Magnesium Deficiency

Magnesium Deficiency

The Condition

Cattle need a continuous supply of magnesium primarily to facilitate numerous energy-generating reactions in their tissues and for the orderly transmission of nerve pulses. The kidneys maintain magnesium homeostasis, and a renal threshold exists below which magnesium excretion is sharply reduced. Cattle rely on absorption of magnesium from the rumen to meet most of their needs. The animal cannot modify magnesium absorption and absorbs only a small proportion (<30%)>

There are two types of hypomagnesaemia, hypomagnesaemic tetany in calves, which appears to be due to a straightforward deficiency of magnesium in the diet, and lactation tetany, in which there may be a partial dietary deficiency but in which nutritional and metabolic factors reduce the availability, or increase the body loss, of magnesium.

Slower grass growth and lack of artificial fertilisation on organically managed pastures may be protective against lactation/grass tetany in cows.

Lactation Tetany

The occurrence of lactation or grass tetany is related to three sets of circumstances. Most common is the occurrence in lactating dairy cows after turnout in the spring onto lush, grass-dominant pasture following winter housing. Most cases occur during the first 2 weeks after the animals leave the housing. Wheat pasture poisoning may occur in cattle of any age grazed on all types (including barley and oats) of green cereal crops in early stages of growth. The third occurrence is in beef or dry dairy cattle running at pasture in the winter time, usually when nutrition is insufficient and where no shelter is provided in changeable weather, rather than in severe, prolonged cold weather (Blood and Radostits, 1989).

Seventy per cent of the magnesium is relatively tightly bound in the skeleton and can only be released during general bone absorption. However, bone turnover decreases in adult animals. The body does not have efficient homeostatic mechanisms such as those which maintain calcium levels. Magnesium levels are therefore also more likely to be seasonably low in large numbers of animals than calcium levels. If this seasonably low level is suddenly exacerbated by a short period (24-48 hours) of starvation, such as during transport, hypomagnesaemia may occur. Hypocalcaemia is often present concurrently and there is evidence that the actual onset of clinical tetany may be associated with a rapid fall in serum calcium levels.

There are several factors affecting magnesium absorption in the rumen. Both potassium and rapidly degradable protein have a negative effect on magnesium absorption, as has a high rumen pH. The coincidence, therefore, of high dietary intake of potassium and degradable protein in rapidly growing spring herbage means that conditions for magnesium absorption are critical at this stage. Pasture which has been heavily top-dressed with fertilizers rich in nitrogen and potash is potentially most dangerous.

Reduced levels of serum magnesium have been observed in adult cattle exposed to cold, wet windy weather with little sunshine and with no access to shelter or to supplementary feed. It is possible that failure to eat during bad weather may be the basic cause of hypomagnesaemia. There is also a suggestion that cold weather stress may increase urinary excretion of magnesium (Shiga et al., 1979). It most often occurs in dry dairy cows and beef cattle kept outside during the winter months in moderately cold climates.

The clinical signs of lactation tetany are muscular spasms and convulsions, and death due to respiratory failure. Although effective treatment is available, the mortality rate is high because of the short course. Since animals die before they can be observed to be ill, the mortality rate is difficult to estimate. It is probably in the order of 20%.

Hypomagnesaemic tetany of calves

Hypomagnesaemic tetany in calves is common and is often accompanied by hypocalcaemia. It is caused by a dietary magnesium deficiency exacerbated by a high intake of calcium, which causes depletion of magnesium stores and lower serum and bone levels of magnesium. Milk is an adequate source of magnesium in very young calves. However, the efficiency of magnesium absorption decreases up to about three months of age, when maximum susceptibility to the disease occurs.

Other factors may predispose hypomagnesaemic tetany. Scouring reduces magnesium absorption. Chewing fibrous material, such as floor bedding, increases salivation and thus causes greater losses of endogenous magnesium. Hypomagnesaemic tetany in calves often coexists with other diseases, especially enzootic muscular dystrophy.

The disease is most common in housed, inadequately fed animals. Calves aged 2-4 months or over are most at risk when fed solely on a diet of whole milk, as with veal calves. However, the disease also occurs in calves running at pasture with their dams. Calves receiving the greatest quantity of milk and growing most rapidly are more likely to be affected, as they have a greater need for magnesium.

Tetany does not occur until serum magnesium levels fall below 0.8 mg/dl (0.33 mmol/l), and is most severe at levels below 0.6 mg/dl (0.25 mmol/l), although calves may have levels even lower than this and show few clinical signs. It is probable that hypocalcaemia precipitates tetany, in animals rendered tetany-prone by low serum magnesium levels.

Methods of Control and Prevention

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.

Methods of Treatment

Lactation tetany is not usually diagnosed in cows until individuals have developed clinical signs. They should be treated without delay by a veterinary surgeon. Cows with lactation tetany are often also low in calcium as well as magnesium. It is therefore wise to use a combined treatment of calcium borogluconate and magnesium hypophosphite. There are several products on the market for cattle. The initial injection is administered intravenously, followed by a subcutaneous injection. If subcutaneous medication is given, care should be taken not to overdose. A 48-hour meat and milk withdrawal needs to be observed in organic herds (7 days if certified with the Soil Association). Recovery is generally rapid but relapses are not uncommon and the treatment may have to be repeated. It is common practice to give a large intra-muscular dose of an ataractic drug before commencing specific treatment, to prevent convulsions during treatment.

Homoeopathy may help aid recovery (Elliott and Pinkus, 1993).

The affected cow should be treated in the field and, after it has settled down and stopped convulsing, transported to the steading. The suckler calves should be fed by hand until the cow has completely recovered, since attempts to suck may precipitate another convulsion.

Hypomagnesaemic tetany in calves can be treated with magnesium injections, but the effect is only transitory because of severe depletion of bone reserves of magnesium. Follow-up supplementation of the diet with magnesium oxide or carbonate is advisable. Tranquilization with an ataractic drug may be essential to avoid death due to respiratory paralysis.

Good Practice based on Current Knowledge

Lactation tetany

• Use plants in the sward (e.g. legumes) containing a higher level of minerals, including magnesium.
• Avoid high potassium levels in the sward.
• Consider moving calving dates away from risk periods.
• Provide adequate shelter for the animals.
• Avoid long periods of fasting.
• Cows in early lactation should be supplemented (epsom salts, dolomitic limestone, magnesite, magnesium phosphate, mineral licks, etc.).
• Frequent dusting of pasture during periods of risk with dolomitic limestone or calcined magnesite may be helpful.
• On farms where lactation tetany is a problem, dressing the pasture with kieserite may be a long-term solution.
• Treat affected cows with products containing both calcium borogluconate and magnesium hypophosphite, both intravenously and subcutaneously, administered by the veterinary surgeon.
• Homoeopathy may be used to aid recovery.
• Feed calves by hand until the cow has completely recovered.
• 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.
• Supplement the affected group with magnesium oxide (magnesite) (120 g/day), magnesium phosphate (54 g/day) and epsom salts (MgSO₄).

Hypomagnesaemic tetany of calves

• Offer hay to stimulate rumination and salivation.
• Supplement calves at risk with magnesium oxide or magnesium carbonate.