Be Aware: Grass Tetany

By Tracy Brewer

I was just in Southern States this past weekend and grass tetany is already rearing it’s ugly head for some producers. Here are the facts so you can be prepared as we go into spring

What is grass tetany and when does is occur?

Grass tetany is a nutritional or metabolic condition in beef cattle and sheep triggered by low amounts of magnesium (Mg) in the blood serum. This disorder occurs most frequently in the spring when livestock graze young, succulent, cool season grasses. It intensifies in warm periods, five to 10 days after a cool, wet period when grass is growing rapidly. Although it is less prominent, grass tetany can also occur in the fall when regrowth of cool season grasses occurs.

What causes grass tetany?

Factors that can increase the occurrence of grass tetany in livestock include stress, drought, diets low in Mg and phosphorous (P), diets with nutrient imbalances that interfere with Mg metabo- lism, high levels of nitrogen (N) or potassium (K) in feed or soils, increased Mg demand during lactation, minimal availability of standing dead forage, and forages with a “tetany ratio” {K/ (Calcium + Mg)} of greater than 2:2. Because it is a function of soils, plant species, harvested feed, environmental factors and animals, the severity of this disorder varies between states, coun- ties, ranches and pastures.

What are the symptoms?

Early symptoms of grass tetany include uncoordinated gait, staggering, nervousness, excitability and muscle spasms. In many cases, animal mortality is the first sign of grass tetany identified, due to a short, four - to eight - hour time interval between the onset of early symptoms and death.

Which animals may it affect?

Female animals have been shown to be more prone to grass tetany than males. Older animals, animals that have recently given birth, those nursing young less than eight weeks of age, high milk producers and fat animals are also more susceptible to grass tetany than other classes of animals. However, it has also been seen in young or dry cows and in growing calves in extreme conditions. Older animals have a diminished ability to absorb Mg, Mg requirements increase greatly directly post - partum and during heavy lactation periods, and fat cows have less Mg available for absorption in their body fluids than lean cows.

Grass Tetany in NC Several recent surveys have shown that the major minerals most likely to be deficient in the eastern US are salt and magnesium. The three trace minerals most likely to be deficient across the en- tire country are zinc, copper, and selenium. When cows are defi- cient in trace minerals they appear unthrifty and in poor body con- dition, and when they are deficient in magnesium they can die suddenly for no obvious reason.

Many cows sent to the diagnostic lab in North Carolina for necropsy are diagnosed with magnesium tetany (also called grass or winter tetany), and also often have very low selenium and copper status. In other areas of the country different minerals such as phosphorus may be deficient, so local recommendations should be used in developing mineral supplementation programs. Traditionally, cattle producers have used trace mineralized salt blocks (“red salt”), sulfurized salt (“yellow salt”) or just plain white salt as their mineral program. The problem with these supplements is that the only mineral they provide in any signifi- cant amount is salt. For example, common “red salt” contains salt and some trace minerals, but does not provide magnesium or selenium, nor adequate levels of either zinc or copper. Because of a general risk of grass tetany on high fertility pastures in the eastern US, a “high mag” complete mineral (8 to 12% magnesi- um) might be used at all times unless in an area where grass tetany is rare.

When tetany is rare, we still recommend that producers use a “high mag” product from 30 days before the cows calve until the end of the lush growth period in late spring. For simplicity, most producers with small herds choose to use “high mag” mineral all year. There are many “high mag” product choices on the market. Producers should buy a product that is labeled for consumption at 4 oz/day per cow. It should have 8% or higher magnesium, at least 0.09% copper (900 ppm, from copper sulfate or chloride), at least 0.18% zinc (1800 ppm), and at least 0.0026% selenium (26 ppm).

The mineral supplement should be available at all times in a covered feeder, should be kept fresh and dry, and intake should be monitored to make sure cows consume close to the recom- mended amount. Cows used to salt may consume a large amount of the complete mineral supplement at first but should slow in- take to near target levels after several weeks. If cows don’t eat enough mineral, mixing in dry molasses or grain can encourage initial consumption. If problems with over - or under - consumption continue, check with the manufacturer for advice, or switch products

Source:ncsu.edu