Grass Tetany 

Ivan Caple,Professor of Veterinary Medicine University of Melbourne, Veterinary Clinical Centre, Princes Highway, Werribee 3030

GRASS TETANY has been the main cause of deaths of adult beef cows on farms in Victoria over the past 30 years. The problem is worse in some years than others, and there are horror stories told of losses as high as 50% in individual mobs of lactating beef cows.

Grass tetany is an annual worry for the beef producer, and those not wanting to take any risks put out supplements each year. The traditional method for preventing grass tetany by feeding out hay and Causmag is messy, and is difficult to do when paddocks are wet. Many producers wait until they have lost cows, or hear of others losing cows from grass tetany in the district before supplementing cows.

Diet risk factors for grass tetany:


What is grass tetany?

Grass tetany is a nervous disorder seen as muscle spasms, tetany, convulsions and death. Cows may be found dead in the paddock, others may show the nervous signs when disturbed or being yarded for calf marking. The nervous disorder arises when the magnesium concentration of the cerebrospinal fluid which surrounds the brain and spinal cord, decreases below a critical level.

There are several types of grass tetany syndromes which can be diagnosed on farms in southern Australia. These differ according to the ages of the cows affected, and the factors causing the nervous disorder. Knowing what type of grass tetany occurs on a farm enables more cost-effective prevention methods to be used.

  • Cows older than six years are more commonly affected with grass tetany than younger cows if they are overfat at calving (body condition scores 4 to 5 on a 1 to 5 scale) and lose liveweight (up to 1 kg/day) during lactation when pasture is very short.
  • Cows younger than six years are also affected in beef herds with complex types of grass tetany syndromes associated with high potassium intake, and low sodium and phosphorus nutrition.

Lactating cows are most at risk because they lose magnesium in milk. There is only a small reserve of magnesium in body fluids. Cows have to absorb enough each day to maintain their blood and brain levels while losing magnesium in milk. Essentially no magnesium is obtained from the body reserves mobilised to support lactation when cows lose bodyweight. Unless these cows are fed additional magnesium, the magnesium levels in blood and brain will decrease and they will become very susceptible to grass tetany. A beef cow requires about 4 kg hay a day to prevent an weight loss of 1 kg a day.

The selection of beef bulls based on growth rate of calves means that cows are being bred for high milk yields. Since the milk production of cows fed pasture usually reached a plateau after four years of age, it was usually only cows older than this which are prone to grass tetany.


The occurrence of grass tetany is seasonal, and its severity varies from year to year depending largely on climatic conditions, pasture availability, the proportion of dry residues in winter pastures, and the clover and grass composition of green herbage.                                                                 '

Autumn calving beef cows are always going to be at risk to grass tetany. These cows usually become fat over spring and summer and, after calving in the autumn, lose bodyweight during lactation between May and September.

Inclement weather, and management procedures such as yarding and transport which result in reduced magnesium intakes, predispose lactating cows to the disorder. Commonly, grass tetany occurs when cattle re­commence grazing lush pastures or cereal crops after being fed on hay during inclement weather. 

What are the important factors causing grass tetany?

Magnesium is absorbed mainly from the rumen and forestomachs in cows.

(a) low Magnesium intake

This can arise simply through a reduction in food intake when cows are grazing short grass dominant pastures containing less than 0.2% Magnesium (DM basis). Low herbage availability (less than 1000 kg DM/ha) results in liveweight losses during lactation, and may result in losses of older cows from grass tetany even though pasture Magnesium concentrations are within normal limits.

(b) factors which reduce Magnesium absorption

Excess potassium.

Potassium is the main factor which reduces magnesium absorption from the rumen.

Potassium concentrations in the rumen increase when:

  • Cows graze pastures on natural soils high in potassium;
  • Cattle graze pastures fertilized with potash;
  • Cows are deficient in salt (sodium); or
  • The diet is changed from hay or dry feed to lush pasture.

 (ii)        Phosphorus deficiency.
This leads to low concentrations of phosphorus in the rumen and exacerbates the effects of high potassium intakes on magnesium absorption from the rumen. Low phosphorus also results in reduced absorption of calcium from the rumen.

(iii) High ammonium ion concentrations in the rumen also reduce magnesium absorption and this effect is additive and independent of that of potassium. High ammonium ion concentrations may occur following ingestion of herbage treated with nitrogen fertilizers.

(c) The cow's ability to maintain blood calcium

Low blood calcium concentrations also play a role in the development of grass tetany in cows in some herds. When blood calcium decrease this may trigger the brain levels of magnesium to decrease and cause grass tetany. The ability of the cow to maintain plasma calcium depends on her acid-base balance which may be affected by roughage intake. Feeding hay to cows is one way of ensuring cows absorb sufficient calcium to maintain their blood calcium levels.

(d) Other factors involved in grass tetany include grazing management and provision of shelter, and husbandry procedures which involve a reduction of food intake in high risk cows. Some herds only experience losses from grass tetany when cows are taken off paddocks for a day at calf marking.

