By now, we are all very aware of the effect of drought on available forage and hay in the southwest. There are other hidden effects that may not be felt until next winter. One of these is increased risk of nitrate poisoning.
Nitrates naturally occur in most plants. At normal levels, rumen microflora convert ingested nitrates into microbial proteins that are then used by the animal. The process is as follows:
Nitrates > Nitrites > Ammonia > Amino Acids > Proteins
When nitrates are very high in forages or hays, nitrate is converted to nitrite at a rate that exceeds the rate at which nitrites are converted to ammonia. This results in accumulation of nitrites which are then absorbed into the blood stream. Nitrites then bind to hemoglobin forming a compound known as methemoglobin. This conversion eliminates the oxygen carrying capacity of hemoglobin. The net effect is a reduction of oxygen reaching body tissues.
Forages that have the greatest potential for nitrate accumulation are those that have the potential for very rapid growth. These plants have evolved (or were selected for) the ability to rapidly accumulate nitrogen in the form of nitrates. Under normal conditions, this is favorable. However, when uptake of nitrates exceeds the rate of use to sustain rapid growth, excessive accumulation may occur.
This is typically the case when plants are stressed due to drought or cold weather. Accumulation may be amplified when high rates of nitrogen fertilizer were applied prior to the stress period. Though nitrate toxicity is often associated with sorghum, sudangrass, johnsongrass, sorghumsudan hybrids, and carelessweed (pigweed), it can also occur in many other grass, row crop, and forb (weed) species.
If you have high risk pastures (containing johnsongrass, pigweed, etc.), avoid grazing during or immediately after periods of plant stress. If you produce or plan to use sorghum, sudan, johnsongrass, or sorghum-sudan hays, we strongly recommend testing for nitrates. Many commercial laboratories offer this service. Most laboratories report results as nitrate or NO3. However, some report results as nitrate nitrogen (NO3N) or potassium nitrate (KNO3). Conversions are listed below.
- Nitrate (NO3) = Nitrate nitrogen (NO3N) x 4.4
- Nitrate (NO3) = Potassium nitrate (KNO3) x .6
- Percentage (%) = Parts per million (PPM) / 10,000
As a rule of thumb, forages with nitrate levels above 1% are considered toxic. Harvested forages (hays, etc.) with nitrate levels between 1% and 3% can be utilized if diluted with low nitrate forages and/or feed grains. Forages with more than 3% nitrates should not be used. When diluting high nitrate forages with low nitrate forages, be careful. The mixture must be complete.
Often grinding or chopping is required. When sampling forages and hays, identify the source location of each sample. Do not use a single plant or bale as the sole indicator of nitrate levels in a field, pasture, or hay lot. Though the most commonly observed symptom of nitrate poisoning is death, the condition is treatable if detected early.
Symptoms often include staggering and darkening of mucous membranes. If symptoms are observed, contact your veterinarian immediately. Affected animals may be treated with intravenous injection of methylene blue at a rate of 1 gram per 250 pounds of body weight. Administration of epinephrine may also be helpful.