Nitrate Toxicity
by Dr. Clay Zimmerman
Death from nitrate poisoning has been documented for many years; however, under normal conditions it is not a problem. Ruminants such as dairy cattle are more susceptible to nitrate poisoning than nonruminants because the toxic form of nitrate is actually nitrite which is formed in the rumen (Figure 1).
| NO3- (nitrate) | → | NO2- (nitrate) | → | NH2OH (hydroxylamine) | → | NH4+ (ammonium) | → | Amino Acids | → | Protein |
Under normal conditions the nitrite in the rumen will be reduced and may eventually form protein to be used by the animal. The conversion of nitrate to nitrite is faster than the conversion of nitrite to ammonia, so if a high level of nitrate is fed too quickly, nitrite will build up in the blood and become toxic. Hemoglobin carries oxygen to tissues, however nitrite causes the conversion of hemoglobin to methemoglobin which can't carry oxygen to tissues, so the animal in essence suffocates. Blood high in methemoglobin has a chocolate brown color instead of its normal red color.
Nitrates are normal plant nutrients taken up by the plant to make protein. Plants accumulate nitrate under certain conditions such as:
- 1. Drought - Especially after rapid early growth. Accumulation is also a problem after a drought breaking rain which may have occurred in some areas this year. The problem is greatest in drought stricken crops with heavy nitrogen fertilization.
- 2. Low light intensity - This can be due to cloudiness or shading by heavy plant populations. The plant needs light energy to convert nitrates to protein. Therefore, low light can cause nitrates to accumulate.
- 3. Excessive nitrate levels in soil - Due to high levels of nitrogen fertilization.
- 4. Plant species and variety - Grasses and weeds are most susceptible. Grains and legumes are low. Particularly troublesome are small grains (especially oats), corn, ryegrass, sorghum, sudan or their hybrids.
- 5. Stage of plant growth - Generally nitrate levels decrease as the plant becomes more mature. The low portions and stalks or stems have a higher proportion than upper parts such as leaves or grain.
- 6. Other environmental factors such as frost or soil mineral deficiencies - Especially phosphorus, magnesium, or manganese or NPK imbalance.
What are the effects of nitrates on dairy cows?
In addition to being lethal, sublethal doses of nitrates have been shown to decrease feed intake, milk production, and reproductive performance. High nitrate rations can increase abortion rates. Georgia workers showed that a high nitrate diet (1600 ppm Nitrate-N) caused decreased blood progesterone in cycling cows and early pregnant cows. If progesterone is too low in these cows then pregnancy will not take place or be maintained.
There is also conflicting evidence that high nitrates cause poor utilization of Vitamin A and carotene which can lead to Vitamin A deficiency. Whether this actually occurs in ruminants is unclear, however an animal that is deficient in Vitamin A is more susceptible to nitrate poisoning.
What are the recommendations for feeding high nitrate forages?
Forages suspected of being high in nitrates should be sent to a feed testing laboratory for analysis (our lab does not analyze for nitrates, but they will sent your sample to an outside lab if one comes in). Most forages containing nitrates can still be used if fed properly. Specialists at Penn State use the following guidelines for feeding forages of various nitrate levels, pay particular attention to the unit of measure.
| Content of Nitrate Ion (Dry basis) | Content of Nitrate Nitrogen (Dry basis) | Comment |
|---|---|---|
| 0.0 - 0.44% | 0.0 - 0.1% or 0 - 1000 ppm | Safe levels if on good feed. |
| 0.44 - 0.66% | 0.1 - 0.15% or 1000 - 1500 ppm | Safe for non-pregnant animals. For pregnant animals, limit use to 50% of total ration on a dry basis. |
| 0.66 - 0.88% | 0.15 - 0.20% or 1500 - 2000 ppm | Safe if limited to 50% of total ration dry matter. |
| 0.88 - 1.54% | 0.20 - 0.35% or 2000 - 3500 ppm | Limit to 35-40% of total ration dry matter. Do not use for pregnant animals. |
| 1.54 - 1.76% | 0.35 - 0.40% or 3500 - 4000 ppm | Limit to 25% of total ration dry matter. Do not use for pregnant animals. |
| Over 1.76% | Over 0.4% or over 4000 ppm | These feeds are potentially toxic. Do not feed. |
| 1R.S. Adams and S.B. Guss, Pennsylvania State University. | ||
Nitrates in water can also be a problem. Below are guidelines for using water with various nitrate levels.
