The cause of grass tetany (grass staggers, milk tetany, lactation tetany, winter tetany, wheat pasture poisoning, crested wheatgrass poisoning, barley poisoning) has been poorly understood, yet annual death losses cost beef producers millions of dollars. It affects mature cattle grazing lush forage, after weather changes like freezing early spring pastures or sudden growth after rainfall following drought.
This disease was first described in Britain in 1930, associated with magnesium deficiency and calcium deficiency (“milk fever”) and excess potassium in the blood of affected animals. During cool, wet conditions or regrowth after frost or drought damage, sodium levels in certain forage plants plummet, while nitrogen and potassium levels spike. The dead cattle have high levels of potassium in their eye fluid.
Recommended prevention has been supplemental dietary magnesium. Standard treatment has been to give affected cows oral and/or intravenous magnesium.
After examining cattle lost in 2001 following spring frosts in the Midwestern U.S., and analyzing the pastures, Dr. Thomas Swerckzek (a veterinary pathologist in Kentucky) found clues about the cause and prevention of grass tetany.
“Each spring, farmers were advised to get magnesium into their cattle when pastures were growing fastest,” he says. “Producers tried this, but it didn’t work.”
Then nutritionists said farmers should start four to five weeks before peak pasture growth. “That didn’t work either,” says Swerckzek. “By the 1980’s they advised year-round supplement, to get it into the animals’ bones.” Theoretically, cattle could pull it out of bone storage when blood levels dropped due to sudden pasture changes.
As a pathologist, Swerczek did diagnostic necropsies on livestock until the early 1970s. Then for 15 years he worked as an equine diagnostic research pathologist. In 1986 he continued his necropsy research on all classes of livestock. When he resumed doing necropsies on cattle, he was shocked to see how much worse the grass tetany situation had become.
“The dead cattle were arriving in a wasted condition I’d never seen before,” he says. “When I’d stopped doing cattle necropsies in 1971, the dead cattle were in good shape, so I noticed how much they’d deteriorated.” After coming back to cattle necropsies in 1986 he hated to be on necropsy duty after a cold spell because so many dead cattle were coming into the lab.
“By 1995 I started seeing adult cattle with salmonellosis, coccidiosis and other calfhood diseases, which didn’t make sense. Perhaps the cows were eating something the calves were not,” he says. He began looking at diets and mineral mixes, because that’s what had changed over the years.
He collaborated with a bovine veterinarian, Dr. William McCaw, who was working with several purebred herds trying to find answers. McCaw thought there was something related to diet that was causing wasting and opportunistic diseases. Although reluctant, they found a few producers who agreed to stop feeding minerals to help them in their research.
When Swerczek started looking at herds throughout the state, he found a farm with very healthy Hereford crossbreds. The owner was feeding loose salt rather than mineral mixes. Most farmers in that area fed mineral mixes and salt/mineral blocks instead of loose salt. Cattle often overate mineral mixes, to get the little bit of salt that was in them.
The farmer with the crossbred cattle had a salt house in every pasture. “He wasn’t feeding any magnesium. He’d been in the cattle business more than 40 years and hadn’t had a case of grass tetany,” says Swerczek.
Not a magnesium issue
“This was a hint that maybe it wasn’t necessary to feed magnesium to prevent grass tetany. Later, when I got several herds off the mineral mix, they quickly started to turn around. Most of the cows had been suffering from diarrhea, wasting away, and within 24-48 hours they improved, after giving them plain loose salt instead of mineral.”
He was working with a herd of about 1,000 Angus and driving through that farm with the manager. “We came across a cow that had been down for several days in spite of multiple treatments with magnesium and calcium.”
Swerczek had some sea salt, and put it in front of that cow. After ingesting a couple handfuls of salt the animal got up and was able to rejoin the herd.
There were other cattle in the herd showing signs of grass tetany and going down. “I told the manager to put a handful of salt in front of them. Those cows came out of it. I asked if he’d be willing to take away all the minerals and just feed salt. He was afraid to do that, but agreed to put a small group in a different pasture and try it. The next cold spell, cows in the pasture where he had the salt were not affected.”
“We had massive losses in Kentucky one year, due to an unusual winter with many warm spells. Grass and clover grew early. Then we had a hard freeze in April. Cattle went down by the thousands with grass tetany and bloat. People were using bloat blocks but this didn’t help. Cattle were actually dying while eating those, because they didn’t have salt,” he says.
“We’d been taught for many years (and people still believe) that nitrate is not toxic — that nitrite is the problem. In the 1940’s when nitrate was discovered as the cause of corn stalk toxicity, it was nitrites causing shortage of oxygen in the blood. But I found that nitrate is 100 times more important in grass tetany syndrome than nitrite,” Swerczek says.
Body looks for salt
The body must get rid of the nitrate and it does this through the cations, especially sodium. “When there isn’t adequate salt in the blood, the body grabs onto the most available cation, which would be magnesium and then calcium,” Swerczek says. When the spike of nitrate occurs — when the cow consumes frost-damaged forage the body immediately uses magnesium in the blood to get rid of the nitrate, which depletes the body, and this is why the cow goes down.
“If there’s enough salt available the body can grab onto the sodium and cows don’t go down with grass tetany or milk fever. If you don’t have salt out on the day this hits, they go down. It has to be there all the time and it can’t be hard salt blocks because cattle can’t eat enough when they suddenly need it.”
The body usually has the ability to keep sodium levels within normal range, but when it drops lower, you only have a few hours before that animal dies. “If you feed salt, however, and the animals eat it, they’ll be fine—as long as they have plenty of water,” says Swerczek. “British scientists in the 1930’s noticed that salt could prevent grass tetany, but no one put it all together. Grass tetany should be called nitrate toxicity/salt deficiency leading to hypomagnesia/hypocalcemia.” †