High potassium and protein levels, both of which contribute to magnesium deficiency, are less common under organic management systems. Organic farming also promotes the inclusion of legumes in the pasture, which contain a higher level of magnesium than does grass. Shelter is required for animals kept outside, to protect them from adverse weather conditions, which predispose cows to hypomagnesaemia. Therefore, magnesium deficiency is less likely to be a problem under organic management. Recently, new grass varieties have come onto the market, bred for high magnesium content (Binnie et al., 1996; Crawford et al., 1998; Hemingway, 1999; Moseley and Baker, 1991).
To prevent further cases of lactation tetany, the herd should be moved off the danger area and onto some permanent pasture or longer-term ley. This is usually enough to prevent further cases, but the cows should not return to the original pasture until appropriate remedial measures have been taken. Hay can be offered to cows, as it stimulates rumination and salivation, which prevents excessive build-up of ammonia in the rumen and aids magnesium absorption.
In areas where the incidence of the disease is high, it may be advisable to avoid calving during the cold winter months when seasonal hypomagnesaemia is most likely to occur. It is important that long periods of fasting, e.g. during movement or yarding, should be avoided, especially in lactating cows and when seasonal hypomagnesaemia is likely to be present.
The average dietary requirements of magnesium for cows is 1.3 g Mg/kg DM for pastures high in potassium (K = ³ 4 g K/kg DM) and 0.6 g Mg/kg DM for pastures low in potassium (K = £ 2 g K/kg DM) (Suttle, 1998). The feeding of magnesium salts to cows during the danger period is universally adopted. In-feed magnesium oxide (magnesite) (120 g/day), magnesium phosphate (54 g/day) and epsom salts (MgSO4) are a safe and effective way of ensuring a good intake of magnesium. They can also be included in mineral licks. Frequent dusting of pasture during periods of risk with dolomitic limestone or calcined magnesite is also useful. Longer-term solutions are a better way of dealing with the problem, for example, by dressing the pasture with kieserite (Thompson and Reid, 1981) under the right circumstances to give protection for several seasons, and increasing the clover content of the sward. Farmyard manure or cattle slurry can maintain soil magnesium levels in grass/white clover ley (Humphreys et al., 1997).
The provision of hay in the diet may help to prevent hypomagnesaemic tetany of calves. Supplementary feeding of magnesium to at-risk calves should begin during the first 10 days of life to prevent excessive falls in serum magnesium and should continue until at least 10 weeks of age. Dosing has to be fairly accurate to avoid scouring or inefficient protection. For calves with an average growth rate, appropriate dosages are 1 g daily of magnesium oxide for calves to 5 weeks, 2 g for calves 5-10 weeks and 3 g for 10-15 week calves, or twice this dose of magnesium carbonate.