Hypomagnesemic tetany in ruminants (Proceedings)

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Hypomagnesemic tetany in ruminants (Proceedings)

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Aug 01, 2009

Ruminants are uniquely susceptible to hypomagnesemic tetany (HMT) caused by a variety of factors. Grass tetany, wheat pasture tetany, lactation tetany, and milk tetany are just a few of the terms used to describe HMT. A marked decrease in the extracellular fluid (ECF) concentration of magnesium (Mg) is the primary metabolic abnormality. The low ECF Mg or low Mg/Ca ratio in the ECF will potentiate the release of acetylcholine at the neuromuscular junction and is thought to be the cause of the tetany.

The factors thought to be important in causing HMT are:

1. Lactation: cows at the peak of lactation are at greatest risk. There is a substantial amount of Mg that is secreted into milk. Mg is in milk at about one-tenth the amount of Ca or P.

2. Potassium in feed and/or soil: potassium (K) interferes with Mg absorption in both the plants and in the ruminants. Many forages are high in K, particularly lush cereal pastures such as wheat, barley, and oats. Using the following ratio: K ÷ Ca + Mg; Dutch researchers have shown that when this value is >2.2 there is an increasing risk of HMT.

3. Nitrogen fertilization: the use of ammonia/ammonium based fertilizers increase the susceptibility to HMT. This is due to increased dietary crude protein and K; both of which interfere with Mg absorption in the ruminant and lead to an increased risk of HMT. Also, NH4+ competes with Mg uptake by the plant and thus lowers the Mg content in the forage diet of ruminants.

4. Stress or fasting: fasting, anorexia, and/or stress can cause decreases in serum Mg and ECF Mg that may not return to normal for several days. Epidemics of HMT commonly occur in lactating beef cows on foothill pastures in California following spring storms.

5. Milk diets in calves: whole milk and some milk replacers are Mg deficient. Long term (> 2 months) milk diets without access to other Mg sources can be associated with episodes of tetany in calves.

6. Rations high in fat: fat tends to form Mg soaps and decreases Mg absorption. This can be a practical consideration in some dairy rations.

HMT clinical syndromes

Grass tetany. Associated with grazing lush pastures by cows early in lactation. The cows, and rarely the calves, are found dead or having convulsions. The etiologic factors include:

Low Mg content of pasture forage

b. High K content pasture forage

c. High crude protein content (ruminal NH3 and NH4+ concentrations)

d. Bad weather, stress, fasting

e. Lactation losses—Ca and Mg in the milk

f. Organic acids (citric, trans-aconitic, etc) which may bind Mg

g. Ammonia fertilization of irrigated pastures

h. Combinations of the above factors

Wheat pasture tetany. Young, rapidly growing cereal grain pastures (wheat, barley, oats) with low Mg content and very high K and crude protein levels. Mg absorption is interfered with by all of these factors.

Milk tetany in calves. Whole milk diets in some dairy and veal calves for prolonged periods of time or in beef calves nursing cows predisposed to HMT are most at risk. Diarrhea often precipitates the onset of tetany.

Transport tetany. Stress and fasting associated with transport can precipitate HMT.

Low Mg diets. Some diets are extremely low in Mg content such as dry winter pasture or low quality roughage. These diets can cause frank Mg deficiencies and HMT.