Grazing practices to prevent grass tetany

grazing cattle

As I gaze out of my farm office dreaming of warmer days and greener grass, I can’t help but get excited about the upcoming growing season. I have always enjoyed early spring and often amazed how quickly life begins to flourish with just a few additional degrees and a few more minutes of sunlight each day. 

As a youngster, I have fond memories of turning the Holsteins out to grass after being cooped up in the free stall barn all winter. There were many reasons to laugh during this time, watching a 5.5-foot tall 1,600-pound lanky Holstein cow jump and kick after being confined on concrete all winter reminded me of the national geographics videos of giraffes running through the African savanna. 


With all the comedy of turning cattle out to fresh green grass came the concern of potential health hazards that follow. Grass tetany can be a major problem for livestock managers grazing livestock in early spring and fall as well. 

Grass tetany is a fast-moving and potentially fatal disorder caused by low magnesium level in the blood. This condition is referred to as hypomagnesemia. High-risk livestock are older lactating females, most commonly with high milk-producing females. Ruminants such as cattle, goats and sheep grazing young, succulent, green grass in early spring have a potential risk of developing the disorder. 

Symptoms of the disorder include imbalance, stiffness, uncontrollable muscle movement, convulsion and potential death. 

Why it happens

As an Extension educator, I often get asked why does grass tetany seem to occur more often in certain years or on different pastures. To answer that question, one must understand what causes the forage to have such a low amount of magnesium. 

I have heard my whole life that spring grass is nothing more that green water, and I would have to agree partially with that statement.  But grass still has some nutrition in early spring although of course moisture levels of the forage is higher depending on the weather. 

Cool, wet springs slow down the process of magnesium uptake and absorption in the plant. Cool season grasses tend to have less need for magnesium and, in turn, lower in magnesium levels in fresh forage or dried hay. Pastures that received high amounts of potassium and nitrogen fertilizers or manure tend to have higher risk potential. Acidic soils with pH levels below 6.5 will tend to inhibit magnesium uptake by plants. 

How to prevent

Now that we know a few potential reasons for the problem you might be wondering where does grazing management lies in the prevention of grass tetany? Believe it or not, proper grazing management practices can go a long way in preventing grass tetany. Here are a few considerations and practices to implement during your early spring grazing season. 

1. Look at the soil test — If you have a past soil test within 3 years that would work, but I recommend collecting a new sample this spring before grazing. The test will allow you to know just how much potassium and magnesium is in the soil available for plant uptake. 

A recommended pH for pastures should be 6.0-6.5. According to the tri-state fertility guide, the optimum levels for potassium is between 120-170 ppm and magnesium levels equate to 35-50 ppm depending on soil CEC levels. By knowing these numbers, this can help prevent unneeded fertilizer applications to pastures that could potentially increase the risk of grass tetany. On the flip side, it could help make adequate fertilizer rate to bring nutrients or soil ph. in balance. Dolomitic lime applications can be a great way to increase soil pH and magnesium levels at the same time but be patient because this is not a quick fix.

2. Add some legumes — Cool season legumes such as alfalfa and clover tend to have higher magnesium content. Mid-February through mid-March is a great time to frost seed clovers to boost pasture legume percentages. I recommend not going over 30% legumes. Higher percentages could cause a higher risk of bloat. 

3. Don’t forget about animal nutrition — A common practice is to feed high magnesium minerals. This is a good practice, but the problem is that many high magnesium minerals have poor palatability due to having a bitter taste. Addition of mixing salt to the mineral or addition of energy sources such as corn and dried molasses can not only make the mineral more appealing to livestock but improve digestibility and absorption. 

The two main sources of magnesium available to producers include magnesium oxide and magnesium sulfate. Read your mineral tags and making sure mag-oxide percentage equals 14% and maintaining a consumption rate of 1 ounce per head a day (for a lactating mature cow). 

Magnesium based minerals should be fed to livestock 45-60 days before grazing and it is key to move the mineral feeder as you begin grazing to allow fresh mineral to be available to 15- 20 cows per feeder. Before turning cows out to pasture make sure they are not hungry. Feeding dry hay throughout the beginning of the grazing season can help with preventing grass tetany (also keeps stools solid). 

4. Stage your grazing — As you begin grazing allowing younger animals to begin grazing early in the season first. Allow weaned calves, heifers, cows less than 5 years of age, younger ewes and does to graze pastures first. This group of livestock are considered lower risk and as forages mature, older lactating livestock can be turned out. 

Rotating the younger animals through paddocks at a quicker pace, grazing the top 2 inches of forage, and then allowing the older animals to graze the remainder of forage afterwards is another option. The key is to allow forages to reach the right height and stage of growth before grazing. Six inches should be minimum height. 


Grass tetany can be a scary and fast-moving health hazard associated with early spring grazing. With the extra management steps grass tetany issues can be avoided. Mineral supplements are just one tool in the toolbox but should not be depended on solely. Additional management steps such as proper grazing management and managing high risk livestock separately from other members of the herd or flock can go a long way in keeping livestock happy and upright this coming spring. Happy grazing! 


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