Nitrate toxicity in forages is a concern for many livestock owners. It can have significant negative economic impacts, as well as negative effects on livestock health and performance. Horses, cattle, sheep, goats, llamas, alpacas and other grazing animals can be harmed by nitrate toxicity.
Symptoms of nitrate toxicity vary depending on whether it is a severe, one-time ingestion, or a chronic, long-term ingestion. Both situations can be fatal to the animal, but an acute ingestion can result in death within six to nine hours. Other signs of acute toxicity include difficulty breathing, frothing at the mouth, tremors, collapse, and discoloration of the mucous membranes and blood. Signs of chronic toxicity include watery eyes, decreased appetite, decreased production, and abortion. If any of these signs are observed, or if there is a chance of accidental ingestion, alert your veterinarian immediately.
Critical to controlling and preventing nitrate toxicity is knowing the cause. Nitrogen is an integral component of plant growth and production, critical for plant survival. Nitrogen is important for plant structure, growth, and reproduction. Under normal conditions, nitrogen in the chemical form of nitrate is taken up from the soil by the plant and converted to plant protein. However, when plants become stressed they cannot convert all of the nitrate to plant protein, resulting in excess plant nitrate levels in the stems and leaves. This typically occurs during conditions such as drought or excess fertilizer application, and can be affected by factors such as time of day, plant maturity, plant part, or herbicide application. Additionally, species such as oats, wheat, and barley are known as “nitrate-accumulators,” and develop nitrate toxicity at increased rates compared to other forage species.
How does one decrease the effects of nitrate accumulation on plant and animal production goals? The first thing is to ensure appropriate amounts of fertilizer are being applied to forages. Soil testing is a key factor in proper application and should be done prior to planting. Knowing soil phosphorus, nitrogen, and pH levels is especially important in managing nitrate toxicity, as phosphorus-deficient and acidic soils increase the chances of nitrate accumulation. Only apply as much fertilizer as outlined in the test results, which will save money as well as decrease chances of nitrate toxicity. Do not harvest or graze forage plants immediately after fertilizer application, as nitrate levels will be elevated at this time.
Second, avoid cutting forages after a drought-ending rain or irrigation application, as this is when nitrate levels peak. Nitrate accumulation in forages follows a diurnal pattern, peaking in the morning and dropping during the evening. It is safer to graze or harvest in the afternoon or evening to avoid forage plants when they are at their highest daily nitrate levels.
Thirdly, test forages for nitrates. Get a sample that is representative of the entire field, meaning sampling plants in different areas of the field and combining them into one larger sample to send to a laboratory for analysis. Make sure to clip plants at harvesting or grazing height, which is typically around three inches. Local county or reservation Extension agents can provide addresses of reliable forage testing laboratories.
It is important to remember that the lower six inches of a forage plant can contain up to three times more nitrate than the top part of the plant, so it may be advantageous if harvesting for hay or silage to cut at a taller height (at six or more inches high), or not allow animals to graze plants too closely. Also keep in mind when grazing fresh forage that immature plants have a greater potential for nitrate accumulation than mature plants, so caution must be used when animals are grazing young forage plants early in the growing season.
If there is concern about nitrate toxicity in stored forages such as hay, it is advisable to take samples and send them in for analysis before feeding the stored forage to animals. Use a hay corer and randomly sample approximately 10 percent of stored forage, ensuring the sampling is from several areas of the haystack. It is advised to have at least 20-25 samples to combine and send in for analysis.
A Nitrate Quik Test was developed at Montana State University which enables certified Extension personnel to quickly test fresh forage samples prior to harvesting to determine whether forage plants have reached toxic nitrate levels. However, care must be taken as this is only a qualitative measurement and not a quantitative measurement, and additional testing will be needed to determine the exact nitrate levels of plants. For more information contact your local MSU Extension office.
If forage tests high for nitrates, ensiling may be an option. Ensiling is the process of fermenting the soluble sugars that are contained in the forage using microbes in an oxygen-free environment. During the ensiling process, one-third to one-half of the nitrates within the forage are converted to gaseous nitrous oxide, reducing the nitrate concentration of the forage. Therefore, silage is the safest feed, and should be considered whenever nitrate toxicity is of concern.
Once you obtain the nitrate levels in forage, it is important to know what constitutes “toxic levels.” Nitrate levels can be reported as NO3-N (nitrate-nitrogen) or as NO3 (nitrate). The difference between these is how the nitrogen ion concentration is reported. When using NO3, it is referring to the entire nitrate ion, which includes both nitrogen as well as oxygen. On the other hand, NO3-N, refers only to the nitrogen that is contained within the nitrate ion. This means NO3 numbers are always 4.5 times greater than NO3 N numbers, so you need to know which is being reported. As a rule of thumb, any level above 1130-2260 ppm NO3-N (5,000-10,000 ppm NO3) should be avoided by pregnant animals and fed in limited quantities to non-pregnant animals. Levels above 2260 ppm NO3-N (>10,000 ppm NO3) should not be fed to any class of livestock.