The causes of grass tetany have been poorly understood, while annual death losses still cost stockmen millions of dollars. Yet new information, combined with older knowledge, can help reduce problems with this ailment.
Grass tetany is also variously called grass staggers, milk tetany, lactation tetany, magnesium tetany, winter tetany, wheat pasture poisoning, crested wheatgrass poisoning and barley poisoning.
It affects mature cattle, especially cows in late gestation or early lactation, which are grazing lush forage such as early spring pastures or sudden growth after rainfall following drought or weather changes.
Classically recommended prevention is supplemental dietary magnesium. The purpose is to flood the system with a lot more magnesium atoms during risky situations. Standard treatment is oral and/or intravenous magnesium.
Woody Lane, of Lane Livestock Services at Roseburg, Oregon, says the low blood levels of magnesium in affected cattle and which affect nerve impulses cause the dramatic symptoms like incoordination and convulsions. As a rule of thumb, he says that magnesium in forages above 0.18% are generally safe, while levels down to 0.12% are risky, and levels below 0.12% are high risk, although these percentages can be influenced by high levels of potassium.
Other nutrients in forage can influence magnesium uptake and metabolism, such as potassium and calcium levels, Lane adds. Forage analysis of what the cattle are eating can give the percentages of potassium, calcium and magnesium, and if the ratio is skewed and magnesium levels are marginal, cattle are at risk for tetany.
Lane says the levels can change in grass plants, depending on several factors. Magnesium tetany is usually associated with fast-growing, highly fertile and lush grass because it contains a lot of nitrogen. However, he adds it doesn’t have to be lush and green. Sometimes winter tetany when cattle are eating grass hay high in potassium, with marginal magnesium levels.
Grasses are the culprit, as opposed to legumes, because grass picks up extra potassium in what is called luxury consumption.
“Grasses love potassium just like some people love chocolate,” Lane says. “Grass requires potassium for growth but if the soil contains excess potassium the grasses take in more than they need.”
“A common recommendation for reducing risk of magnesium tetany is to add legumes to the pasture, but tetany usually occurs during earliest spring growth when soil temperatures are still low,” Lane says. “We often see tetany in the early spring because grass gets started ahead of the legumes that haven’t started to grow yet. The grass is lush and young and high in nitrogen, and maybe high in potassium and low in magnesium. This sets the stage for problems, especially if the animals are not getting enough salt.”
The new twist.
Salt (sodium chloride) is important in the diet of cattle and becomes even more important in helping prevent the complex chain of events that result in grass tetany. High levels of potassium can reduce magnesium uptake by plants. High potassium also can 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 a low level of sodium.
A few years ago, Thomas Swerczek, a veterinary pathologist in Kentucky, stated grass tetany was somehow related to the amount of salt in the diet after many years of observations and study.
“Swerczek postulated that lack of salt was a major factor in the occurrence of this syndrome,” Lane says. “His conclusions were based on his extensive field observations with beef cattle, noting that herds with good access to salt showed fewer cases of magnesium tetany than herds where cows did not eat much salt. Sometimes those herds were consuming standard trace mineral mixtures, but those mixtures contained only low percentages of salt.
“I was skeptical until I reviewed the scientific literature and found that scientists in Europe have conducted good laboratory experiments on the effects of sodium chloride and have developed sound physiological models to support this theory," Lane says.
Swerczek’s observations were looking at magnesium absorption and the effect of sodium levels in the gut. There is a definite physiological, biochemical mechanism by which this occurs. Together with field observations, it makes sense, Lane adds.
Tetany is a complex syndrome in which many factors play a role.
“They interact, and we try to keep cattle safe by including a lot of magnesium ions in what the cattle are eating. Salt (sodium chloride) is part of this picture. Sodium is part of the physiological system for absorbing magnesium from the gut,” he explains.
How to apply the salt knowledge.
Salt is cheap, so during grass tetany season beef producers should make sure their trace mineral mixtures contain enough salt and that the cattle are actually eating it, says Woody Lane, a livestock consultant from Oregon.
However, getting adequate consumption is not so simple. It also depends on what is in the mineral mix.
“If the driving force of intake is salt, then the animals will eat enough of it, desiring the salt," Lane says. "If there are other ingredients that are tasty, like soybean hulls or distillers grains, they will eat the mix because of that—and if the salt level in the mineral mix is low they may not be getting enough salt."
Some mixes, called pre-mixes, may not contain salt, Lane says. They are designed to be added to salt, as stated on the label. If you follow label directions the cattle will have plenty of salt, Lane says.
The other key is whether the animals are eating the mineral mix. The typical way stockmen provide trace minerals is in a block or loose mineral feeder in the pasture, hoping the cattle will eat it. Some do and some don’t. Those which do not are at risk for tetany, Lane says. It helps to have more than one mineral feeder, especially if the pasture is large or there are many cattle.
Sometimes producers add other ingredients, such as a grain mix, to improve performance. “If they add straight corn it’s generally not a problem, but if it’s a commercial grain mix it probably contains salt. If consumption of a mineral mix is primarily driven by salt intake, and then you are feeding something else that has salt in it, there will be more variability on consumption and some animals may not eat enough of the mineral mix,” Lane says.