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The Basics

Grass tetany (GT) is also referred to as grass staggers, wheat-pasture poisoning or hypomagnesaemia. It is a metabolic disorder of cattle and is generally thought to be related to a deficiency of magnesium (Mg). It occurs primarily in ruminant animals and mature lactating cows (heifers on occasion) are the most susceptible. Older cows are considered more prone to the problem than those with only their first or second calves. The reason is thought to be that older cows are less able to mobilize Mg reserves from the bones than are younger cows. Other theories are that as the cow ages and if mineral supplementation has not been consistent, her normal body stores of Mg may not be adequate to supply for the deficiencies that can occur in the diet when on these pastures.

Very seldom if ever is GT found in growing cattle (stockers). It is also not related to bloating although this seems to be a common misconception. Many producers seem to correlate Mg issues with bloating tendencies.

Conditions Leading to Grass Tetany

Generally, GT occurs when animals are grazing lush pastures in the spring, but it can occur in the fall and winter as well. Grass tetany is common during cool, cloudy and rainy weather and often occurs when cool weather is followed by a warm period. Mature and especially lactating cows develop GT most often while grazing cool-season grasses or small-grain pastures in spring or fall. Rapidly growing, lush grasses create the greatest problem. Grass tetany has occurred on orchard grass, perennial ryegrass, timothy, tall fescue, crested wheatgrass, bromegrass, and winter annuals. Small grain pastures include wheat, oats, barley and rye. It also occurs when livestock are wintered on low Mg grass hay or corn stalks. Fortunately, GT is not usually a problem on legume pastures such as clovers, alfalfa, etc. or in animals wintered on legume hay.

Grass tetany is most likely to occur on pastures grown on soils that are low in available Mg and high in available potassium (K). Advance soil testing can provide indicators of potential problems. High rates of nitrogen (N) and K fertilizers are sometimes associated with increased tetany problems. Forages should be analyzed immediately when a potential problem is suspected. Forage containing less than 0.2 percent Mg and more than three percent K and four percent N (25 percent crude protein) are likely candidates to exhibit GT problems. Forage that is high in K and N also should have at least 0.25 percent Mg. Some research has also shown that low phosphorus (P) levels can also contribute to increased problems on these types of pastures. So a plant profile that is high in protein (N), high in K, low in P and low in Mg are the greatest potential problem for mature cows. This is not unusual on pastures that are fertilized predominantly with only N and not a balanced fertilizer used according to soil testing.

What do Symptoms Look Like?

Unfortunately, in many cases, symptoms are not observed far in advance and the only evidence is a dead cow. Some of the symptoms that MAY be seen include:

�      In mild cases, milk yield is decreased, and the animal acts nervous.

�      Animals affected by acute GT may suddenly stop grazing.

�      Appear uncomfortable and show unusual alertness such as staring and keeping their heads and ears in an erect position.

�      Staggering, exhibit twitching skin, especially on the face, ears and flanks.

�      Lying down and getting up frequently.

�      Easily disturbed with any stimulation leading to startling reactions such as continuous bellowing or running.

As the condition advances a staggering gait pattern develops followed by collapse, stiffening of muscles and violent jerking convulsions with the head pulled back. Animals lie flat on one side; the forelegs pedaling periodically; the eyes and ears twitch; and chewing motions produce froth around the mouth. Between these convulsions, the animal may appear relaxed. Also during this period, sounds or touching the animal, as when administering treatment, may result in violent reactions. This subsequently results in an increase in body temperature and respiratory rate. Heart sounds become audible. Animals usually die during or after a convulsion unless treatment is given very soon.

Some Measurements

In recognizing and diagnosing GT it is helpful to understand levels in both the plant and the animal. Generally, plant magnesium (Mg) levels of 0.2 to 0.25 percent constitute a safe level to prevent tetany. Most nutritionists have accepted a level of 0.2 percent Mg in forage dry matter unless some antagonist is present at a high level, i.e. high potassium levels. The following criteria have been used to evaluate tetany danger of forage Mg for lactating or pregnant beef cows.

Prevention

Some or all of the following preventative steps may be helpful:

1. Applying Mg fertilizer and dolomitic (high Mg) limestone to the soil may increase the magnesium concentration in plants. The effect of Mg fertilizer or dolomitic limestone is generally greatest on coarse-textured acid soils that are low in potassium. Soil analyses and local recommendations by a qualified agronomist should be obtained before fertilization. This is the case even when GT is not a potential issue.

