Historically, grass tetany has been attributed to a magnesium deficiency brought about by cattle grazing spring pastures when there is a superabundance of young, rank, quick-growing grasses rich in proteins. It is sometimes seen in the fall when new growth of cool season grass or wheat 
pastures occurs.
Similar conditions can arise when rain finally comes to drought stricken pastures and the new growth sets the stage for a metabolic syndrome that may be more severe than spring-time grass tetany. Horses and small ruminants can be affected as well as cattle.
Typical signs of grass tetany range from an uncoordinated gait to convulsions, coma, and death. Animals on pasture are often found dead without illness having been observed, but with evidence of thrashing around the carcass.
Recommendations:
• The standard recommendation to prevent grass tetany is to feed Mag Ox in salt or the feed at the rate of 1 ounce / day. Mag Ox is easily absorbed, but it is relatively unpalatable, and if mixed with salt the animals may not eat it unless they are lacking in salt. A mixture of magnesium oxide along with the more easily tasted magnesium sulfate solves the dilemma and allows the animals to exercise their own judgment with regard to their mineral needs.
• Keep dry hay available at all times.
• Provide loose white salt free choice and separate from other minerals.
• Provide a wide variety of free choice minerals.
• Use dolomite or high Mg limestone on pastures and include legumes in pasture mixes. Avoid over application of nitrogenous fertilizers including manure.
• Steers, heifers, dry cows, and cows with calves over 4 months old are less likely to develop tetany. Graze less susceptible animals on high risk pastures.
• Over feeding of protein can be somewhat alleviated by supplying adequate calcium, magnesium and sodium in complete rations, but these minerals also should be available free choice.
Other factors besides magnesium deficiency may be involved in grass tetany.
Consider the following:
•
Excess nitrates (associated with excessive nitrogen fertilization of pastures and feeding high protein rations and non-protein nitrogenous compounds) are bonded to cations – Sodium, Calcium or Magnesium. If sodium is deficient and potassium is high, the body “washes out” some of the essential cations, calcium, magnesium and sodium in the urine, feces and milk. NH+ may also interfere with the absorption of magnesium from the gut.
• Adequate dietary sodium protects against nitrate toxicity, grass tetany syndrome in herbivores, and other metabolic and reproductive disorders induced by nitrate or other excess protein sources in the ration. It has been observed that if cattle have access to adequate loose salt, they rarely die from grass tetany or acute bloat.
• High nitrates and low sodium in the ration can cause low blood calcium and thus contribute to the occurrence of milk fever and the ‘downer cow’ syndrome. The clinical signs of grass tetany or hypomagnesemia are unlikely to occur unless there is also a hypocalcemia.
The common denominator of all of the above is that grass tetany is a complex condition involving the interplay of many different minerals, anions and cations. One of the best safeguards is to make available a wide range of free choice minerals, and let the animals utilize their intrinsic nutritional wisdom to balance their own intake for optimum health.
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