As cows age they tend to lay down more body fat. Cows older than 6 years are at most risk to grass tetany, particularly if they were overfat at calving and lose liveweight during lactation. Fat cows have less available magnesium in their body fluids on a weight basis than lean cows.


What type of grass tetany causes losses on your farm?

A history where lactating beef cows have died suddenly after showing signs of tetany and convulsions is suggestive of grass tetany. It is important to establish what factors are involved before recommending any general preventive measures be applied to all cows on a farm. The benefits, costs, efficacy and ease of integrating the options with current or innovative management practices should be assessed.

It is important to establish that grass tetany is the cause of death in cows found dead in the paddock. Usually there are signs of scuff marks on the ground caused by the cow thrashing around during a convulsion. Samples of cerebrospinal fluid can be analysed to provide a diagnosis, and this is a reliable indicator for up to 12 hours after death. An eye can be removed and sent to the laboratory as the magnesium concentrations in the eye fluids provide a reliable indication for up to 48 hours after death providing environmental temperatures do not exceed 23°C after 24 h. 

Prevention of Grass Tetany

The beef producer has to make several decisions concerning the prevention of grass tetany:

1.Do I need to supplement my cows?

2.If so, a) what do I supplement with?

  1. b) which cows do I supplement?
  2. c) how long do I supplement them for?

3.What is the cost of grass tetany on my farm?

This should be assessed by considering losses over 5 to 10 years: . number of cows lost. did they all die from grass tetany?

(Other conditions causing sudden death in lactating cows in winter include: clostridial diseases; mycotoxicosis; mastitis; milk fever; bloat)

4.Can any management strategies causing grass tetany be identified, and changes made to prevent the problem?

(i) stress situations - avoid yarding and running cattle around at high risk times

(ii) time of calving - would a change in calving time prevent the problem, and if so what is the effect of changing time of calving on net farm income

Example herd: 100 cows; 1991 - lost 8 cows, but usually lose about 2/year; cause not always certain. Estimate lost 16 cows over past 5 years from grass tetany.

losses = 16/500 or 3.2%/year

cost: $500/cow ($450 cow, $50 calf) losses = $1600 per year (or $16 per head)

Were there "high risk" factors associated with these losses?

In the example herd, all cows lost were 6 years or over, except for one 4 year-old cow lost in 1990. This information becomes important in deciding which cows to treat, and on the method of supplementation.

Options for prevention of grass tetany

There is no single preventative strategy for all farms. An analysis of past losses on a property can indicate which age group of cows are most at risk, and can substantially reduce the cost of labour and supplements required for prevention

Feeding hay alone may be all that is required to prevent grass tetany in some herds where only old cows are affected. As an alternative to feeding hay, older cows may be grazed on paddocks containing the most dry standing residues. Some herds may require addition of magnesium either as Causmag, by slow release capsules, or in licks.

It is more common to see younger cows affected (younger than 4 years of age) when more than one complicating factor is present.

In more complex situations we have had to provide cows with hay, magnesium, salt and phosphorus. Correction of sodium deficiency aids prevention of grass tetany, but provision of salt to cows receiving adequate sodium intakes may be detrimental.

Since there is no readily available store of magnesium in the body, magnesium supplementation has to be given daily to animals at risk to grass tetany. Most Magnesium salts are quite unpalatable and an important practical aspect in feeding supplemental magnesium is combining it with other palatable ingredients such as molasses, concentrates and hay. 

Magnesium supplements.

Hay treated with magnesium oxide (Causmag) is the most common supplement. Daily Causmag required for cattle is 60 g/cow/day. After supplementation commences it takes 2 to 3 days before cows are protected, and protection ceases as soon as this supplementation is stopped. 

Medicated water troughs

Addition of soluble magnesium salts (magnesium sulphate or Epsom salts) to water troughs at a rate of 3 g/litre can be effective if cows drink the water. It is usually an unreliable method because water intake by cows is generally low when they are grazing lush tetany prone pastures. 

Magnesium Licks

Magnesium licks are considered unreliable for 100% protection because some cows are not good lickers, and licking behaviour may be intermittent. More cows accept licks if they have been exposed to licks as calves.

Home-made recipes for licks for extensive grazing situations include:

Causmag:Molasses                                                                    1:1

Salt: Molasses: Causmag                                                           5:3:2

Salt: limestone: DCP:Molasses:Causmag                                  1:1:1:1:1

Most commercial Magnesium licks and blocks contain up to 80% molasses. Licks and blocks should be placed near watering points or stock camping areas. Crusts which develop on licks should be removed, and licks should be moistened before cows are allowed access. 

Treating a cow showing early signs.

An injection of one or two packs of solutions containing calcium and magnesium under the skin should be given as soon as cows show the nervous signs.

It is best not to disturb the cow until you are ready to treat her as excitement can precipitate convulsions. So tie the dogs up before you attempt to treat the cow. The cow should be left after treatment to get up by herself.

Start feeding the cow hay as soon as possible after she recovers, and consider adding Causmag and supplementing the rest of the herd. 

Does your management contribute to the problem of grass tetany ?

Older lactating beef cows should not be grazed on short green pastures on paddocks fertilized with potash.