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.

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. 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%.


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