Wheat pasture poisoning is caused by dangerously low magnesium levels in the blood of cattle grazing lush, rapidly growing small-grain pastures like wheat, oats, or rye. The condition, also called grass tetany or hypomagnesemic tetany, develops when these fast-growing forages fail to provide enough absorbable magnesium to meet the animal’s needs. Without treatment, mortality rates range from 50 to 100%, usually from kidney failure.
Why Lush Wheat Pasture Depletes Magnesium
The root cause is a mineral imbalance in the forage itself. Young, rapidly growing wheat and other cool-season grasses are high in potassium and nitrogen but low in magnesium and calcium. High potassium levels in the plant actively interfere with magnesium absorption in the rumen. So the animal is eating plenty of forage but absorbing far less magnesium than it needs.
Unlike calcium or other minerals, cattle have very limited ability to pull magnesium from their bones or other reserves when blood levels drop. They depend almost entirely on a continuous daily intake from their diet. When the forage they’re eating blocks that intake, blood magnesium can fall to dangerous levels within days.
Weather Patterns That Trigger Outbreaks
Specific weather conditions make wheat pasture poisoning far more likely. Cool, cloudy, rainy weather drives the kind of rapid, lush growth that produces the most dangerous forage. The classic trigger is a warm spell following a stretch of cool weather, which causes a flush of new growth that is especially high in potassium and low in magnesium. Spring and fall are the peak risk seasons, when cool-season grasses and small-grain pastures are growing fastest.
Which Cattle Are Most Vulnerable
Lactating beef and dairy cows are by far the most susceptible, particularly those in early lactation four to eight weeks after calving. Milk production creates a huge demand for magnesium. A cow producing milk may need two to three times as much magnesium daily as a dry cow, and since she can’t mobilize meaningful reserves from her body, she’s entirely dependent on what she eats and absorbs that day. Older cows are at higher risk than younger animals, likely because magnesium absorption efficiency declines with age. Steers and young calves rarely develop the condition.
What Happens Inside the Animal
When blood magnesium drops low enough, it also falls in the fluid surrounding the brain and spinal cord. Magnesium normally acts as a brake on nerve activity. Without enough of it, neurons in the central nervous system begin firing spontaneously, which is what produces the hallmark muscle cramps and excitability. Calcium levels often drop simultaneously, compounding the problem by further destabilizing nerve and muscle function.
The progression can be alarmingly fast. Early signs include restlessness, loss of appetite, and unusual excitability. These quickly advance to muscular trembling, teeth grinding, staggering, and loss of coordination. In severe cases, the animal goes down, develops rigid arching of the head and neck, and begins paddling its legs uncontrollably. Death typically occurs within 2 to 6 hours if the animal isn’t treated. In the most extreme cases, the heartbeat can be audible from several feet away.
How It Differs From Milk Fever
Wheat pasture poisoning is easily confused with milk fever, especially when both conditions occur around calving. The key difference is behavior. A cow with milk fever (low calcium alone) typically appears calm but unable to stand. A cow with low magnesium appears agitated, nervous, and shows visible tremors when trying to rise. When both conditions overlap, which happens frequently, diagnosis becomes much harder and a veterinarian may need blood work to distinguish them.
Prevention Through Mineral Supplementation
The most reliable prevention is feeding a mineral mix containing magnesium oxide before and during the grazing period. A mineral mixture with 10 to 14 percent magnesium, consumed at about 4 ounces per head per day, provides adequate protection for most cattle. Standard mineral mixes typically contain only about 1 percent magnesium, which is nowhere near enough when grass tetany is a concern. The supplement needs to be available and palatable well before cattle are turned out onto lush pasture, not after animals start showing signs.
Magnesium oxide has a bitter taste, so palatability can be a real challenge. Mixing it with dried molasses, salt, or other feed ingredients helps encourage intake. Placing mineral feeders near water sources or loafing areas increases the chance that every animal in the herd gets adequate exposure.
Grazing Management to Reduce Risk
How you manage the pasture matters almost as much as mineral supplementation. Avoiding grazing on very young, immature wheat forage reduces risk because more mature plants have better mineral balance. Rotational grazing systems that move cattle among paddocks help by preventing animals from grazing only the youngest, most lush regrowth. Allowing adequate forage residual height, rather than grazing down to the ground, also reduces the proportion of dangerous new growth in the diet.
Providing dry hay alongside lush pasture slows the rate of passage through the rumen, which gives the animal more time to absorb what magnesium is available in the forage. Some producers delay turnout onto wheat pasture until the forage has more growth, or they limit grazing hours during the highest-risk weather windows. Keeping lactating cows off the most dangerous pastures entirely during the four-to-eight-week postpartum window is the most conservative approach, though not always practical.