Thursday, July 18, 2024
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Managing to Prevent Grass Tetany

Cattle grazing on lush green pasture. Photo courtesy of Kay Dee Feeds

Is grass tetany a threat to your herd? It could be if you’re dealing with cool conditions and lush spring forage growth. It is what we call “washy” grass. This new growth is very high in water and nitrogen. It can also be fairly high in potassium. Fields that have been fertilized with nitrogen may actually be even more imbalanced. The big problem is, these forages can easily be low in magnesium, and without adequate magnesium, livestock are susceptible to grass tetany.

How Do You Protect Your Stock?

Pastures with a good grass-legume mix, generally at least 30 percent, are normally a little better than monocultures of grass because legumes carry more magnesium than grasses.

Grass tetany is generally more of a problem on fields that were grazed short last fall leaving very little or no dry matter behind to mix with the washy spring grass, and also in fields that have had early spring applications of nitrogen.  Once the forages start catching up to where they should be for this time of the year, and start maturing more, dry matter will increase in the forage, and nutrients will be more in balance.

I’ve watched cows, after two or three days in a lush spring field, start going to fence rows looking for some dry matter. If they really are needing some dry matter and it is available, most will seek it out and consume it eventually. This dry matter early on may be from left over forage from last year, leaves off trees, and even some twigs. You can help by having some hay available for grazing livestock to eat as dry matter along with the lush spring forage can also help keep the grazing animal’s rumen in balance.

Supplemental Feeds

I asked Dr. Lemenager about supplemental feeds, specifically corn gluten and its effect on grass tetany, because I had been asked the same question several times and he said, “Adding a protein source like corn gluten might seem odd when N is already high, but it is actually adding rumen undegradable protein (RUP) and a fiber-based energy supply which can be positive from a cow’s nutrition perspective, especially lactating cows and even more so for replacement heifers that have been gaining 1.5 pounds per day all winter and now they crash on energy. Without energy (carbon chains) the soluble nitrogen cannot be efficiently incorporated into microbial protein.  A problem with “washy” grass is that the rate of passage is very fast (hence manure can go through a screen door). This rapid rate of passage and lack of carbon chains is not conducive to good rumen fermentation of structural carbohydrates further aggravating the problem of energy balance. Bottom line is that feeding a dry feed like corn gluten helps slow rate of passage, provides an energy source to use the rumen degradable protein (RDP) fraction, and provides RUP and an amino acid supply to the small intestine. By itself, adding corn gluten doesn’t necessarily prevent grass tetany, but it should improve overall animal nutrition and help reduce the incidence of grass tetany.”

Feeding a high-magnesium mineral under these cool, wet, lush forage conditions is always a good practice. Lemenager recommends supplementing this mineral during times like these to make sure they are taking in enough.  Magnesium is bitter and cows don’t like it, so additives can help improve intake. If salt is fed free choice at the same time as the high-mag mineral, they may not get enough magnesium to help prevent grass tetany.

Dr. Lemenager also commented that soybean hulls, either fed alone or as a mixture with corn gluten, would be a better choice than corn gluten alone. The hulls aren’t as high in protein, but provide a readily fermentable fiber (no starch) and the carbon chains to use the rumen degradable protein. He likes a 50:50 mix which runs about 17.5 percent crude protein and on a forage diet has almost the same energy value as corn without the starch that reduces rumen pH lowering fiber digestibility.

Keep on grazing!

Want more details on grass tetany, how it works, and the kind of threat it represents? Check out this article!

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Victor Shelton
Victor Shelton
For more than 25 years, Victor Shelton, Indiana agronomist and grazing specialist with the Natural Resources Conservation Service, has provided advice about grazing’s best practices. He travels across the state conducting pasture walks, working one on one with farmers and participating in grazing talks. He also writes a newsletter called "Grazing Bites" as a way to talk about current and seasonal grazing issues and what farmers need to be prepared for.


  1. Thanks to Charlie Krauss for his comments. My wise dairy-farmer neighbor says this: “Always have dry available for cows when they are grazing.” I’ve noticed that my cows may graze on gorgeous grass/clover pasture and then see or hear me in the barn; they return and fill up on hay which I fork out for them. Also, this reference– –suggests that dandelions may help provide Mg and Ca for cows. We watch our cows enter a new paddock and invariably–after checking out the borders–go first for dandelions.

  2. A hundred years ago, dairy farmers around here followed a practice that protected them from grass tetany and made their lives easier.

    Just as soon as there was green grass, they would let the cows out to graze after the morning milking. They gathered them for the evening milking. At night, they fed dry fodder in a dry-lot so they wouldn’t have to gather them for the morning milking.

    Lush grass during the day and dry fodder at night made a better balance.

    A lot of cattle illness boils down to eating too much of something good – too much clover, too much grain, too much milk, etc.

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