What is grass tetany?

Source: Michelle Arnold, extension ruminant veterinarian

Spring in the Bluegrass is a great time of greening and warming, but it’s also a time when livestock producers need to watch out for grass tetany, also called spring tetany or grass staggers. Some people also refer to it as wheat pasture poisoning, winter tetany or lactation tetany. Regardless, it’s a condition caused by an abnormally low level of magnesium in the blood.

Maintenance of normal blood magnesium is completely dependent on absorption of magnesium from the diet. Deficiencies occur most often in beef and dairy cows in early lactation that graze lush pastures high in potassium and nitrogen and low in magnesium and sodium. Affected cattle often have low blood calcium concurrently. Typically grass tetany occurs when grazing ryegrass, small grains (i.e. wheat, rye) and cool season perennial grasses in late winter and early spring (February through April), although it can occur in fall-calving cows. Fast-growing spring grass is usually high in potassium and crude protein, and low in sodium and magnesium.

The earliest signs are twitching of the facial muscles, shoulder and flank. As the blood magnesium level drops, muscles stay contracted, so legs are stiff and rigid and eventually cause the cow to stagger and fall. Rapid convulsions or seizures quickly follow, with chomping of the jaws and frothy salivation.

Affected animals lie with the head arched back and the legs paddling. The heart rate may reach 150 beats per minute (normal is 60 to 80) and can often be heard without the use of a stethoscope. Respiratory rates of 60 breaths per minute and a rectal temperature as high as 105 degrees F may result from the excessive muscle activity. Animals may stand up and repeat these convulsive episodes several times before they finally die. A moderate form of grass tetany can occur with milder signs of reduced feed intake, nervousness and reduced milk production.

A veterinarian makes a diagnosis based on history, clinical signs and low magnesium levels in the blood or spinal fluid.

Animals with grass tetany need immediate veterinary treatment. Response to treatment depends largely on the length of time between onset of symptoms and when treatment begins.

Producers who want to prevent grass tetany should start supplementing feed with a high magnesium mineral at least 30 days prior to calving. Cows require 20 grams of magnesium daily or four ounces per day of a 15-percent magnesium mineral mix during the late winter and early spring. Don’t let your mineral feeders get empty because consistent intake is important for prevention. UK Beef IRM mineral recommendations for free-choice supplements for grazing beef cattle include 14 percent magnesium in the complete mineral mix — all from magnesium oxide (no dolomitic limestone or magnesium mica). At least a third of your magnesium oxide should be in the prilled form to increase palatability.

In addition to supplying supplemental magnesium, you can do several things to decrease the risk of grass tetany. These include:

1. Soil test and apply fertilizer based on soil test results and use no more potassium than recommended, since grasses are luxury consumers of potassium.

2. Feed legumes that are high in magnesium to help offset the problem, although their growth is often limited in late winter.

3. Feed small amounts of hay and/or grain to cattle on lush pasture during susceptible periods or limit grazing to 2-3 hours per day.

4. Graze the less susceptible or non-lactating animals (heifers, dry cows, stocker cattle) on the higher risk pastures.

For more information on grass tetany, contact the Clinton County Cooperative Extension Service at 606-387-5404.

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