Staggers (Magnesium Deficiency)
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%) of the magnesium consumed.
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.
