As spring moisture and fluctuating temperatures drive a surge in forage growth, conditions are aligning for an increase in grass tetany risk across many beef operations. The same environmental shifts that are jumpstarting wheat pasture and other small grains can also create the mineral imbalances that trigger sudden losses in lactating cows.
With rapid pasture growth underway in many areas, grass tetany risk is rising in susceptible herds, according to Paul Beck, Extension specialist for beef nutrition with Oklahoma State University. High-quality forage is often directed toward cows with the greatest nutritional demands, placing early-lactation animals directly into higher-risk environments.
Fertility and Forage Growth Driving the Issue
Cool-season annuals are a valuable resource, particularly when they reduce reliance on hay and supplemental feed. But as pasture quality improves, mineral balance can shift in ways that are not immediately visible.
“Our best managed cool-season annual pastures have had adequate fertilizer high in nitrogen and potassium, both of which are necessary for grass growth. But high nitrogen and high potassium interacts with the marginal magnesium level in these forages and create issues with beef cows as they begin lactating,” Beck says.
Nitrogen and potassium fertilization support aggressive forage growth, particularly during periods of favorable moisture. At the same time, they can interfere with magnesium uptake, leaving cows vulnerable even when forage appears nutritionally rich.
Magnesium absorption occurs primarily in the rumen and can be impaired by high potassium levels, which reduce transport across the rumen epithelium. This is why fertilized, rapidly growing forages create a consistent risk pattern.
Periods of rain followed by rapid pasture growth can further amplify the risk, especially when cattle are transitioned quickly onto highly digestible forage.
Early Signs Easy to Miss as Cases Develop Quickly
Grass tetany remains a neurologic condition driven by low blood magnesium, and clinical signs can escalate rapidly once levels fall.
“Cows will start shaking and have uncontrolled muscle movements. They will lose their balance. That will be one of the first signs you see,” Beck says.
In the early stages, affected cattle may appear nervous or uncoordinated. As the condition advances, animals can go down and become unable to rise, with death occurring shortly after if intervention is not successful.
Because of this rapid progression, cases are often first recognized only after severe signs appear.
Early treatment with intravenous or subcutaneous calcium-magnesium solutions can be effective, particularly before animals become recumbent. Relapses are possible, and animals should be monitored closely following initial treatment.
Grass tetany should be differentiated from other causes of neurologic signs and sudden death, including hypocalcemia, polioencephalomalacia, and lead toxicity. History, pasture conditions and response to magnesium therapy can help support a presumptive diagnosis in the field.
Prevention Hinges on Timing, Not Reaction
Despite the speed at which grass tetany can develop, the risk itself is highly predictable. That makes prevention the most effective strategy, particularly during periods of rapid pasture growth.
“The best way to counter the problem is to act before we get to it,” Beck says.
That means preparing ahead of turnout, not reacting after symptoms appear. In practical terms, that looks like identifying high-risk pastures and production stages in advance, then ensuring supplementation is in place before cattle enter those environments.
This is especially important during spring transitions, when forage conditions can change quickly over a short period of time.
Mineral Intake Remains the Weak Link
While most producers are aware of the need for high-magnesium mineral, consistent intake remains the primary challenge.
“Magnesium oxide does decrease the palatability of mineral mixes, making it important to manage the feeding of these minerals,” Beck says.
Magnesium oxide is widely used due to its availability and cost-effectiveness, but reduced palatability can limit voluntary intake. Without active management, even well-designed mineral programs may fall short.
Management Focus as Risk Window Opens
With pasture conditions improving and turnout underway or imminent in many areas, attention is shifting toward practical prevention.
Risk mitigation should focus on:
- Ensuring high-magnesium mineral is available before and during turnout.
- Monitoring intake closely, rather than assuming consumption.
- Placing feeders in high-traffic areas to encourage consistent use.
A Seasonal Risk That Follows Predictable Patterns
Grass tetany tends to emerge when rapidly growing forage, high-producing cows and inadequate magnesium intake intersect. Spring conditions consistently bring those factors together, making this a predictable — yet preventable — challenge.
Outbreaks often affect multiple animals within a short timeframe, particularly when herd-level mineral intake is inconsistent. This makes grass tetany both an individual animal emergency and a herd management issue.
Timely supplementation and close management of intake can help you stay ahead of the problem before clinical cases begin to appear.
