Grass Tetany (hypomagnesaemia) may occur throughout the year on both tropical and temperate pastures. Grass tetany has been known under a variety of names including magnesium tetany, lactation tetany, grass staggers, magnesium deficiency and pasture flush staggers. Grass tetany is usually confined to medium and high-rainfall regions where improved pastures have been established. It is regarded as economically significant because, when it strikes, mortality rates can be high and it can be the biggest cause of deaths in any given herd in any given year. Grass tetany is a nervous disorder and occurs when the magnesium (Mg) concentration in the cerebrospinal fluid (CSF - fluid surrounding the brain & spinal cord) falls below a critical level. In the development of grass tetany, the level of Mg in the blood decreases before the level in the CSF.
Signs and Symptoms
The normal magnesium content of blood serum in cattle is within the range of 17-40 mg of magnesium per litre of blood serum, but levels below 17 frequently occur without clinical symptoms of disease. Hypomagnesemic tetany is usually preceded by a fall in blood serum magnesium to about 5 Mg/L (McDonald, 1995). The low Mg levels in blood serum and CSF leads to hyper-excitability, muscle spasms, convulsions, respiratory distress, collapse and death. For most farmers, the first sign that grass tetany is a problem is when cows are found dead in the paddock. There is usually froth from the mouth and nose and the ground is rubbed where the animal’s legs have moved violently before death.
Grass Tetany risk factors:
- Stock grazing immature, fast-growing, grass-dominant pastures or lush cereal crops such as Rye, Oats, Wheat or Barley.
- Cool and cloudy weather conditions reduce plant uptake of magnesium.
- Anything that reduces feed intake, reduces magnesium intake (e.g. yarding, transport and stress). Increased demand for magnesium by stock during lactation or late pregnancy and stress periods such as weaning is a key note to remember.
- Many soils are low in calcium and magnesium. Soils high in magnesium are still a risk for Grass Tetany during periods of lush green growth.
- Elevated levels of potassium or aluminium in soil or the heavy use of nitrogen and potassium fertilisers can reduce the availability of magnesium in pasture.
- Magnesium absorption may be reduced by high potassium levels in the rumen. This effect increases if phosphorus is also deficient.
- Increases demand for magnesium by stock during lactation or late pregnancy and stress periods such as weaning.
- Grasses that contain high oxalate levels impact on magnesium absorption.
- High risk animals are often fat and losing condition. Older cows with young calves are most vulnerable.
Elliott (2009) stated that grass tetany prone pastures have the following features:
- Magnesium (Mg) concentrations < 2 g/Kg of dry matter.
- Calcium (Ca) concentrations <3 g/Kg of dry matter.
- Sodium (Na) concentrations <1.5 g/Kg of dry matter.
- Potassium (K) concentrations >20 g/Kg of dry matter.
- Nitrogen (N) concentrations >50 g/Kg of dry matter.
Prevention is vital as grass tetany often occurs without warning
The best prevention strategies are:
- Increase magnesium intake of at risk groups of animals with a daily supplement. MegaMin Extra Magnesium and MegaMin Extra Magnesium Sweet to be used for stock grazing lush pastures and experiencing stress to help meet the extra magnesium requirements during these times.
- Because mature fodder has a higher magnesium level in the stem, it is recommended to give animal's access to roughage and legume hay during high risk periods.
- If possible avoid grazing high risk animals (older, pregnant and lactating animals) on high risk pastures.
- Avoid subjecting animals to unnecessary stress (mustering, yarding and transporting) during risk periods.
Good management is essential to help minimize the losses that can be associated with Grass Tetany. Preventing one death can more than cover the cost of supplementation with MegaMin Extra Magnesium.
Shannon Godwin BAppSc GDTL
Elliott, M. (2009). Grass Tetany In Cattle. Taree: NSW Department of Primary Industries.
Hungerford, T. (1990). Diseases of Livestock (Vol. 9th Edition). Roseville, NSW, Australia: McGraw-Hill Book Company Australia Pty Ltd.
Kahn, M. (. (2005). Merck Veterinary Manual (Vol. 9th Ed.). Whitehouse Station: Merck & Co., Inc.
McDonald, P. E. (1995). Animal Nutrition (Vol. Fifth Edition). Essex: Addison Wesley Longman Limited.
Sackett, D. &. (2006). Assessing the Economic Cost of Endemic Disease on the Profitability of Australian Beef Cattle and Sheep Producers. Sydney: Meat & Livestock Australia.
How To Maximise Weight Gain When Feeding Lush Green Fodder
It is common for stock to have low weight gain, or even lose weight in the first 2-3 weeks of grazing lush fodder.
Fibre is essential for rumen digestion
Adequate effective fibre in the diet is essential to slow the rate of rumen digestion. The other critical factor is that Rumen Forage Fibre Digesting Microbes' take 3-4 weeks to adapt to the change in fodder.
Because these microbes multiply very slowly it is good practice to supply reasonable quality hay or daily access to dry grass at least one week prior to grazing lush fodder as well as the next 2 -3 weeks to ensure sufficient fibre for cud chewing and a healthy rumen.
After this period, the green fodder will gradually mature and the level of plant fibre will increase. The fibre digesting microbes should also have adapted and multiplied to better ferment the new fodder, assisting stock to perform to their potential. The manure will always tell you if there is enough fibre in the diet.
Mineral deficiency can be a limiting factor
With energy and protein in adequate supply, minerals then become the next limiting factor.
There is obvious need to supplement livestock to maintain condition or avoid losses during winter and dry conditions. Not so obvious are the clinical or sub-clinical mineral deficiencies that can restrict animal performance during good seasons or periods of lush pasture growth.
For example, with lush feed, phosphorus is required at an increased rate to maximise weight gain.
Magnesium supplements are also vital. Plant uptake of magnesium is reduced in the cooler months and because magnesium is the only mineral absorbed in the rumen (and lush fodder passes through the rumen quickly), the rate of absorption may be reduced, leading to a potassium: magnesium imbalance which can cause 'staggers' or grass tetany.
To avoid clinical or subclinical mineral deficiencies that limit productivity, it is important to provide broad spectrum minerals for microbial growth and animal performance.
With the cost of MegaMin Extra Magnesium being approximately 12-15 cents per day for cattle, providing minerals and trace minerals is a 'cheap insurance' to bridge the mineral gap that may be lacking in pasture, helping to ensure maximum performance for your livestock.
Gary Zerner (Sales Manager)