Veterinary advice: Any form of stress can trigger a grass tetany case

The phone rang at 6.30 am.
“Come quick, I’ve a cow down with tetany.”

I threw on the clothes and as I was going through the kitchen, the phone rang again. “She’s gone.”

I put on the kettle, deciding to have some breakfast before the day got busy. Ten minutes later the phone rang again.

“I’ve another one down!”

The grass tetany season had started with a bang.

Grass tetany is the clinical manifestation of magnesium deficiency. Hypomagnesemia is the sub-clinical version.

Things can proceed from hypomagnesemia to full blown tetany very fast, and a very careful watch on stock is needed at this time.

Magnesium is one of the minerals essential for the normal working of a cow, or an ewe. Among other things, it is needed to help prevent the excessive transmission of nerve impulses.

A cow/ewe has a lot of magnesium in her body, but the most of it is in the bones and soft tissue, with only 2% circulating in the blood. This means that they need to take magnesium in every day.

In the normal course of events, the cow/ewe takes in her daily requirement of magnesium in what she eats, but at certain times of the year, what she is eating is unable to supply her requirements.

This usually happens when the cow is at grass.

When the pasture is growing fast, it will have a magnesium content lower than needed. This will be exacerbated by a high potassium or nitrogen content.

Usually, the application of fertiliser followed by rain and warm weather will drive on this type of grass, leading to grass tetany.

The cow is now in a finely balanced situation, where any form of stress (inclement weather, milking, weaning of sucklers in the autumn, sudden dietary changes leading to elevated rumen pH etc), will trigger a case of grass tetany.

We are all aware of the signs of full blown tetany, with the cow down, and kicking, thrashing about with the eyes rolling in her head, and no one wants to see this scenario.

Lesser cases involve the cow staggering and looking nervous, walking stiffly or slowly. or maybe just away from the rest of the herd. and not interested in grazing.

This cow can come in empty in the belly and slack in the dug.

At this time of year and again in the autumn, when the diet of the cow is likely to be deficient in magnesium, we need to supplement the diet with magnesium in order to protect the cow, both dairy and suckler.

There are a number of ways to do this.

The most common way in dairy cows is to feed concentrates with magnesium included.

This is a good way to ensure optimum levels, but sometimes cows leave the nuts after them, potentially leading to a crisis.

The common way in suckler cows is to give them the magnesium bolus which lasts six weeks.

It is an easy way of providing normal levels for six weeks, but odd cows regurgitate a bolus, and there is no way of knowing which cow did this.

Magnesium can be incorporated into mineral licks or tubs of molasses.

Again, we are dependent on cows making use of these.

Cows uniformly eat grass and drink water, so dusting the pasture before the daily grazing, or putting magnesium into the water trough, means that every cow will ingest magnesium, every day.

These methods are not without problems, such as causing extra labour.

The paddock has to be dusted each morning before leaving out the cows. The water trough has to be adjusted, depending on the weather.

No matter what your situation, there is a method to suit you.

If you need to talk to anyone for clarification or advice, then your local vet will only be too happy and willing to talk to you.

Whatever you do, don’t take a chance.

Cutting costs can lead to a lot of problems. and thereby. major losses.


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