For cows diagnosed with grass tetany, prompt treatment usually gets the affected cow back on her feet. Unfortunately, the first symptom of grass tetany may be a dead cow that appears to have struggled

Grass tetany is caused by low magnesium in the blood. When blood serum magnesium is low, the symptoms, such as not eating right and not producing adequate milk, are sometimes hard to see, according to Ryan Lock, a graduate student studying the problem at the University of Missouri. In acute cases of grass tetany, the magnesium in the blood serum is so low that the cow's body takes magnesium from the cerebrospinal fluid, which can lead to paralysis, coma and even death.

Most cases of grass tetany occur during periods of stressful weather changes especially when it's cool, damp and cloudy. “Over the years we've often seen these downer type cows from January into April,” says Eldon Cole, livestock specialist at the University Outreach & Extension Center in Mt. Vernon, Mo.

Mr. Lock noticed that that magnesium concentrations are typically at their lowest levels in tall fescue during the early spring – a time when many cows are beginning lactation and require nearly three times more magnesium per day to produce milk. Researchers believe magnesium levels are lower at this time because plants may not be picking up the needed magnesium (Mg) necessary from the soil, especially if the soil phosphorus level is low. Mr. Lock drew on previous MU research that indicated phosphorus applications increase the level of magnesium in tall fescue.

At MU Southwest Research and Education Center near Mount Vernon, Mo., Mr. Lock is conducting research to determine if phosphorus applications to pastures can increase magnesium uptake in fescue and increase Mg intake by cattle. He grazed three groups of cattle in separate pastures for 56 days.

The control group grazed on tall fescue pasture with a soil test of five pounds of available phosphorus per acre. Another group grazed on tall fescue pasture with a soil test of nearly 30 pounds of available phosphorus per acre. The third group was put in a pasture with only five pounds of available phosphorus per acre, but those cows had free-choice access to magnesium in block form.

The cows from the phosphorus-fertilized pasture and the magnesium supplement groups had magnesium blood serum levels 24 percent and 20 percent higher than that of the control group. "In both those groups, the magnesium blood serum level was significantly greater," Lock said.

The experiment indicates that tall fescue in pasture with 30 pounds per acre of available phosphorus "provided the same protection against grass tetany as supplying free-choice magnesium supplement," Lock said.

The researchers also noted that the high phosphorus pastures "greened up" sooner in spring, he said. "We think that when plants have enough phosphorus around the roots, it opens a 'gateway' in the roots that's specific to allowing the magnesium to get in and hasten the process of chlorophyll production."