Hypomagnesaemia (Grass Staggers/ Tetany)
Grass tetany, or hypomagnesia, is caused by low blood levels of magnesium. It is most prevalent when cows and ewes that are in late gestation or heavy lactation graze lush spring growth, usually in April and May, but cases can be seen in the fall. The clinical signs can occur very quickly because the cow does not store magnesium, and is thus reliant on its daily dietary intake. Hypomagnesaemia is most commonly a disease of lactating cows at grass, because grass can be very low in magnesium and the output in milk is high.
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| Grass tetany-an overview of clinical signs |
Like most nutritional diseases, the animals showing clinical signs are just the tip of the iceberg, for every cow with obvious disease many more will be affected sub clinically.
The increase in disease in spring occurs because rapidly growing grasses, particularly ryegrasses, are very low in magnesium. This low concentration of magnesium can be further reduced by the application of fertilizer, partly because of the increase in growth rate, but also because application of potassium directly inhibits uptake of magnesium by the grass. Slower growing forages, particularly clovers and other broad-leaved plants contain much higher levels of magnesium.
• Symptoms
- Restlessness
- Staggers
- Over-alert appearance
- Excitable or aggressive
- Animals may fall down and go into convulsions
- In many cases animals may die without any prior sign of disease
• Treatment
Treatment must be prompt to be effective. It is best to inject a combined calcium and magnesium solution 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.
Animals should not be moved during treatment, but once they have responded to treatment it is best to move them off the pasture. In some cases, repeat treatment may be needed.
• Prevention
Daily supplementation of magnesium should be given to animals at risk. This can be through a mixture of oral supplements including salt licks, molasses concentrates or hay. Fertilizers containing magnesium are effective in increasing herbage.
It is recommended that dry cows receive a diet containing 0.35 percent Mg, and lactating cows 0.28 percent Mg. There are a number of different sources of magnesium, and methods of adding these into a cow's diet. Common methods include drenching, pasture dusting, hay slurries, through water, and as magnesium bolus.