain blood levels through this vulnerable window. The average direct treatment cost for a case of milk fever is modest (around $39 per case for organic farms in one U.S. study), but the real expense lies in reduced milk production, delayed breeding, and increased risk of being culled from the herd.
Prevention Through Pre-Calving Diet
The most effective prevention strategy starts three to four weeks before a cow is due to calve. The goal is to “train” her calcium regulation system to be active before the surge in demand hits.
One well-established approach is feeding a low-calcium diet during the dry period. If calcium intake stays below 20 grams per day, the cow’s body is forced to upregulate its bone-mobilization and gut-absorption pathways. Then, when calving triggers a sudden need for calcium, those pathways are already running at full speed.
The more widely used modern strategy involves adjusting the dietary cation-anion difference, or DCAD. This means shifting the balance of minerals in the pre-calving diet to create a mildly acidic internal environment, which makes the cow’s calcium-regulating hormones work more effectively. The target DCAD is around negative 100 milliequivalents per kilogram of feed, achieved by adding anionic salts (chloride and sulfate sources) while keeping potassium and sodium low. Farmers monitor success by testing urine pH, aiming for a range of 5.5 to 6.2.
Magnesium also plays a critical role. Pre-calving diets should contain adequate magnesium because it is required for parathyroid hormone to function properly. Without enough magnesium, even the best DCAD program can fail. Keeping dietary phosphorus in check before calving further reduces risk, as high phosphorus interferes with vitamin D activation.
Milk Fever in Dogs and Cats
Milk fever isn’t limited to cattle. In dogs, the equivalent condition is called eclampsia, and it most commonly strikes small-breed dogs nursing large litters. Peak risk is two to three weeks after birth, when milk production is highest, though it can occasionally occur during delivery itself.
The signs in dogs differ somewhat from cattle because the neurological effects are more prominent. Early symptoms include panting, restlessness, and pacing. As calcium drops further, dogs develop muscle tremors, a stiff or uncoordinated gait, and behavioral changes like whining, aggression, or hypersensitivity to touch and sound. Without treatment, eclampsia progresses to full-body seizures, coma, and death. Cats can also develop the condition during early lactation, though it is uncommon.
Eclampsia in dogs is treated with intravenous calcium, just as in cows. Puppies are typically weaned or supplemented with formula to reduce the drain on the mother. Unlike cattle, where prevention centers on pre-calving diet manipulation, the main preventive measure in dogs is avoiding calcium supplementation during pregnancy. Excess calcium before birth actually suppresses the hormonal pathways that regulate calcium, leaving the dog less able to adapt when lactation begins.