When Pastures Turn Deadly
Courtesy of University of Missouri
When drought stops plants making protein, nitrate poisoning can kill grazing livestock
Drought-stricken forages that accumulate nitrates can kill grazing livestock, and quickly, warns a University of Missouri plant scientist.
"We’re getting reports of cattle dying," says Rob Kallenbach, MU Extension forage specialist. "As hot weather without rain continues, we expect to hear of more death losses. It happens at the start of every drought."
Large grasses, such as corn, sorghum and sudangrass hybrids, are most often the cause of problems, Kallenbach said on a statewide teleconference Thursday morning. Many plants, even ryegrass and fescue, can accumulate nitrates when soil moisture becomes short.
Johnsongrass and other common weeds can be deadly also.
Nitrogen is essential for forage and grain-crop production. Nitrates are in the plants all the time, creating normal growth. Nitrogen picked up by plant roots from the soil moves up into the plant. Eventually the plant stores that energy in the seed heads as protein.
Nitrates are converted into amino acids, which are building blocks for plant proteins. Protein is an essential part of animal diets.
Lack of moisture stops the flow of nitrates up the plant and the conversion to protein. The roots continue to bring nitrogen into the plant, where it accumulates first in the stalks. Too much unconverted nitrate can become toxic.
In a drought, producers needing forage turn cows to graze corn, sorghum or other large grasses. Usually the only time a farmer grazes corn would be when it is obvious the plant will not make ears of corn for grain harvest. Grazing is considered when drought stops conversion of nitrate into protein.
That’s when deadly trouble occurs.
Cornstalks and other plants can be given a quick test for nitrates. A few drops of test solution on a split stalk turn deep blue when high levels of nitrate are present.
Most MU Extension county offices have test kits to provide quick nitrate checks. This test gives only rough indications of potential problems. It’s a warning.
A more accurate, quantitative test must be done in a laboratory, but that takes time. The lab test works best on stored forages such as bales, balage or silage.
On the teleconference, a regional specialist asked Kallenbach about corn being chopped and fed to cow herds. That is being done already in dry areas of southern Missouri.
"That works well—if it is done quickly," Kallenbach said.
The worst thing is to chop a load of cornstalks, then let the forage sit on the feed wagon overnight. In that time, the deadly nitrates convert into even deadlier nitrites.
"If you feed a load of high-nitrite corn to your cattle in the morning, by noon you can be out of the cattle business. The cows put their four feet into the air," he added.
Nitrites tie up the oxygen-carrying capacity of blood hemoglobin. Without oxygen, the cow suffocates.
At even low levels of nitrate, pregnant cows can lose their calves.
Grazing drought-stressed cornstalks is safer than chopping, if managed right. Cows prefer eating corn leaves first. Usually, leaves have less nitrate content than stalks.
Management-intensive grazing works when a strip of a cornfield is fenced off with an electric wire. When the herd eats all of the leaves, but before they start eating nitrate-rich cornstalks, the cows are moved to a new grazing paddock.
Even after rains come, the water won’t clear up problems overnight. It takes the plant at least five days to convert nitrate to safer levels of amino acids. If there are no ears of corn on the standing stalks, conversion takes longer.
When cattle run out of pasture, farmers turn to alternative forages, Kallenbach said. Slow down and make a quick nitrate test to ensure safety of the herd. It is so long between severe droughts that people forget lessons learned in the last drought.
