Drought conditions throughout many areas in the intermountain west and great plains can cause several plants and harvested forages the potential to accumulate excessive levels of nitrates. All plants contain some nitrates, however, in years such as 2002, forages grown under stress can accumulate toxic levels of nitrates. Nitrates in the soil are a primary source of nitrogen which is essential for plant growth. Under normal conditions nitrates are converted to protein in the plant at about the same rates as they are absorbed into the root system. Plant stress is the major course of nitrate accumulation and can be caused by one or several of the following factors :
- weather extremes such as drought, frost or hail
- shading or low light intensity
- herbicide application or disease.
Adverse weather conditions such as the hot, dry weather experienced this past year can cause nitrates to accumulate at high levels. In plants that are drought stressed, nitrate levels can be especially high for several days after a break in dry weather. Frost, hail and low temperatures will interfere with normal plant growth and can cause nitrate accumulation. Plants require temperatures at 55E or above for active photosynthesis. If photosynthesis is not occurring due to low temperatures, frost or hail damage to leaves, nitrate can then occur in the stems or stalks. Several plant species such as pigweed, lambs quarter, oats, millet, sorghums, sudan grass and corn are often high in nitrates. Under extreme conditions, even grasses and legumes can accumulate nitrates.
Existing levels of nitrate are not particularly toxic to animals in their present state. Nitrates consumed by ruminants are normally reduced to ammonia, absorbed and excreted. Nitrite, an intermediate product is the culprit in nitrate poisoning. As some of the nitrate is absorbed into the blood it converts the blood hemoglobin to methemoglobin. The conversion of hemoglobin to methemoglobin reduces the oxygen carrying ability of the animal. The oxygen carrying capacity of the blood is severely reduced. Younger ruminants are more susceptible than older animals.
Nitrates in Water
Water sources which would normally not cause problems can provide a compounding effect when feeding forages high in nitrates. Table 1 gives the guidelines for nitrites in water.
|Table 1. Guidelines for nitrites in water|
|ppm of NO3||Effect|
|45-120||Slight possible effect|
|121-220||Risk over long periods of time|
|221-440||Increased risks possible|
|221-440||Increased risks possible|
|441-660||Serious levels, possible acute losses|
|661-880||Increase acute losses|
|881 and higher||Heavy acute losses|
Testing for Nitrates
Most commercial forage test laboratories will run a nitrate test along with standard nutrient analysis. Reporting terminology varies across labs. Terms in Table 2 and 3 show the formulas to convert lab reports to a standard measure.
|Table 2. Methods of expressing nitrate and nitrite contents of feeds and water, atomic weights of the various substances, and a conversion factor.1|
|Nitrogenous substance||Chemical formula or designation||Atomic, molecular, ionic weight||Multiplication factors|
|1 To convert parts per million (ppm) to percent, divide by 10,000 (e.g., 1,500 ppm is .15%).|
|Table 3. Conversion table.|
|% KNO3||=||% nitrate (NO3) x 1.6|
|% KNO3||=||% nitrate nitrogen (NO3N) x 7.0|
|% NO3||=||% KNO3 x .6|
|% NO3N||=||% KNO3 x .14|
|% NO3N||=||% NO3 x .23|
|Parts per million (ppm)||=||Percent (%) x 10,000|
|Percent (%)||=||(ppm ) 10,000|
Feeding Forages with Higher Levels of Nitrates
Feeding cattle or sheep nitrate levels of less than 1% or 10,000 ppm are not toxic. Diets of pregnant beef cows should not exceed 5,000 ppm nitrates on a dry matter basis. Sheep appear to be able to tolerate higher levels of nitrates even while pregnant.
Table 4 lists guidelines for nitrate in feedstuffs.
|Table 4. Guidelines for nitrate in feedstuffs (expressed on 100% dry matter basis).|
|Nitrate Nitrogen content %||Comment|
|0.0 to 0.44||This level is considered safe to feed under all conditions.|
|0.44 to 0.66||This level should be safe to feed to nonpregnant animals under all conditions. It may be best to limit its use to pregnant animals to 50 percent of the total ration on a dry basis.|
|0.66 to 0.88||Feeds safely fed if limited to 50 percent of the total dry matter in the ration.|
|0.88 to 1.54||Feeds should be limited to about 35 to 40 percent of the total dry matter in the ration. Feeds containing more than 0.88 percent nitrate should not be used for pregnant animals.|
|1.54 to 1.76||Feeds should be limited to 25 percent of total dry matter in the ration. Do not use these feeds for pregnant animals.|
|over 1.76||These feeds are potentially toxic. Do not feed.|
- Have forages tested for nitrates, it takes only one dead cow or sheep to pay for the test.
- Test water sources if there is suspicion of high nitrates or a history of water problems.
- Blend high nitrate feeds if possible with feeds known to have a lower level of nitrates.
- This is the year to test corn stalks and late cuttings of sorghum or Sudan grass.
Example: Laboratories typically
report nitrate levels as a percentage, or as parts per million (ppm). A
sample that is reported as 1800 ppm when divided by 10,000 (1800/10000) =
.18% nitrates. The reverse
calculation can also be made (% NO3 x 10,000) = ppm.