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Virginia Cooperative Extension - Knowledge for the CommonWealth

Grass Tetany Season is Here

Livestock Update, April 2009

Mark A. McCann, Ph.D., Animal & Poultry Sciences, VA Tech

Early spring is usually the peak period for the occurrence of grass tetany in lactating beef cows in Virginia. Grass tetany is caused by low blood levels of magnesium and is worsened by high levels of nitrogen and potassium and low levels of calcium and magnesium intake. The lush new growth of cool season perennials and annuals consumed by spring calving cows is a recipe for trouble. Heavy nitrogen and potassium fertilization intensify the problem. This makes it more of an issue in the poultry production areas where litter is used routinely as a pasture fertilizer.

Grass tetany can be prevented by supplementing lactating cows with .5-1.0 ounces of supplemental magnesium per day during the high risk period. Magnesium is not effectively stored in the body, as such; supplementation is only cost effective during the high risk periods. There are several commercial high magnesium mineral options available from feed and farm suppliers across the state. Commonly called High-Mag blocks or mixes, these should contain about 14% magnesium with a targeted intake of 4ounces per head per day. For small herds, it is usually more practical and easier to purchase these supplements than to mix individual ingredients.

Cattlemen also have the option of mixing their own magnesium-based mineral mix on the farm.

A mature cow needs to consume 4-5 ounces of the mineral mix daily. With this level of intake, a cow will receive 1-1.2 ounce of magnesium oxide daily. Magnesium oxide is about 60% magnesium so cows will receive .6-.7 ounces of magnesium daily from this mineral mix. In most situations this should control grass tetany. When using this mix or any High-Mag mineral mix, be sure to remove all other sources of salt from the cattle’s diet.

Monitoring consumption of any magnesium supplement is important to insure that cattle are consuming the supplement at a level to provide protection. Magnesium oxide is an unpalatable mineral and the salt and corn/molasses are added in an effort to improve palatability and consumption. Monitoring intake will allow modification in the recipe. It is a mistake to believe that lower than expected consumption is due to a lack of need on the cow’s part. More likely it is an indication of the palatability of the mineral mix.

Other than palatability, low mineral consumption can be caused by poor mineral feeder placement or an inadequate number of feeders for the number of cows. Locate the feeders in the high traffic or loafing areas and provide one feeder for every 20 cows. This will enable cows at the bottom of the herd pecking order to consume enough minerals.

High-Mag minerals are only necessary and effective during high risk situations. There is no advantage in using them during the other times of the year.

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