WINTER TETANY AND GRASS TETANY by Mike Mehren Ph.D.

WINTER TETANY AND GRASS TETANY
BY MIKE MEHREN

We don’t see much written about winter tetany, because most of the tetany occurs in the spring. Most typical tetany happens when spring-calving cows with young calves (less than 3 months old) are turned out to lush spring grass. The cows in most danger are mature cows raising the biggest calves, so it really hurts! Old cows or heifers rarely get tetany. However winter tetany occurs before calving, when cows are being fed grass hay, grain hay, or straw.

Winter tetany is similar to grass tetany that occurs in the spring because it is caused by a lack of magnesium and calcium along with an abundance of potassium. Most grass hays, grain hays, and straws are abundant in potassium but are low in calcium and magnesium. When fed for a long period of time, the cows are not able to mobilize enough calcium and magnesium from their bones to offset the amount of potassium.

The following table shows the mineral content for Calcium, Magnesium, and Potassium of different hays.

Hay Name	Calcium	Magnesium	Potassium
	…percent of dry matter…
Grass Hay (meadow)	0.44	0.13	1.77
Oat Straw	0.23	0.11	2.53
Wheat Straw	0.17	0.09	1.41
Alfalfa Hay	1.24	0.22	2.40

Sometimes a formula is used to predict the probability of tetany occurring. It is referred to as the ‘tetany ratio’. The potassium content of the feed is divided by the total of calcium plus magnesium. For example:
for oat straw the formula would be 2.53 divided by .23 + .11 or .34. ( 2.53
divided by .34 = 7.44). On the other hand, alfalfa would be 2.40 divided
by 1.46 = 1.64. Any feed having a tetany ratio of 2.2 or more is considered suspect. Our oat hay is 7.44 or highly probable, while our alfalfa is 1.64, which would be very safe. You might use this info before you begin winter feeding. Most of you have your hay tested at a forage lab to determine if there is enough protein. (In my dreams!) Rather than stopping there, have the lab check the ‘macro’ minerals. This will give you information on Calcium, Phosphorus, Magnesium, Potassium, Sodium, and Sulfur. Having these figures helps you decide if you need extra protein, extra phosphorus, and you can calculate the chance of getting winter tetany with your hay or straw. If the chance of tetany is high, you can feed some alfalfa hay in addition to your hay or straw. The alfalfa will provide protein, phosphorus, and higher levels of calcium, and magnesium. In most instances a ‘high mag’ or spring time mineral will further improve your tetany protection. In some areas, such as the Wallowa Valley of Northeastern Oregon, it is almost taken for granted that winter tetany will occur when feeding grass hay unless something is done to provide extra calcium and magnesium. If you are new to an area that has this problem, don’t assume that your hay will be ok because you’re farther up the valley, or have fertilized your pasture. Chances are that your cows will be in danger too. This winter, due to really high alfalfa hay prices, I’ve seen more winter tetany in areas where it is not considered normal. I think this is due to feeding grass hay and straw for a longer period of time and in greater amounts than is normally fed as the cows near calving.

Grass Tetany primarily occurs after calving. The symptoms are the same
for either kind of tetany. In most instances the cow isn't seen until after she is dead. The time from onset of tetany to death is between 4 and 8 hours. Symptoms include a staggering gate, extreme excitement and aggressive behavior, thrashing and death. If an animal is spotted in the early stages, a solution of calcium and magnesium can be administered I.V. Administering this solution too fast may cause death; care should be taken. The cow can also be given an enema of magnesium chloride. Although this presents it’s own set of problems, it is quite safe...as far as the animal is concerned.

If you have to treat or lose an animal to tetany, you should assume that the entire cowherd is at risk. Minerals high in magnesium and calcium should be provided to all immediately. Magnesium sulfate (Epsom’s salts) can be dissolved in water if the cattle are watering from a tank. You need the size of the water tank and an estimate of daily water intake to calculate the amount of Epsom’s salt to add to the water. Unfortunately, cattle may not drink water from a tank because the forage has such a high moisture content and a creek or pond rather than a tank provides water. Allowing limited access to the tetany-prone forage can also work. The cows are filled up on hay and then turned out to graze for several hours before being locked up again. This is continued for at least a week, increasing the grazing time each day. If a more mature pasture is available this may also serve as temporary fix. As the lush spring grass matures, the chance of tetany declines. I’ve never seen an explanation for this; we know that protein and some of the organic acids decline as forage matures, which may implicate them as factors in the cause of grass tetany.

Tetany, whether during the winter or shortly after calving, is a costly nutritional disease. It can be prevented by feeding management. Feeding some alfalfa hay along with meadow hay or grass straw during the winter is the first step. When coupled with a mineral supplement high in magnesium and calcium it can normally be prevented. Grass tetany in the spring is usually controlled by feeding a mineral with high levels of calcium and magnesium. One very important point: cows don’t build up a store of magnesium and calcium in their bodies that can be drawn on when a tetany challenge occurs. They need to consume the supplement almost every day. Magnesium fed last week or last month is of no benefit.

Michael J. Mehren, Ph.D. is a livestock nutritionist trying his darndest to avoid either giving or receiving a magnesium enema; his present location is unknown. He may be contacted by Email at mehrens@eotnet.net.