| Nitrate Ion (ppm) | Nitrate Nitrogen (ppm) | Comments |
|---|---|---|
| 0 - 44 | 0 - 10 | Not harmful. |
| 45 - 132 | 10 - 30 | Safe if feed is low in nitrates and nutritionally balanced. |
| 133 - 220 | 30 - 50 | Risky. Could be harmful over a long period of time. |
| 221 - 660 | 50 - 150 | Cattle at risk and possible death. |
| 661+ | 150+ | Unsafe, possible death. Should not be used as a water source. |
| 2National Research Council, 1974. | ||
An area of confusion is the form in which test results are reported. Make sure that you look under the correct column for either nitrate ion or nitrate nitrogen and don't confuse the two. To convert from nitrate ion to nitrate nitrogen, you multiply nitrate ion by 0.23. To convert from nitrate nitrogen to nitrate ion, you multiply nitrate nitrogen by 4.43. Nitrate levels in feed and water are often reported in ppm. To convert from ppm to %, move the decimal point 4 places to the left or vice versa. (example: 4000 ppm = 0.40%)
How should you manage forages that contain nitrates?
- 1. Avoid harvesting drought stricken forages for 3-5 days after a recovery rain.
- 2. Cutting the stalk high will leave much of the nitrate in the field since nitrate levels are highest in the bottom of the stalk.
- 3. Chop the entire plant as opposed to allowing selective consumption of stalks (in a grazing situation).
- 4. Dilute high nitrate feeds with low nitrate feeds.
- 5. Feed a ration balanced in energy, protein, minerals and vitamins. It is well documented that high energy diets decrease susceptibility to nitrate toxicity. Adequate protein levels also decrease toxicity of nitrate, but gross overfeeding of protein, especially NPN sources such as urea can aggravate the problem. Vitamin A levels are also important.
- 6. Gradually introduce suspected forages into the ration over a period of 1-2 weeks. The rumen can adapt to a small degree to higher nitrate levels.
- 7. Feed suspected forages in smaller amounts at more frequent intervals. High nitrate intake over a short time period is the most critical problem.
- 8. Put the crop up as silage to decrease nitrate concentration. Fresh forage that is grazed or green chopped may be more toxic than if made into hay or silage. Nitrates are converted to silo gas which is toxic to humans and animals if inhaled. Be very cautious of these gases. After 30-60 days in the silo, nitrate levels should drop to about 50% of the original amount. Don't feed out of a newly filled silo for at least 3 weeks. Analysis of silage before feeding is recommended if high nitrates are suspected.
- 9. Don't allow hungry animals free access to high nitrate forages.
- 10. Seek professional guidance from the herd veterinarian if you suspect nitrate poisoning.
What are the symptoms of nitrate poisoning?
- Rapid pulse and respiration rates
- Muscle tremors and general weakness
- Diarrhea and frequent urination
- Abortion
- Blue color of mucous membranes, muzzle and udder
- Unsteady gait and convulsions
- Death
- Chocolate brown blood
An emergency treatment for nitrate poisoning is a 4% methylene blue solution in a 5% glucose or 1.8% sodium sulfate solution administered i.v. by a veterinarian at 100 ml/100 lbs of body weight.
If you suspect high nitrates, please have the sample analyzed for nitrates so that we can find the proper use for the forage.
© Blue Seal Feeds – September, 2001