Grass tetany (cattle), grass staggers (sheep) and hypomagnesaemia 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 occurs when blood magnesium levels fall below a critical level - hence the term hypomagnesaemia. 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 or late pregnancy.
Signs of the disease include restlessness, staggers, an over-alert appearance, being excitable and in some cases, aggressiveness. In severe cases, animals may fall down and go into convulsions or just die without warning.
There are some common clinical signs that are always present with grass tetany:
- Animals are usually grazing grass dominant pasture or lush cereal crops, often without any hay supplementation.
- Cold and wet windy weather with little or no shelter, resulting in short periods of fasting. The danger period in Tasmania extends from May to October.
- Animals are either fat and losing condition or very thin.
- Animals recently moved to a different paddock.
- Heavy use of nitrogen and/or potash fertiliser on pasture.
Dairy cows in peak lactation are most commonly affected, but dry cows and, under certain conditions, beef steers, may also suffer.
Treatment must be prompt to be effective. It is best to inject a combined calcium and magnesium solution (350ml for cattle, 100ml for sheep) under the skin in the area behind the shoulder and over the ribs. Massage the area well after injecting the solution to spread the fluid and aid its rapid absorption into the blood stream. Treated animals should be given adequate shelter and identified so that a response to treatment can be judged. In some cases, repeat treatment may be needed.
A frustrating aspect of grass tetany is that animals often relapse and die, or become 'downers' and eventually have to be euthanised. The success rate depends on getting to the animals soon enough AND reducing stress from the weather. Often affected animals do not eat - this can be a very serious complication, especially in pregnant ewes, which then succumb to pregnancy toxaemia and death.
Prevention is preferable to treatment as grass tetany often occurs without warning. Prevention involves supplementing the animals with magnesium during the period of greatest risk. The provision of hay during periods of stlush, rapid pasture growth may also reduce the incidence of the disease. For dairy cows, the risk period is one month before calving to two months after calving.
Magnesium stored in the body is not rapidly available so it must be supplied at least every second day during the 'danger period'.
- There are several magnesium supplements available from rural suppliers or veterinarians that can be used - magnesium oxide, magnesium chloride, magnesium sulphate (epsom salts) and grass tetany blocks. They may be given in several ways:
- Dust pasture with magnesium oxide at the rate of 60g per cow per day or 10g per sheep per day. Apply powder early in the morning and strip graze through the week. Redusting may be required if heavy falls of rain (25mm or more) occur.
- Magnesium oxide may be sprayed on hay. This is one of the cheapest and most reliable methods of providing magnesium. A mixture of 600 grams of magnesium oxide and molasses mixed in two litres of water is poured evenly onto the cut edge of the bale about 12 hours before feeding. Feed this treated hay at the rate of 10 cows or 100 sheep per bale. Don't feed untreated hay until this hay has been eaten. Hay may be treated with this magnesium oxide solution before baling - allowing 600 grams of magnesium oxide, to a 'bale length' of windrow. Magnesium oxide may also be added as a dry powder (600 g/bale) during baling, by means of a salting applicator attached to the baler. The treated bales should be put in a separate stack to allow them to be used during the danger period.
- 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 manufacturer's instructions concerning the number of cows per block. When buying blocks, be sure that they are recommended for the prevention of grass tetany.
- Epsom salts or magnesium chloride may be added to the water supply. This is best done through a Dosatron® at a rate of 500g per 100 litres of drinking water for Epsom salts, or420g per 100 litres of water for magnesium chloride. The salts can also be added at the rate of 60g per cow per day (60g is about two level tablespoons). The dose must be split and added to the water on two occasions during the day. The normal water flow should be maintained. The capacity of the trough should be at least nine litres per cow so that the salts are sufficiently diluted. Cattle will scour if they get more than 140g of Epsom salts per day. Also, because cattle don't like the taste, the Epsom salts need to be added gradually over 2-3 weeks. There are several disadvantages in using this method. Epsom salts are unpalatable and not readily accepted by stock. In winter, water consumption is variable due to the high moisture content of the feed and as a result insufficient salts may be ingested.
- Drenching stock with magnesium oxide or Epsom salts mixed in water is an effective but time consuming, method. The daily rate is 60g/cow mixed in 100ml water. Epsom salts may be mixed with pluronic-type bloat treatments but the volume of water will need to be increased if such a mixture is used. The magnesium oxide drench mixture must be constantly shaken to prevent it settling out.
- Magnesium oxide may be added to feed fed in the bail at the rate of 45-50g per cow per day, but there are indications that levels greater than 30g per cow per day may predispose the cows to Salmonella.
- Fertilizers rich in potassium and nitrogen reduce the availability of magnesium from the pasture and increase the risk of grass tetany. Avoid grazing pastures soon after fertilizer application.
If magnesium is fed over a long period, it is important to add phosphorus (dicalcium phosphate powder or bone flour) as a precautionary measure because magnesium can reduce phosphorus absorption.
Grass tetany is difficult to treat, however with magnesium supplementation, it is easy to prevent. Saving one cow will pay for the cost of supplementation for at least a year.
For concerns regarding the prevention or treatment of grass tetany, contact your local veterinarian.