Grass Tetany - What is it and how do you avoid it.

30/05/2011 3:23:38 PM

Dr Colin Trengove

Grass tetany is the main cause of deaths of adult beef cows on farms in south-eastern Australia. The problem is worse in some years accounting for losses as high as 50% in individual mobs and an estimated 1% of all lactating beef cows in this region annually.

 

What is grass tetany?

Grass tetany is a nervous disorder seen as muscle spasms, tetany, convulsions and death as a result of low blood magnesium concentrations.  Cows may be found dead or only show signs when disturbed.  The nervous disorder arises when the concentration of magnesium in the fluid which surrounds the brain and spinal cord falls below a critical level. 

 

Cows older than six years are more commonly affected with grass tetany than younger cows as their milk yield is generally higher and they are usually fatter - both factors reduce blood magnesium concentration. Magnesium intake is limited if pastures are very short (ie less than 4 cm average height) or it is deficient in magnesium. Blood magnesium levels in many 4-8 yo lactating beef cows decrease below critical levels in May - August in south-eastern Australia, but most do not develop grass tetany.  It usually takes a "triggering event" to cause deaths from grass tetany and these include:

• short-term starvation - such as yarding the cows for calf marking,
• putting the cows on a fresh grass paddock low in magnesium,
• application of potash fertiliser in late autumn,
• inclement weather reducing grazing time & magnesium intake.

 

When cows graze lush grass pastures typically low in fibre they do not salivate as much as when they are fed dry pasture or hay.  Early pasture growth is also high in nitrogen & potassium which compete with the uptake of magnesium. As a consequence, dietary magnesium is restricted, electrical activity in the brain increases, and the cows develop signs of grass tetany - excitement, convulsions, collapse, respiratory distress and death.

 

If the diet allows lactating cows to maintain adequate blood calcium grass tetany does not develop even though they may have low blood magnesium levels.  This is why it is recommended to provide hay daily to older lactating cows in the critical months to prevent grass tetany as it helps to keep their blood calcium concentration at the desired level. It also assists in maintaining adequate magnesium concentration in the brain and central nervous system.

 

Cows younger than six years can also be affected by more complex forms of grass tetany associated with high potassium intake and low sodium and phosphorus nutrition.  Soils and pastures high in potassium e.g. red gum soils (solodised solonetz) as well as low in phosphorus and sodium are high risk for grass tetany in all ages of lactating cows. Hence, application of potash fertiliser in autumn in grass paddocks is strongly discouraged in autumn/winter calving herds.

 

 

Incidence

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

 

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.  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 per day to prevent a 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 and therefore higher output of magnesium.  Milk production of pasture fed cows usually plateaus around four years of age, while the ability to absorb magnesium decreases after this age – explaining why older cows are most prone to grass tetany. 

 

Risk assessment

As part of their risk assessment, the concerned producer needs to consider the following:

 

(i)  Soil / Pasture analysis

• soil test prior to the risk period to assess the soil calcium, magnesium, potassium and sodium  balance and predicted availability of magnesium for uptake by pasture;
• pasture test during winter to determine the grass tetany risk index and best prevention strategies; and
• feed on offer assessment to estimate daily feed / magnesium intake by grazing cows

 

 (ii) Stress situations

• avoid yarding and running cattle around at high risk times;
• avoid grazing older lactating beef cows on short green grass pastures; and
• avoid high potash soils

 

(iii) Time of calving

• changing to a later calving can prevent the problem, but
• consider the effect on cash flow and net farm income?

 

(iv) Herd age structure

• Herds with a high proportion of old cows can expect a higher incidence of grass tetany.  These herds typically:

-        calve heifers down at 3 years of age;

-        have an extended joining of 4 months or more;

-        do not pregnancy test and cull empty cows; and

-        are understocked with excessively fat cows (at the beginning of autumn)

 

 

Treatment

A cow may show nervous signs when disturbed or being yarded.  Initially the cow may walk stiffly, or gallop madly or bellow loudly, before going down and remaining recumbent.  The cow has an exaggerated response to sight, sound or touch, and may begin convulsing, with muscle spasms, kicking, rolling eye-balls, head arched back and frothing at the mouth.  Death can occur within minutes of the first signs.   An injection of one or two packs of calcium and magnesium solution should be given under the skin as soon as cows show the nervous signs.  It may take 1-2 hours for cows to recover after treatment because of the time required to restore the magnesium balance in the fluids around the brain.

 

Start feeding the cow hay as soon as possible after she recovers, and consider the following strategies for supplementing the rest of the herd.

 

 

Prevention

There is no single preventative strategy for all farms.  Soil & pasture tests are useful tools to assess the grass tetany risk and develop preventative strategies. An analysis of past cow losses on a property can indicate which age groups are most at risk allowing a substantial reduction in the cost of labour and supplements by targeting the high risk mobs.

 

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 amount of dry standing residues.  Some herds may require addition of magnesium either as Causmag, 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 cows may need hay, magnesium, salt and phosphorus supplementation.  Correction of sodium deficiency aids prevention of grass tetany, but provision of salt to cows receiving adequate sodium intake 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.

 

The following is a list of options used to control and prevent grass tetany and it is recommended that some or all should be used depending on the risk factors identified in each situation:

 

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. Daily treatment is essential for the duration of the risk period.

           

Medicated water troughs

Addition of magnesium sulphate (Epsom salts) or magnesium chloride (Mag C) 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 grass pastures. 

 

Magnesium Licks

Magnesium licks are considered to offer variable 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.

 

Magnesium capsules

Intraruminal Magnesium devices (Sire Sine Magnesium Capsules) are marketed as an aid to grass tetany prevention in cattle and release about 1.5 g Magnesium/day for 90 days after a stabilising period of one week.  It is recommended that cattle which receive these devices are also fed hay and the devices used as an alternative to adding Causmag to the hay.

 

Foliar Sprays

Magnesium sulphate can be applied as a foliar spray at 3 kg/ha to supplement pasture magnesium & should be repeated monthly during the high risk period May - September. The inclusion of 1kg/ha Solubor boron will help to reduce the high pasture nitrogen from impeding the uptake of magnesium. Trace elements such as copper sulphate 0.5kg/ha and cobalt sulphate 0.1kg/ha are also included to assist in the uptake of calcium.

 

Topdressing

The application of dolomite or magnesium sulphate as a fertiliser at 100 – 200 kg/ha prior to winter has also served as a successful preventative in some situations. However, an initial soil test is recommended to assess the risk of grass tetany and establish the best fertiliser strategy for disease prevention.