When to Feed High Mag Minerals? Your Guide to Preventing Grass Tetany

January 9, 2026 •

4 min read

Grass tetany, a fatal metabolic disorder caused by low magnesium levels, remains a significant threat to grazing livestock, with older lactating cows being most susceptible. A proactive high mag mineral supplementation program is a simple and reliable method to prevent this condition, but timing and correct administration are critical.

Quick Summary

This guide outlines the critical periods and environmental factors that necessitate high magnesium mineral feeding for livestock. It details the causes of grass tetany and offers practical advice on supplementation strategies, intake monitoring, and best practices for herd management to mitigate risk during vulnerable times.

Key Points

Pre-Grazing Supplementation: Start feeding high mag minerals at least two to four weeks before turning cattle out onto lush, fast-growing pastures in spring or fall to build up protection.
High-Risk Period: The main risk for grass tetany occurs during the early grazing of cool-season grasses when the forage is high in potassium and nitrogen but low in magnesium.
Lactating Cows Are Most Vulnerable: Older lactating cows are at the highest risk due to magnesium depletion through milk production and lower absorption efficiency.
Monitor Intake: Ensuring consistent and adequate intake is crucial. Use palatable mixes and monitor consumption to ensure cattle are eating the target amount, usually 3-4 ounces per head daily.
Consider Year-Round Supplementation: For operations with a history of grass tetany, a continuous, year-round mineral program can help maintain mineral balance and prevent sudden deficiencies.
Avoid Stress: Minimize stress from handling, transport, or severe weather during high-risk periods, as this can trigger a grass tetany outbreak.

Understanding the 'Why': The Threat of Grass Tetany

Grass tetany, or hypomagnesemia, is a nutritional disorder that occurs when an animal's blood magnesium levels drop dangerously low. Magnesium is vital for proper nerve function and muscle contraction. Unlike other minerals, animals have very little stored magnesium they can mobilize to maintain blood levels, making them dependent on daily dietary intake.

Several factors can create a perfect storm for grass tetany:

Lush, Fast-Growing Forages: Rapidly growing pastures, especially cool-season grasses in spring or fall, are often high in potassium and nitrogen but low in magnesium. The high potassium interferes with magnesium absorption from the rumen.
Lactation: Older, lactating cows are particularly vulnerable because they lose a significant amount of magnesium in their milk, and their bodies are less efficient at mobilizing mineral stores.
Stress and Weather: Cold, wet, or windy weather can reduce feed intake and cause stress, triggering an outbreak. Stressful events like transportation, handling, or yarding can also be triggers.
Fertilizer Application: Heavy application of potash (potassium) and nitrogen fertilizers can decrease the magnesium available in the forage.

The Critical 'When': Timing Your High Mag Mineral Supplementation

The timing of high mag mineral feeding is arguably the most important aspect of a successful prevention program. You must begin supplementation before the risk period starts to ensure consistent intake and build up a level of protection.

Before Spring Turnout

For herds grazing lush spring pastures, the high-risk period begins with the first flush of new grass. It is recommended to start feeding a high-mag mineral at least two to four weeks before cattle are moved to these pastures. This acclimates the cattle to the supplement, mitigating palatability issues, and ensures they have sufficient magnesium onboard as dietary intake drops.

During Fall Grazing

While spring is the most common time for grass tetany, the risk can also be high in the fall, especially on lush cereal crops or with the regrowth of cool-season grasses. Fall calving herds, with their increased magnesium demand from lactation, are especially susceptible. Start supplementation several weeks before grazing these high-risk forages and continue until the threat subsides.

Throughout Winter

In some regions, grass tetany can occur when cattle are fed low-magnesium dry forages or silage in winter, especially if other minerals like calcium are also low. In this scenario, high mag mineral supplementation should be part of the winter feeding program.

For High-Risk Herds or Conditions

Consider year-round, low-dose supplementation if you have a history of grass tetany or graze high-risk pastures consistently. A continuous mineral program helps maintain a baseline level of minerals and prevents sudden deficiencies, particularly for older lactating cows.

Practical 'How': Feeding High Mag Minerals Effectively

Ensuring adequate intake of high mag minerals is the biggest challenge due to the unpalatable nature of magnesium oxide (the most common source).

