Grass Tetany/ Milk Fever/Winter Tetany/ Wheat Staggers
These local names describe either the symptoms,the physiological state of the animal, the season of the year, or the type of feed consumed as related to the occurrence of tetany.
Grass tetany is a syndrome, primarily involving mature cattle grazing on succulent forages. It is seen in almost all parts of the world. The most consistent clinical sign of grass tetany is hypomagnesemia, and typical grass tetany is frequently low in Mg. In fact, the tetany-prone areas may be mapped by the Mg content of the forage. Often these tetany-prone areas are also the same areas that have crested wheatgrass seedings.
Hypocalcemia, rather than hypomagnesemia, is probably the major problem associated with wheat pasture poisoning. The availability of Ca or Mg may be modified by other dietary factors. High dietary intake of nitrogen (N),potassium (K), and aconitic acid have been associated with tetany even with a marginal intake of Mg and/or Ca.
Tetany usually occurs when the grass or cereal grains are immature and rapidly growing; usually this means that most cases are observed in the spring months. Tetany usually occurs near parturition or until approximately 2 months postpartum, and the frequency usually increases with older cows. A cow in peak lactation (6–8 weeks following calving) needs a constant source of magnesium to replace the large amount lost from the body in milk. Even when feed levels of magnesium are low, the loss of magnesium in the milk remains the same. Young animals and steers rarely get tetany. Tetany occurs most frequently in beef cows grazing pastures. Cool, rainy weather generally accelerates the occurrence of this syndrome. Tetany seldom occurs when legumes or legume grass mixtures are a major portion of an animal’s diet. Legumes may contain twice or more the concentration of Mg as do grasses grown on the same soil. Pastures heavily fertilized with high levels of nitrogen or pastures with high soil K are more likely to produce tetany-prone pastures, but tetany will occur under other conditions also.
Contributing causes are:
- magnesium levels are lower in cool season grasses and cereals than in legumes or weeds.
- levels are low in grasses grown on leached acid sandy soils.
- levels are low when potash and nitrogen fertilisers are used and growth is vigorous.
- high moisture content in grass causing rapid gut transit and low uptake.
- reduced absorption of magnesium resulting from high rumen potassium and nitrogen and low rumen sodium.
- low energy intake, fasting or sudden changes in feed.
- bad weather, especially winter storms.
- stress such as transport or yarding.low roughage intake (young grasses have low roughage and often poor palatability).
- low intake of phosphorus and salt
Although tetany is caused by a metabolic deficiency of Mg and Ca, it is usually not possible to prevent it with a normal supplemental Mg and Ca in mineral mixes. Cattle should have access to a high Mg mineral mix while grazing tetany-prone areas.
Many animals afflicted with grass tetany die before symptoms are seen. The interval between the first signs of tetany and death can be as short as 4 to 8 hours. When symptoms are observed, there is little opportunity to get other help or medicine if they are not readily available to treat the animal.
- Immediately seek treatment of affected animals by your veterinarian.
- Get supplemental Mg into all of the animals in the herd as soon as possible. At this time, mineral supplementation is not the method of choice. This step does not guarantee that all cows will get Mg immediately. If the animals are familiar with concentrates, feed this fortified with Mg so that each animal consumes 1 to 2 ounces of magnesium oxide daily or the equivalent. If the animals are not familiar with concentrates, they will probably not eat it soon enough to prevent an outbreak. Adding Mg to the water under the conditions described earlier will work.
- Increase energy and roughage intake. Good quality hay and silage are suitable.
- Provide salt if a natural source is not available.
- Move lactating cows (especially older animals) to high legume and high dry matter pastures.
- Provide shelter.
- Reduce stress factors (yarding, transport).