Factors influencing the availability of dietary magnesium include magnesium levels in the soil and grass which vary considerably. High levels of potassium (application of potash fertilisers) disrupt the absorption of magnesium. High levels of ammonia (from nitrogenous fertilisers) inhibit magnesium absorption. Lush pastures are low in fibre and increase the rate of passage of food material through the rumen reducing time for the absorption.

Clinical signs:

Sudden death.

Seizures.

In acute disease there is initial excitability with high head carriage, twitching of muscles (especially around the head) and incoordination ("staggering gait"). Affected cows become separated from the group and have a startled expression, show an exaggerated blink reflex and frequent grinding of the teeth There is rapid progression to periods of seizure activity.

Diagnosis

Plasma magnesium less than 0.8mmol/l indicate subclinical hypomagnesaemia and an increased risk of developing acute hypomagnesaemia.

Treatment:

Sedative first administered to prevent seizures.

Administration of 400ml of 40% calcium borogluconate plus 50 mls of 25% magnesium sulphate by slow intravenous injection once seizure activity has been controlled. The remainder of the 400ml bottle of 25% magnesium sulphate is given by subcutaneous injection.

Prevention/control measures

The total diet should contain 2.5g/kg DM of magnesium to meet requirements of the majority of lactating cows at pasture. The best method is to use 60g magnesium oxide (calcined magnesite) per cow per day in high-magnesium cobs. Magnesium salts are relatively inexpensive and the cost of supplementation 100 cows for two months will be less than the loss on one animal due to hypomagnesaemia.

The sole water supply can be medicated with soluble magnesium salts such as chloride, sulphate or acetate. Pastures may be dusted during high-risk periods with finely ground calcined magnesite every 10-14 days. Intra-ruminal boluses give a slow release of relatively small amounts of magnesium into the rumen over a period of four weeks. Magnesium salts and minerals are unpalatable therefore ad-lib minerals are not satisfactory. Supplementation is especially important during stormy weather when roughage, such as straw, can be beneficial for beef cows.