In the spring and fall, the risk of Hypomagnesemia in cattle unfortunately increases.

Hypomagnesemia is similar to milk fever, but in the case of Hypomagnesemia we speak of a magnesium deficiency. 

This occurs when nitrogen doses to the grass are too high, and/or the feed ration is too potassium-rich (grass). 

A high potassium content in the grass reduces the absorption of magnesium in the intestine.


The disorder is more common in cow-calf herds that graze on the pasture outside day and night.

Dairy farmers with balanced feed rations rarely have head Hypomagnesemia among cows. 

It is more frequent in pasture-raised and extensively farmed cow-calf herds.

Hypomagnesemia is a direct result of magnesium deficiency in the bloodstream. 

It has several causes:

1. Due to the fact that (lactating) cows graze on unrestricted pasture grass during cold and wet weather conditions, the food in the intestines has an excessively high passage rate.

The magnesium in the grass is therefore not absorbed sufficiently into the bloodstream.

This reduced absorption capacity is caused by "water grass", (grass with a high leaf content, high moisture content, too much unstable protein and not enough structure).

2. In cold weather, grass absorbs little magnesium and calcium from the soil.


Lactating cows actually absorb more grass in cold spring and fall weather and therefore give more milk.

This puts them in a vicious circle. A grazing cow that suckles a healthy calf has a daily milk production of between 10 and 20 liters.

With the milk, the cow gives away a lot of magnesium and calcium.

She MUST replenish both of these minerals with the daily ration!




Normally pasture grass contains sufficient of these minerals but in 3 situations there is serious risk for lactating cows of deficiencies:

- on heavy clay soils, the magnesium content in the grass is low because it can be absorbed poorly from the soil.

- on pasture grass that has grown after (too) heavy fertilization with nitrogen or with slurry from pigs or cows.

- on pasture grass in spring or autumn because the grass then has little structure ("water grass"). The mineral content of magnesium and calcium in this grass is too low.


If one or more of the above situations occurs, the first symptom in the cow is (very) watery manure.

If you see this in lactating cows you should immediately adjust the ration. Feed dry matter with lots of structure-rich hay.

This will restore the mineral utilization in the cow. 

Magnesium-poor soils can be fertilized with magnesium-containing fertilizers.

Feeding magnesium and calcium in concentrate is also an option.



In cases of Grass tetany, the cow eats less, the abdominal size increases and the cow has diarrhea.

The milk production has decreased, the cow separates from the group and her ears are tightly pointed backwards.

The animal walks wobbly and stiffly. This can quickly progress to lying completely flat on one side.


Sometimes the earlier symptoms are not clearly visible.

The animals have cramps and the neck is stretched backwards. Some animals thrash their heads and are unable to stand.

The body temperature at this stage is too low, (37.5 degrees Celsius). Tremors and cramping of the muscles occur.

In blood tests, you will see that the magnesium level in the blood is normally 0.66 - 1.07 mmol/L.

In cows with Grass tetany, this magnesium level is sometimes less than 0.12 mmol/L.

The blood calcium level is normally 2.43 - 3.10 mmol/L.

With significantly lower blood calcium levels (e.g., 2.05 mmol/L.), combined with low magnesium levels, the probability diagnosis of Grass tetany can be confirmed.




Treatment consists of a calcium-magnesium infusion. This is the same infusion used in the treatment of milk fever. 

Often the animals recover quickly after the infusion. At the same time, the feed ration must be adjusted immediately to prevent recurrence.

If Grass tetany is diagnosed in one animal, please also pay attention to the other animals in the herd!


Your Cowmunicator,

Ronald Rongen