Nitrate Toxicity in Cattle

As we increase our knowledge of modern agronomic practices to help improve forage quality, there are factors to consider for grazing livestock. Nitrate toxicity is a non-infectious condition that can be fatal and results from overconsumption of plants fertilized with nitrogen or from water contaminated with nitrate-containing fertilizers (Thompson 2026). Nitrate poisoning, which can especially affect ruminants, occurs when rumen microbes rapidly convert nitrates in forage to high levels of toxic nitrites.The effects of nitrate poisoning can be chronic or subacute and can cause major setbacks in your grazing cattle herd. This article reviews the Merck Veterinary Manual and other sources to discuss the basics of nitrate toxicity in cattle.

The Cause

Under normal conditions, plants absorb nitrogen from the soil in the form of nitrates during the nitrogen cycle Figure 1. Through the process of photosynthesis and plant metabolism, plants use nitrates to synthesize amino acids, proteins, and other essential compounds required for growth or simply assimilated as non-protein nitrogen. Plants that experience stressors, such as drought, frost, or hail, cannot utilize these nitrates and tend to accumulate them. Many factors, such as soil type, availability of nitrogen in the soil, and chemical damage, will also affect the amount of nitrate uptake by the plant. Certain crops and plants are also more likely to naturally concentrate nitrates (Agricultural Research Service, USDA). See Table 1 below.  Plants that experience stressors typically concentrate nitrates in the lower stem or vegetative part of the plant and not the seed (Agricultural Research Service, USDA).

Adapted from "Nitrate and Nitrite Poisoning in Animals." Merck Veterinary Manual

The Effects

When livestock ingest plants with high nitrate levels, the nitrates are broken down by rumen microbes into nitrites, which are rapidly absorbed. The body then forms methemoglobin, a dysfunctional form of hemoglobin. Unlike normal hemoglobin, methemoglobin cannot carry oxygen.  Because of this, animals can experience labored breathing, muscle tremors, staggering, excessive salivation, blue or purple mucous membranes, bloat, weakness, and even death. Typically, these symptoms appear within 6 to 8 hours of exposure, but can appear as early as 30 minutes or as late as 6 days, depending on the amount of feed ingested. If the animal survives, nitrate poisoning can cause secondary effects such as abortions, which can be seen 5 to 4 days after exposure, poor weight gain, and reduced milk production.

Diagnosis and Treatment

Nitrate poisoning can be diagnosed with the appearance of clinical signs. Serum, ocular fluid, fetal fluid, or urine can be collected and sent to a laboratory for diagnosis. Affected animals often present with chocolate colored blood. Testing of suspected forages can be done in a forage testing laboratory.  If clinical signs are identified early enough, veterinary intervention may be an option. Affected cattle can be treated with intravenous methylene blue to reduce methemoglobinemia. Multiple injections of methylene blue may be required depending on the severity of the toxicity. Methylene blue must be prescribed by a veterinarian. Always seek care from a veterinarian for this treatment.  The source of the toxicity should be identified and removed to reduce exposure. It is also recommended to reduce stress when handling affected animals to help reduce the effects of labored breathing and anxiety.

Vitamin A deficiency has also been noted in livestock exposed to nitrates (Agricultural Research Service, USDA). It may be beneficial to supplement vitamins if worried about exposure to nitrates through mineral supplementation or a complete feed.

Prevention

Environmental stressors regarding forages and plants are sometimes inevitable. Ensure proper nitrogen application practices to crops and grazing pastures. If a crop experiences stress, it is recommended to have it tested at a forage testing laboratory before feeding. If high nitrates are confirmed, dilution may be the solution. Diluting a high-nitrate forage with a low-nitrate forage will reduce the overall nitrate intake (Agricultural Research Service, USDA). Ensiling affected forages can also reduce nitrate content by up to 50%. This, in conjunction with raising the cutter heads during harvest and leaving more of the stalks in the field, can help reduce nitrate exposure. Take caution when considering the type of feed to make from forage exposed to stressors. Hay and green chop have been noted to be more hazardous for nitrate toxicity than pasture. Heat can increase the conversion of nitrate to nitrite, so feeding hay or straw that is wet should be avoided.

Making good forages at harvest is the key to preventing nitrate accumulation. Timely harvesting at optimal moisture levels will help prevent dry matter and nutrient loss, as well as disease. When harvest conditions aren’t favorable, have a plan to mitigate the effects of potential problems for the health of your cattle and your farm profits. Forage testing is an important tool to use when nitrate toxicity is a potential risk.

Sources:

"How Do Plants Sense and Acquire Nitrate?" Plantae, American Society of Plant Biologists, 27 April, 2018, accessed 21 May 2026.

"Nitrate-Accumulating Plants." Agricultural Research Service, U.S. Department of Agriculture, accessed 21 May 2026.

Thompson, Larry J. "Nitrate and Nitrite Poisoning in Animals." Merck Veterinary Manual, Merck & Co., Inc., modified April 2026,