Grass Tetany

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Grass tetany (cattle), grass staggers (sheep) and hypomagnesaemia (hypo = low, = low magnesium) are names given to a condition that can affect stock in late autumn, winter and spring. It can cause significant losses in production, even when there are no signs of illness.

Grass tetany or grass staggers occur when blood magnesium levels fall below a critical level. This occurs when animals are running on pasture, which has low available levels of magnesium, or as a result of increased body demands for magnesium during lactation. Most commonly this occurs in animals grazed primarily on fresh, lush, rapidly growing pastures, which have often been heavily fertilised with nitrogen and/or potassium and have a low DM (dry matter) and Magnesium content. Younger pastures are more likely to contribute to grass tetany than older pastures.


Signs of grass tetany include:



Muscle twitches

An over-alert appearance


And in some cases aggressiveness

In severe cases animals may fall down into convulsions or just die without warning


Common Factors that are usually present with grass tetany:

Animals are usually grazing lush young pasture or lush cereal crops often without hay supplementation

Cold, wet and windy weather with little or no shelter, resulting in periods of reduced feed intake

Animals are often fat and losing condition or are very thin.

Animals have recently been moved to a different paddock

Heavy use of nitrogen and/or potash fertiliser on pasture

Cows in lactation are commonly affected, but dry cows and beef steers may also suffer.



Treatment must be prompt to be effective

 It is best to give either a 4 in 1 flow pack or a mag-sulphate flow pack under the skin in the area behind the shoulder and over the ribs. Massage the area well to spread the fluid and aid its rapid absorption into the blood stream. Then call a vet. If the animal is still up, do not leave the cow in the crush. If the animal is convulsing, care should be taken when approaching the animal and vet advice should be sought immediately.

Treated animals should be given adequate shelter and identified so that a response to treatment can be judged. Cows with grass tetany do not respond as quickly to treatment as a cow does with milk fever. So repeat treatments and care of the animal (i.e. providing shelter etc) are very important aspects of treatment.

If you have an animal you think might have grass tetany please give us a call so we can discuss options with you.



Prevention is preferable to treatment as grass tetany can occur without warning. Prevention involves supplementing the animals with magnesium during the period of greatest risk. The provision of hay during periods where there is lush, rapid pasture growth can also help reduce the incidence of the disease. The major risk period is one month before calving to two months after calving.

There are several magnesium supplements available from vets and rural suppliers that can be used:

Magnesium oxide

Magnesium chloride

Magnesium sulphate (Epsom salts)

Grass tetany blocks

They may be given in several way. 

In situations where cattle are receiving concentrates on a daily basis, the required amount of magnesium can be readily incorporated into the concentrate mix. This is an easy and cost effective means of supply extra magnesium. 30g per cow per day can be added.

Dust pasture with magnesium oxide at a rate of 60g per cow per day or 10g per sheep per day. Apply powder early in the morning and then strip graze. Paddocks can be powdered several days in advance but should be redusted after heavy (>25mm) of rain.

Magnesium oxide can be sprayed onto hay. This is one of the cheapest and most reliable methods of providing magnesium. 600mg of mag oxide and molasses mixed in 2 litres of water is poured evenly onto the cut edge of a bale (a watering can is good for this) about 12 hours before feeding. Feed this treated hay at a rate of m10 cows or 100 sheep per bale. Don't feed untreated hay until this hay has been eaten

Grass tetany blocks provide magnesium as a palatable "lick". A major disadvantage of this method is that all the animals may not consume

sufficient magnesium. Follow the manufacturers recommendations concerning the number of cows per block. When buying blocks be sure they are recommended for the prevention of grass tetany (general mineral blocks won't supply enough magnesium)

Epsom salts or mag chloride may be added to the water supply. This is best done through a Peta dispenser at a rate of 500g per 100 litres of drinking water for Epsom salts and 420g per 100 litres for magnesium chloride. Things to keep in mind; cattle will scour if they receive more than 140g of Epsom salts per day. Also as cattle don't like the taste Epsom salts and mag chloride need to be added gradually over 2-3 weeks. In winter water consumption is variable due to high moisture content of pasture and insufficient treated water may be consumed.

Rumen Magnesium bullets are available. These are given orally and release Magnesium slowly (over a period of 9-12 weeks) into the rumen. They have been used with mixed success as they only provide a small amount of magnesium on a daily basis and in some cases this is not enough to prevent grass tetany.

Fertilizers rich in potassium and nitrogen reduce the availability of magnesium from the pasture and increase the risk of grass tetany. So avoid grazing these pastures soon after fertilizer application.

If magnesium is fed over a long period it is important to add phosphorous (dicalcium phosphate powder or DCP) as a precautionary measure because magnesium can reduce phosphorous absorption. 

Grass tetany is difficult to treat, but with magnesium supplementation it is easy to prevent. Saving one cow will pay for the cost of supplementation for at least a year. If you have any questions about the prevention or treatment of grass tetany, please give us a call at the clinic.