Grazing in spring’s new grass can lead to livestock disease


The ground has firmed tremendously in our area the last two weeks and warm temperatures have launched grass growth again in most fields so that pasture rotations may be started.

As daylight lengthens, weather warms and pastures grow, farm managers should be aware of the term hypomagnesemia or “grass tetany.”

Turning cows or sheep out into new lush pastures can cause lowering of blood levels of magnesium and an imbalance of electrolytes. This dangerous and unwanted condition is increased in pastures if nitrogen or manure is applied in the spring, if soils contain high levels of potassium or low soil pH conditions exist.

There is a relationship between soil phosphorus content and magnesium uptake in forages too. If phosphorus is low, even if soil magnesium is high, the plant may not take up the magnesium in adequate amounts to meet the cow’s needs.


Early spring applications of triple super phosphate (0-46-0) may help in this situation. If soil tests show your pH is low and magnesium is low, adding high-mag limestone (dolomitic limestone 12 percent-15percent Mg) will increase the pH while adding magnesium to the soil.

Soil analysis showing less than 100 pounds magnesium per acre has high potential for grass tetany. If your soil pH is adequate, but magnesium is low, consider adding magnesium oxide to the soil to bring levels up to minimum requirements.

Adding these nutrients to soil will change amounts in the plants, but it will take time and is not a quick solution to magnesium deficiencies.

Grass tetany is associated with cool weather in spring and fall because the metabolism of the plant is slower and its mineral uptake from the soil is lower, leading to lower magnesium in the forage the livestock eats. Grass tetany is more common on grass pasture than legume pastures because legumes tend to have higher magnesium levels in their leaves.


Early symptoms of grass tetany are muscular weakness, followed by uncoordination that progresses until the animal can no longer get up.

Animals do not store magnesium in their bones as they do other minerals. Magnesium is stored in soft tissue and must be ingested on a daily basis.

Cattle most likely affected by grass tetany are older cows and high producing animals after they have calved. Grass tetany can generally be avoided by feeding minerals with high magnesium content or supplemental magnesium.

Commercial mineral mixes that are effective in preventing grass tetany are available, commonly called high-mag mixes; these minerals contain 12 percent to 14 percent magnesium.

Make your own

Magnesium oxide is an inexpensive source of magnesium if you want to formulate your own mix. However, mag-ox is not readily accepted by animals so it needs to be incorporated with something such as dried molasses, minerals, concentrated feeds or salt supplements.

Magnesium oxide is about 60 percent magnesium, so the cow should consume approximately 1-2 ounces of mag-ox per day to maintain acceptable levels of magnesium in their diet during spring and fall.

Intake of supplemental magnesium should be monitored regularly to be sure lactating animals are consuming proper amounts during the high risk periods.


If you are unsure about magnesium levels in your forages, you can have forage samples analyzed to provide the information needed to supplement with adequate levels of magnesium and avoid grass tetany in your livestock.

(The author is an OSU Extension Agriculture and Natural Resources Extension Educator in Monroe County, Ohio. Questions or comments can be sent in care of Farm and Dairy, P.O. Box 38, Salem OH 44460.)


Up-to-date agriculture news in your inbox!


  1. we feed 2kgs.of consentrate; containing is fed during milking. some members of our farming group spread cal mag.poweder on the pastures prior to grazing very good results in ireland

Leave a Reply to derry o donovan Cancel reply

We are glad you have chosen to leave a comment. Please keep in mind that comments are moderated according to our comment policy.

Receive emails as this discussion progresses.