Nitrite toxicity becomes a concern with extreme heat stress, drought

The topic on every farmer or rancher’s mind these days is the dangerous drought of late and the hope for rain in the future. The pastures and ponds are drying up in the High Plains and the vegetation that remains could be a death sentence to livestock that consume it.

Alex Rocateli, Extension specialist for forage systems at Oklahoma State University, said nitrate is the primary nutrient form of nitrogen in most soils; however, extremely dry conditions can turn plants toxic and producers need to be monitoring their pastures and fields.

“Under normal weather conditions, nitrate is assimilated rapidly following soil absorption, and its concentration in plant tissues is low,” he said. “However, nitrate accumulation can be triggered by drought stress where plant growth is restricted, but nitrate absorption from soil continues. Drought combined with high levels of nitrogen fertilization is the frequent cause of nitrate toxicity in livestock during the summer. When nitrate accumulation occurs, the concentration is greater in lower portions of the stems.”

Rocateli said the forages with the highest potential to accumulate nitrates are sorghum, sudangrass, Johnsongrass, corn and pearl millet. The forbs, or flowering, non-grassy plants, that have the greatest possibility of accruing nitrates are horsenettle, kochia, pigweeds and lambsquarter.

“When a ruminant animal ingests nitrate, it undergoes a chemical reduction action and turns into nitrite,” Rocateli explained. “Nitrite is readily absorbed into the bloodstream, where it oxidizes the ferrous iron of the red blood pigment hemoglobin to a modified red blood pigment called methemoglobin. Methemoglobin is incapable of transporting oxygen to various body tissues.”

Cattle and horses that ingest high nitrate feeds may show the following symptoms: difficult and painful breathing, cyanotic membranes, rapid breathing, muscle tremors, weakness, low tolerance to exercise, incoordination, diarrhea, frequent urination, dark or chocolate colored blood, collapse and reduced milk production. The animals will essentially die of asphyxiation due to lack of oxygen.

“Nitrate poisoning may cause death within the one-half hour to four hours after symptoms appear,” he said. “Symptoms usually appear when methemoglobin reaches 30% to 40%, and death occurs when methemoglobin reaches 80% to 90%. Therefore, observe animals closely for signs of toxicity, remove animals from forage if symptoms are present and call a veterinarian immediately if symptoms occur.”

How to avoid nitrite toxicity

Most of the time nitrate levels are not a concern, but the weather most are experiencing this year requires extra monitoring to protect livestock from plant toxicities. Although we cannot inspect every leaf and stem our cattle eat, there are some basic management practices that can prevent deaths from nitrite toxicity.

“Keeping livestock from ingesting forages and forbs with a high potential to accumulate nitrates is challenging,” Rocateli said. “The best approach is to have proper fertilization, weed control and grazing management to minimize undesired forbs and weeds and maximize desirable forages in the pasture. Pay special attention to Johnsongrass because it is an ‘ice cream plant.’ The cattle prefer to graze Johnsongrass over other forages, therefore a drought-stressed pasture with a high occurrence of Johnsongrass is troublesome.”

He said producers who decide to graze a pasture with suspicion of high nitrates should not turn out hungry cattle. Instead, feed the animals with no nitrate forage first.

“Also, allow limited cattle time in the high nitrate pasture. Then, gradually increase the time spent grazing and the amount of high nitrate forage consumed by the animals.”

Nitrate levels are also a great concern when it comes to hay. Rocateli recommends producers test forage whether they are baling their own hay or purchasing from a hay producer.

“If the nitrate concentration is 0 to 3,000 parts per million, it is generally safe for all cattle. Levels 3,000 to 5,000 are generally safe for non-pregnant beef cattle. There is a low risk of reduced breeding performance and early term abortions. The total ration for dairy cattle should be less than 2,500 ppm nitrate. If the forage tests 5,000 to 10,000 there is some risk for all cattle. These levels may cause mid to late-term abortions, weak newborn calves, decrease growth and milk production. Levels greater than 10,000 are potentially toxic for all cattle and can cause abortions, acute toxicity symptoms, and death.”

Rocateli recommends producers wait for rain if their forage tests high in nitrate levels.

“If you decide not to wait for rain and cut forages with high nitrates, the nitrates will not degrade with time,” he warned. “It will be present in the final hay. Therefore, if you decide to cut drought-stressed plants with a high potential to cumulate nitrates, raise the cutting height to 10 to 15 inches. Most of the nitrates accumulate in the lower portions of the stems.”

Additionally, Rocateli said it is important to wait five to seven days after rain to cut hay or introduce cattle into pastures. During this time, the plant will assimilate the nitrates and return to safe levels.

Every state has a forage testing center to available to determine nitrate levels. For example, the Soil, Water and Forage Analytical Laboratory at Oklahoma State University offers nitrate testing. Additionally, Rocateli advised producers to contact their local county agriculture educator, who can assist with proper sampling, lab analysis submission and result interpretation.

Lacey Vilhauer can be reached at 620-227-1871 or [email protected].