GRASS TETANY CAN BE A PROBLEM AS GRASS GREENS UP

by: Stephen B. Blezinger
Ph.D., PAS

There are any number of misconceptions when it comes to understanding cattle nutrition. Some of this is related to the fact that a great deal of animal health and nutrition advice is gathered from the local coffee shop or feed store. And it's not that some of the information on feeding, nutrition, health and management coming out of these group discussions are not good, but some topics are, in fact fairly complicated. Additionally, there are a lot of concepts and areas of understanding that the so-called “professionals” in the feeding industry don't really understand or at least can't account for the variability.

One of these topics always comes up about this time of the year. With grass greening up or cattlemen turning cattle out onto lush, green winter pastures, various problems begin to be of concern. One of the greatest is grass tetany (GT). The following will discuss GT in detail and hopefully provide an understanding of what the problem truly is AND what it is not!

Some basics

Grass tetany (GT), sometimes called grass staggers, wheat-pasture poisoning or hypomagnesaemia, is a metabolic disorder of cattle generally thought to be related to a deficiency of magnesium (Mg). It occurs primarily in ruminant animals and lactating cows are the most susceptible. Older cows are considered more susceptible 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. Very seldom if ever is GT found in growing cattle (stockers). It is also not related to bloating.

Conditions Leading to Grass Tetany

Grass tetany usually 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 (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 magnesium and high in available potassium (very common in small grain pastures). Soil testing can provide indicators of potential problems. High rates of nitrogen and potassium fertilizer are sometimes associated with increased tetany problems. Forages should be analyzed when a potential problem is suspected. Forage containing less than 0.2 percent magnesium and more than 3 percent potassium and 4 percent nitrogen (25 percent crude protein) are likely candidates to exhibit GT problems. Forage that is high in potassium and nitrogen also should have at least 0.25 percent magnesium. Some research has also shown that low phosphorus levels can also contribute to increased problems on these types of pastures.

Symptoms

Unfortunately in many cases symptoms are not noted and the only evidence is a dead cow. In mild cases, milk yield is decreased, and the animal is nervous. These signs indicate the need for preventive measures. Animals affected by acute GT may suddenly stop grazing, appear discomforted and show unusual alertness, such as staring and keeping their heads and ears in an erect position. Also, they may stagger; have twitching skin, especially on the face, ears, and flanks; and lie down and get up frequently. At this point, they are easily disturbed and any stimulation may lead to startling reactions, such as continuous bellowing or running. Sooner or later, 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 pedal 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.

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. 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. Local recommendations should be obtained before fertilization.

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.

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.

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.

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 particularly practical.

Treatment

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. Therefore, cattle growers 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. As a follow-up treatment, 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.

One precaution, 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.

Conclusions

Grass tetany is a common problem in many herds grazing lush winter and spring pastures. Advance recognition of 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.

Dr. Steve Blezinger is a management and nutritional consultant with an office in Sulphur Springs, TX. He can be reached at sblez@verizon.net or at (903) 352-3475. For more information please visit us on at www.facebook/reveillelivestockconcepts.







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