Magnesium Deficiency

Also known as Grass Staggers, Grass Tetany or Hypomagnesaemia

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Cattle Diseases

Magnesium Deficiency in Cattle

Also known as: Grass Staggers, Grass Tetany, Hypomagnesaemia, Lactation Tetany, Staggers

Cattle need a continuous supply of magnesium primarily to facilitate the numerous energy-generating reactions in their tissues and for the correct 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%) of the magnesium consumed.

There are two types of hypomagnesaemia recognised clinically, 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.

Lactation Tetany

Cow on spring grass

Magnesium deficiency is most often seen in lactating dairy cows after turnout in the spring onto lush, grass-dominant pasture following winter housing

The occurrence of lactation or grass tetany can occur in three sets of circumstances. Most common is the occurrence in lactating dairy cows after turnout onto lush, grass-dominant pasture following winter housing. Most cases occur during the first 2 weeks after the animals leave the housing. Second, 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).

Much of the magnesium (about 70%) 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 for maintenance of a steady state blood magnesium concentration, 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 seasonally 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 blood 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 does a high rumen pH. Sodium, vitamin D and possibly saponins have a positive effect on magnesium absorption (Garcia-Gomez and Williams, 2000). The occurrence together 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 treated with nitrogen fertiliser and potash is potentially most dangerous. Slower grass growth and lack of artificial fertilisation on managed pastures may protect against lactation/grass tetany in cows, although slurry and chicken manure are also high in potash and can favour grass staggers.

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. It most often occurs in dry dairy cows and beef cattle kept outside during the winter months in moderately cold climates.

Clinical Signs of Lactation Tetany

The clinical signs of lactation tetany are muscular spasms and convulsions, and death due to respiratory failure. Progression of disease is rapid, with animals often found as a ‘sudden death’. In this case, the major differential diagnosis which must be considered for statutory disease control reasons is anthrax, but clostridial diseases are the second biggest cause for sudden deaths. Hypomagnesaemia can often be suspected based on the clinical presentation of a dead animal surrounded by churned-up ground as a consequence if fitting. 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

Feeding Pooled Milk

Milk is an adequate source of Magnesium for young calves.

While colostrums are high in magnesium,  milk is an inadequate source of magnesium for growing calves. Additionally, the ability of absorbing magnesium from the intestine decreases as the calf grows. Therefore, the condition most commonly occurs in milk-fed calves (e.g., sucklers) at three to four months of ages.

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.

Other factors may predispose hypomagnesaemic tetany. Diarrhoea (scouring) reduces magnesium absorption, and the condition can occur at a much younger age. 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 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.

Clinical Signs of Hypomagnesaemic Tetany in Calves

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 hastens tetany in animals that are tetany-prone as a result of having low serum magnesium levels. Post-mortem, samples can be obtained from the aqueous or vitreous humour of the eye (Kolb, 2003; Edwards and Foster, 2009) to aid in confirmation of diagnosis since blood concentrations do not remain stable after death.

Control and Prevention of Magnesium Deficiency

High potassium and protein levels, both of which contribute to magnesium deficiency, are less common under extensive pasture based management systems. The inclusion of legumes in the pasture, which contain a higher level of magnesium than grass, should be something that needs to be considered when running a pasture based system.  There are grass varieties that have been bred for high magnesium content (Binnie et al., 1996; Crawford et al., 1998; Hemingway, 1999; Moseley and Baker, 1991) that may be considered for incorporation within grass sward mixtures.

Shelter is required for animals kept outside, to protect them from adverse weather conditions, which predispose cows to hypomagnesaemia.

Should an incident occur, to prevent further cases of lactation tetany, the herd should be moved off the danger area and onto permanent pasture or a longer-term grass 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.

Magnesium supplementation

Magnesium supplementation in water is an effective way of ensuring adequate Magnesium intake by all cattle.

The average dietary requirements of magnesium for cows is 1.3 g mg/kg DM for pastures high in potassium  and 0.6 g mg/kg DM for pastures low in potassium (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 although intake from licks can be variable. Alternatively, magnesium can be supplied in water or through the use of magnesium–containing macro-element boluses.

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.

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. This is often achieved by the use of calf concentrates which contain additional magnesium. Supplementary, dosing has to be fairly accurate to avoid diarrhoea or inefficient protection. For calves with an average growth rate, appropriate dosages are 1g daily of magnesium oxide for calves to 5 weeks, 2g for calves 5-10 weeks and 3g for 10-15 week calves, or twice this dose of magnesium carbonate.

Treating Magnesium Deficiency

Lactation tetany is not usually diagnosed in cows until individuals have developed clinical signs. They should be treated without delay by subcutaneous magnesium sulphate. A ‘fitting’ animal may need to be sedated first for safe handling by the vet and the stockperson. Cows with lactation tetany are often also low in calcium as well as magnesium. It is therefore recommended to use a combined treatment of subcutaneous magnesium sulphate first, followed by intravenous calcium borogluconate and magnesium hypophosphite. Care should always be taken when administering magnesium, especially if in a combined product which is given intravenously, as sudden death may be precipitated. Recovery is generally rapid but relapses are not uncommon and the treatment may have to be repeated.

The affected animal should be treated in the field and transported to a more suitable environment for convalescence only after it has settled down and stopped convulsing. Any calves that are suckling 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. Sedation 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 – care should be taken with cattle slurry and chicken manure
  • Consider moving calving periods (seasons) away from risk periods
  • Provide adequate shelter for the animals
  • Avoid long periods of fasting, especially for lactating cows
  • Cows in early lactation should be supplemented (in feed, in water, by pasture dusting or by use of boluses)
  • On farms where lactation tetany is a problem, dressing the pasture with kieserite may be a long-term solution
  • Treat affected cows with magnesium sulphate subcutaneously, followed by calcium borogluconate and magnesium hypophosphite intravenously
  • Feed suckled calves by hand until the affected 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.

Magnesium Deficiency References
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