Feeding Methods and Best Practices:

Free-Choice Mineral Mixes: This is a common and easy method. Use a mineral mix with 10-14% magnesium oxide. To improve palatability and encourage intake, mix with dried molasses or ground corn. Monitor consumption weekly to ensure cattle are eating the target 3-4 ounces per head daily.
Treated Forage: Topdressing hay or grain with magnesium oxide is one of the most effective methods, ensuring consistent intake per animal. This works well for herds being fed stored forages.
Water Medication: Adding soluble magnesium salts like magnesium sulfate (Epsom salts) to the water trough can be effective, but only if cattle don't have access to other water sources. Intake can vary significantly based on water source and taste.
Mineral Blocks vs. Loose Minerals: While blocks are convenient, they can be a less reliable method for ensuring uniform intake, as some animals may consume less than required. Loose mixes are generally more uniformly consumed.

Comparison of High Mag Mineral Feeding Methods


Method	Pros	Cons	Best For
Free-Choice Loose Mix	Easy to provide, adjustable location	Unpalatable, variable intake	Herds with good consumption, smaller pastures
Treated Forage	Consistent intake, good palatability	Labor-intensive, potential for waste	Supplemental feeding during risk periods
Medicated Water	Ensures intake if no other water source	Variable intake with alternative sources, palatability issues	Controlled environments, monitored intake needs
Mineral Blocks	Weather-resistant, convenient, low labor	Variable intake per animal, higher cost	Lower-risk situations, convenient supplementation

What to Do If Cattle Refuse the Supplement

If your cattle aren't consuming enough of the high mag mineral, try these strategies:

Move the Feeder: Place mineral feeders in high-traffic areas, such as near water sources, gates, or loafing areas.
Add Palatability Agents: Mix the mineral with more molasses, dried distillers grains, or corn to make it more attractive.
Use a Different Source: Consider magnesium sulfate, which is more palatable than oxide, though it has a lower magnesium content.
Enforce Consumption: The most reliable method is to mix the mineral directly into a feed source that cattle consume every day, like a small grain mix or hay.

Conclusion

Knowing when to feed high mag minerals is not just about reacting to a problem but strategically preventing it. By understanding the seasonal risks, especially the start of lush spring and fall grazing, and the vulnerability of older lactating cows, you can implement a proactive and successful supplementation program. Remember, consistency is key, and it is far more effective to begin supplementation weeks before a risk period than to try and remedy grass tetany after it has begun. Regular monitoring of intake and careful management can prevent this dangerous disorder and protect your herd's health and productivity. For more information, consult with your local extension agent or veterinarian to tailor a plan for your specific operation and regional risks.

Alabama Cooperative Extension System provides detailed spring supplementation considerations.

Frequently Asked Questions

Grass tetany, or hypomagnesemia, is a metabolic disorder caused by low blood magnesium levels in grazing livestock. It typically occurs when animals graze on lush, fast-growing forages that are low in magnesium and high in potassium, which interferes with magnesium absorption.

It is generally recommended that cattle consume approximately 3 to 4 ounces of high-mag mineral per head per day during high-risk periods. Refer to the specific product label and monitor actual intake to ensure consumption is consistent.

Early signs can include nervousness, twitching of the muscles in the face and flank, and a stiff gait. More severe symptoms can progress to staggering, excitable behavior, convulsions, and eventually death.

No, prevention is the best strategy. Because grass tetany can advance rapidly, waiting until you see clinical signs is often too late. Proactive supplementation starting weeks before the risk period is essential.

The most common source of supplemental magnesium, magnesium oxide, has an unpalatable, bitter taste. To improve intake, try mixing it with palatable ingredients like dried molasses, grain, or salt, or move the mineral feeder to a high-traffic area.

No, different forms have different characteristics. Magnesium oxide is common and economical but has palatability issues. Magnesium sulfate is more palatable but has a lower concentration and can be problematic with high-sulfur water sources. Bioavailability also varies.

Other risk factors include older, lactating cows, recent calving, stressful conditions like cold weather or handling, and pastures fertilized with high levels of potash. Some breeds are also more susceptible.