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High-Nitrate Feed Poisons Cattle
Cool-season fescue is the grass of choice for Missouri beef producers putting up hay for winter feeding beef cattle. But two years of drought have now put fescue hay in short supply, driving producers to find winter forage from alternate feedstuffs like baled pearl millet, sudangrass, and drought-stricken corn.
Recent reports of livestock deaths due to nitrate poisoning raise the red flags all beef producers should watch out for when harvesting and feeding alternative feedstuffs that can accumulate nitrates during dry growing conditions.
By early March, the University of Missouri (UM) Veterinary Medical Diagnostic Laboratory had diagnosed more than 200 deaths from nitrate poisoning over a period of about a month. The deaths were occurring at multiple locations around the state, involving all classes of cattle.
“It’s just a bad luck of the draw,” says UM Extension beef nutritionist Eric Bailey. “We’ve had drought conditions since the fall of 2017, and our stored supplies of fescue hay are stretched thin. People have turned to feeding any kind of hay they can acquire.”
That was the case at one backgrounding lot that lost 100 head of calves after feeding purchased forage. “It was sudangrass hay that hadn’t been tested for nitrate content,” says Will Gentry, a consulting veterinarian from Diamond, Missouri. “It’s a catastrophic loss, yet it’s a number that represents less than 10% of the animals that were exposed to the feed.
“As people are forced to outsource more feed, they risk buying hay that was grown from plants stressed by drought,” adds Gentry. “Nitrate content in the plants tends to increase in dry growing conditions.”
This is especially true for some warm-season crops and even more so when these crops are fertilized. “When out of balance, nitrate-nitrogen – and its more toxic form, nitrite-nitrogen – concentrations may be great enough in either plants or water to harm or even kill animals,” writes UM forage specialist Robert Kallenbach in the online document “Nitrate Problems in Livestock Feed and Water.”
“Under ideal growing conditions, plants do not contain dangerous concentrations of nitrates/nitrites because the plants rapidly convert the nitrate into protein,” he continues. “However, when environmental conditions arrest growth, plants cannot convert the nitrate into protein as quickly as the nitrate is taken up. Thus, nitrate accumulates in plant tissues.”
Factors that predispose plants to nitrate accumulations include “excessive application of manure or commercial nitrogen fertilizers, water-stressed conditions, and cold weather,” says Kallenbach.
Forages particularly prone to accumulating nitrates under stressed growing conditions include corn, sorghum, sudangrass, sorghum-sudangrass hybrids, and johnsongrass.
Yet a wide range of crops and even weeds, if stressed, can accumulate nitrate. Among these, says a North Dakota State University document, are barley, millet, oats, rye, wheat, Canada thistle, dock, kochia, pigweed, and smartweed.
When a cattle’s diet is made up primarily of feeds containing excessive levels of nitrates, the poisoning process can occur rapidly, and the prognosis is poor. “The nitrates bind with hemoglobin in the blood, and the cattle can’t get oxygen,” says Bailey. “They asphyxiate. You might feed cattle and come back 12 hours later to find a number of deaths.”
The mucous membranes turn blue in acutely afflicted animals and the color of the blood turns chocolate-brown. Poisoned animals are weak, have muscle tremors, and labored breathing.
“Because the oxygen-carrying capacity of the blood is decreased, it’s important not to stress the cattle,” says Gentry. “If you need to handle them, do it calmly and quietly.”
In an outbreak of nitrate poisoning in a group of cattle, immediately remove protein supplements and dilute the suspected problem feed with other feed – especially grains. This can help the as-yet-unaffected cattle handle the nitrate concentrations at lower levels in the ration.
“Increase other feeds in the ration so that the problem feed makes up no more than 25% of the ration,” says Bailey. “Increasing the carbohydrates in the diet with corn or other grains stimulates the microbes in the rumen to use nitrate before it can accumulate in the rumen.”
Testing suspicious feeds for nitrate levels at harvest or before feeding in winter is the best way to know whether or not a feedstuff contains an unsafe level of nitrate. “If you plan on feeding warm-season stored forages, you better be on high alert,” says Bailey. “Testing for nitrates can be done at the same time that you have a regular forage analysis done on feedstuffs. But it’s not part of the standard package. You have to request it.”
On test results, nitrate content in forages is reported in parts per million (ppm). Forage with a nitrate level of 2,500 ppm or lower is safe to feed. “But when the nitrate level is between 2,500 ppm and 5,000 ppm, be cautious about feeding it to young calves or pregnant females,” says Bailey. “If it tests above 5,000 ppm, limit that forage to 25% of the diet.” He notes that nitrate does not dissipate from hay over time.
If forages test high in nitrates prior to harvest, ensiling is the safest way to harvest and store the feed. “The fermentation process in ensiling can reduce nitrates in forage by as much as half,” says Bailey.
Patrick Wall, Iowa State University (ISU) Extension beef specialist, adds, “In a feed emergency, green-chopped, drought-stressed corn fed directly to cattle without ensiling has a high risk for nitrate toxicity.”
Acute as well as subclinical nitrate poisoning can occur on pasture as well. Cattle grazing a mix of forages may ingest weeds, for instance, that are high in nitrate. The subclinical nitrate poisoning can result in midsummer abortions.
The Role of Weather at Harvest
Nitrate accumulation in plants results from a complex interplay of weather and growth stage of the plant. As a rule of thumb, drought-stressed plants are prone to accumulating nitrate. But weather conditions right at harvest can rapidly increase the nitrate uptake. This can be particularly dangerous if a crop is to be harvested by grazing.
“In late August or September, for instance, corn has a high demand for water,” says Patrick Wall, ISU beef specialist. “If the conditions have been dry but then it rains, that nitrogen is drawn up into the plant. The crop can be more dangerous to livestock after a rain than it would have been if the plants had stayed dry.
“You have to test for nitrates right before turnout for grazing,” he says. “If it was dry when you tested but then it rains two days later and you turn out after that, you could have trouble.”