2. Dusting pastures with Mg oxide (MgO) as finely powdered calcined magnesite helps increase the intake of Mg by cattle. Rates of 15 to 30 pounds of MgO per acre are recommended, with the lower rates for pastures where cattle are moved every two or three days.

3. Animals should be fed a high Mg supplement or free-choice mineral (containing 8 to 12 percent Mg). Magnesium may be added to a protein supplement, grain mix or liquid supplement. Assuming a 20 percent availability from the supplement, the cow's Mg requirement for maintenance and lactation would be from 13 to 15 grams per day. However, situations may require at least 36 grams to prevent development of tetany in herds.

4. The simplest method of prevention is to not graze susceptible animals (mature cows, pregnant, lactating) on tetany-prone forages. Save these pastures for young growing animals who are very unlikely to develop GT. However, this is not always practical or the goal for nutritional management.

Management of High Mg Mineral Supplements

A potential problem with high Mg mineral supplements is low palatability. Magnesium oxide, the primary source of Mg for these situations is unpalatable and getting a cow to eat an adequate quantity in the midst of a high tetany threat situation may be a problem. Magnesium sulfate is also a good source and is more palatable. A better solution is to feed a more moderate amount of Mg on an ongoing (year-round) basis (include 2.5 to 3.5 percent Mg) to the basic mineral to help support the animals overall Mg stores. This is especially important in herds with a higher percentage of older cows. Maintaining the herd in an adequate to good Mg (as well as all essential minerals) status simply makes sense for overall performance and health of the individual cow and the herd as a whole.

This brings up another potential issue with mineral supplementation is intake management. In many cases, just a simple mineral supplement can exhibit variable intake. This is accentuated if not managed properly. In every group of cattle there will be a range of intakes from very little to excessive. This dynamic may be changed by adding high levels of Mg which can depress supplement intake. In these cases the animals that are on the low end of the intake scale are likely to be the ones affected since they will not be consuming adequate levels of mineral and subsequently Mg. It is critical that when changing over to a high Mg mineral that the product takes necessary steps to improve palatability of the overall product to increase the odds of proper intake. If the ongoing mineral program does not contain at least somewhat elevated Mg levels (2.5 to 3 percent as noted previously), the high Mg product should be offered to the animals several weeks prior to grazing on pasture types previously discussed.

Treatment

It is always wise for the producer to consult a local veterinarian in advance in order to become more familiar with treatments for GT if lush fall or spring pastures are to be grazed. Advance planning and preparation will help offset potential loss that can and do occur.

Cows in the early stages of GT should be handled gently, producing the least stress and exertion possible. Driving, roping or anything producing excitement will often result in sudden death. Two hundred (200) mls/cc's of a saturated solution (50 percent) of Mg sulfate (Epsom Salts), injected subcutaneously, gives a high level of magnesium in the blood in 15 minutes and can be administered by producers under range conditions. Any solution injected into the animal should be sterile, otherwise an infection may be introduced. Cattle producers interested in using Mg sulfate solution should contact a veterinarian. Other treatments used by some vets include intravenous injections of chloral hydrate or Mg sulfate to calm excited animals and then follow with a calcium-magnesium gluconate solution. Intravenous injections should be administered slowly by a trained person, because there is danger of heart failure if they are given too rapidly. The animal should be removed from the tetany-producing pasture and fed hay and concentrates. Also, approximately 30 grams of Mg sulfate should be given daily through feed.

A word of caution, cows that have GT are likely to get it again later in the season or in later years. Yet, these animals may continue to be high producers. Also, in some cases GT can be mistaken for milk fever (low Calcium) or vice versa. This requires a different treatment.

Conclusions

Grass tetany is a common problem in many herds grazing lush winter and spring pastures. Advance recognition of the conditions that produce the threat along with taking appropriate steps to offset the problem is the key to reducing the incidence of the condition and minimizing loss of animals or performance.

Copyright 2015 � Dr. Stephen B. Blezinger. Dr. Steve Blezinger is a management and nutritional consultant with an office in Sulphur Springs, TX. He can be reached at [email protected] or at (903) 352-3475. For more information please visit us on at www.facebook/reveillelivestockconcepts.

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