The cause of grass tetany (also called grass staggers, milk tetany, lactation tetany, magnesium tetany, winter tetany, wheat pasture poisoning, crested wheatgrass poisoning, barley poisoning) has been poorly understood, yet death losses cost producers millions of dollars.It affects mature cattle grazing lush forage such as early spring pastures, sudden growth after rainfall or after weather changes such as freezing.
This disease was first described in Britain in 1930, associated with magnesium deficiency and calcium deficiency (milk fever) and excess potassium in the blood. During cool, wet conditions or regrowth after frost or drought damage, nitrogen and potassium levels increase. Prevention is supplemental dietary magnesium. Treatment involves oral and/or intravenous magnesium.
Woody Lane, Ph.D. – owner of Lane Livestock Services in Roseburg, Oregon – says low blood levels of magnesium (affecting nerve impulses) cause incoordination and convulsions. Magnesium levels in forages above 0.18% are generally safe, while levels down to 0.12% are risky. Levels below 0.12% are high risk, although these percentages can be influenced by high levels of potassium.
For prevention, producers try to make sure cattle have lots of magnesium during risky situations – lush grass pastures that may coincide with cool, rainy weather. It mainly affects mature animals, especially cows in late gestation or early lactation. Other nutrients in the forage can influence magnesium uptake and metabolism.
“Potassium and calcium levels can affect the tetany ratio,” Lane says. Forage analysis can show percentages of potassium, calcium and magnesium, and if the ratio is skewed and magnesium levels are marginal, cattle are at high risk.
The levels can change in grass plants, depending on several factors. “Magnesium tetany is usually associated with fast-growing, lush grass, containing lots of nitrogen. But it doesn’t have to be lush and green; sometimes we see winter tetany when cattle eat grass hay that’s high in potassium with marginal magnesium levels,” he explains.
Grass is the culprit, picking up extra potassium. “Grasses love potassium. They require it for growth, but if soil contains excess potassium, grasses take in more than they need.” Potassium levels in alfalfa and clovers aren’t high enough to interfere with magnesium absorption in the animal.
“Common recommendation for reducing risk is to add legumes to pasture, but tetany usually occurs during early spring growth when soil temperatures are still low. We often see tetany in early spring because grass gets started ahead of legumes. The grass is lush, young and high in nitrogen, and maybe high in potassium and low in magnesium. This sets the stage for problems, especially if animals don’t get enough salt,” says Lane.
The importance of salt
Salt (sodium chloride) is important in cattle diets and becomes even more important in helping prevent the complex chain of events that result in grass tetany. We know high levels of potassium can reduce magnesium uptake by plants, and can also reduce magnesium absorption by the animal, so fewer magnesium atoms cross into the blood from the digestive tract, yet the effect of high potassium levels may be less important than low sodium level.
“This fact is something you won’t find in standard literature about grass tetany. The person who first saw this was Dr. Thomas Swerczek, a veterinary pathologist in Kentucky. After many years of observations, trying to solve the mysteries of magnesium tetany, he said this syndrome was somehow related to amount of salt in the diet.” Swerczek’s conclusions have been controversial in the science world, however, because there were no formal studies to prove his theory.
“What he saw was a manifestation of what was going on physiologically in these animals. Swerczek postulated that lack of salt was a major factor, based on extensive field observations with beef cattle. He noted that herds with good access to salt showed fewer cases of magnesium tetany than herds where cows didn’t eat much salt. Some herds were consuming standard trace mineral mixtures, but those mixtures contained low percentages of salt,” says Lane.
“I was skeptical until I reviewed scientific literature and found that scientists in Europe [who had no connection with Swerczek’s work] conducted good laboratory experiments on effects of sodium chloride and developed sound physiological models to support this theory. Swerczek’s observations were very good; he added another layer of understanding to a very complex problem, giving us one more part of the picture. It wasn’t just his observations, but also the physiology behind his thinking that has been shown in other laboratories. They were looking at magnesium absorption and the effect of sodium levels in the gut, and it all made sense. There is a physiological, biochemical mechanism by which this occurs, explaining why Swerczek was seeing what he was seeing in the field. Then it all makes sense – even without extensive studies and experimentation,” Lane says.
“Salt is cheap, and during tetany season, producers should make sure trace mineral mixtures contain enough salt, and that cattle are actually consuming it,” he says.
It also depends on what else is in the mix. “If the incentive for intake is salt, cattle will eat enough of it, desiring salt. If there are other ingredients like soybean hulls or distillers grains, they eat the mix for that – and if the salt level is low, they may not be getting enough salt. Some mixes, called pre-mixes, might not contain salt. They are designed to be added to salt. If you follow label directions, the cattle will have plenty of salt,” he says.
The other key is whether the animals are eating the mix. The typical way we provide trace minerals is in a salt block or mineral feeder in the pasture, hoping cattle eat it. Some do and some don’t. “If 10 percent of them don’t eat it – for whatever reason – they might be at risk for tetany,” Lane explains. It helps to have more than one mineral feeder, especially if the pasture is large or there are many cattle in it.
Tetany is a complex syndrome; many factors play a role. “We try to keep cattle safe by including a lot of magnesium ions in what they are eating. Salt is part of this picture. Sodium is part of the physiological system for absorbing magnesium from the gut,” he explains.
“Magnesium tetany is a frustrating problem, studied by many scientists for a long time. What happens on a farm when cattle die – occurs when the animals, the weather and the system doesn’t fit perfectly. If there’s not enough magnesium, or if it’s not being absorbed at high enough levels, you’ll have a problem. Usually, this is associated with lush spring growth, lush fall growth, winter feeding grass hay with low magnesium and high potassium levels, etc. Then if some animals in the herd are not eating enough of the trace mineral mix, some will develop tetany,” says Lane.
As an analogy, plane crashes generally occur when multiple things go wrong. “Most of the time, airplanes fly just fine. If they crash, it’s usually because half-a-dozen things combined at the same time. Failure of any one of those would not have caused the problem, but together they contributed – and this is what happens with magnesium tetany,” Lane says. It’s not simple.
“I lived on a beef cattle operation in West Virginia a couple years, and one of the time periods when animals showed signs of tetany was midwinter when we were feeding hay. Cattle are often fed grass hay in winter, and if it was made during a period when grass was high in potassium, it can skew the tetany ratio,” says Lane.
