Nitrate intake is closely related to the levels found in forage and drinking water. Forages may contain elevated levels of nitrate when fields are heavily fertilized with manure and nitrogen-containing fertilizer, crops are environmentally stressed by cold, rainy weather with a lack of sunshine in the spring or fall, and drought. Drought-stricken forage may be highest in nitrates for a period of three to seven days following appreciable rainfall. While corn silage often is associated with nitrate problems, high levels may occur in other forages such as sorghum, sudangrass, perennial grasses, and legumes. Sometimes alfalfa may have a higher nitrate content than corn silage. Annuals grown on heavily fertilized fields that were intended for corn but were plowed down due to drought and silage made from drought-stricken corn or sorghum are the most frequent offenders. Certain weeds such as pigweed and lambsquarter are nitrate accumulators and may increase levels in forage even under normal growing conditions.
Water may contribute considerable nitrate, or the more toxic nitrite, to the diet. Acute toxicity from water alone is rare and may result when levels approach 1,500 to 3,000 ppm nitrate ion (NO3) or 340 to 680 ppm nitrate-nitrogen (NO3-N). Subclinical toxicity in the form of reduced reproductive efficiency may result from water containing 374 ppm NO3 or 85 ppm NO3-N when fed with a diet with relatively normal nitrate content. Weight gains in calves may be reduced on a normal diet when water contains 150 to 300 ppm NO3 or 34 to 68 ppm NO3-N. Nitrate levels in water may be reported as mg/L or as ppm. These units are synonymous.
See Table 1 for a guide to possible toxicity of nitrate in water. Nitrate tests can vary according to moisture conditions. Because definitive calibration studies with graded levels of nitrate in water have not been conducted and rations may vary appreciably in nitrate content, it is recommended that water contain less than 100 ppm NO3 or 23 ppm NO3-N, unless total nitrate intake is determined using forage and feed analysis and animal performance is closely observed. Some researchers feel that a nitrate level as high as 400 ppm NO3 or 91 ppm NO3-N should not appreciably affect performance under normal conditions. The average content of farm water supplies in Pennsylvania is 34 ppm NO3 or 7.7 ppm NO3-N. Nitrate contents over 100 ppm NO3 or 23 ppm NO3-N rarely are encountered. High levels generally are associated with leaching of nitrate through soils that have received heavy applications of manure and nitrogen-containing fertilizer or with contamination of water sources by septic systems. Some discrepancies in the literature on toxic levels of water may be due in part to a greater proportion of total nitrate being in the more toxic nitrite form in some studies. Continuous access to water also reduces risky toxicity.
Table 1. Guide to Toxicity of Nitrates in Water for Cattle.a
|Nitrate level as NO3, ppmb||Nitrate level as NO3-N, ppmb||Possible effects|
|aAssumes normal or close to average nitrate levels in forages and feeds. Test forages and complete a nitrate worksheet when water contains over 100 ppm NO3 or 23 ppm NO3-N See Table 5. |
bppm = mg/L; mg/L x 0.454 = mg/lb.
cShortness of breath, rapid breathing.
dSuffocation signs, incoordination, or staggering.
|0 to 100||0 to 23||None|
|101 to 500||23 to 114||Reduced gains, more infertility|
|501 to 1,000||114 to 227||Gray-brown mucous membranes, distress symptomsc|
|over 1,000||over 227||Acute symptomsd, deaths|
Any discussion or guidelines relating to nitrates are complicated by several factors. A number of recommendations and guidelines still found today are based largely on early field observations and limited research data obtained in the late 50s and early 60s, and have not been updated to more recent research and field experiences. For example, many people indicate that vitamin A status of animals may be adversely affected by elevated intakes of nitrate. However, many well-controlled studies indicate that little or no appreciable effects occur. In addition, nitrate intakes necessary to produce a harmful degree of methemoglobinuria (lack of oxygen-carrying capacity in the blood) are much higher than previously thought. Most studies show little or no adverse effects on feed intake or milk production in dairy cows. In fact, increased milk production has occurred in some trials when appreciable levels of nitrate (and thus more soluble N) were present in the diet. A decrease in feed intake and gains have been noted in some research with growing animals, especially young calves when water was the source of nitrates.
Numerous studies indicate that there is wide variation in the response to high nitrate intake among animals on a given diet and among trials due to apparent differences in environment, diet, and feeding practices. Nitrate problems may occur when these compounds are converted to the more toxic nitrite (NO2) form via rumen metabolism. Conditions that limit the reduction of nitrate to ammonia (NH3) in the rumen or tax this system and allow for a buildup of nitrite make animals more susceptible to nitrate toxicity. For example, animals on a high forage or forage alone ration may be at greater risk due to lack of readily available carbohydrates in the rumen. At least three to five pounds of grain intake may be necessary to minimize toxic nitrite production.
Some minerals such as molybdenum, copper, iron, magnesium, and manganese are involved in the complete reduction of nitrate to ammonia, which avoids nitrite accumulation. Cattle on some rations, especially all-forage diets in some areas, may lack some of these or other minerals that are important for normal rumen metabolism. Rumen microorganisms often adapt to higher nitrite levels over a period of time. Thus the rate at which higher nitrate feeds or water are introduced to the diet may considerably influence the effects noted. Most importantly, the rate at which even adapted animals consume nitrate is the critical factor. Rate of nitrate intake is a function of both forage nitrate content and the rate of forage dry matter consumption. Thus, less dry matter from high nitrate forage can be eaten in a single meal.
Grazing animals often are at less risk than those consuming dry or ensiled forages because they eat less dry matter per unit of time, and may have less buildup of nitrite in the rumen and, consequently, the blood stream. There also may be differences in effects between species of grass or possibly annual hybrids at similar nitrate intakes. Methods of feeding also may influence effects. The TMR system of feeding may be less risky at a given nitrate intake than conventional feeding, which results in several peak intakes of forage during the day.
Numerous factors beyond those discussed here are known to affect the reduction of protein degradation and other nitrogen-containing compounds to ammonia, and the synthesis of microbial protein in the rumen. For example rumen by-pass protein is needed in the diet. It is likely that many of these same factors affect nitrate reduction and levels of nitrite that may accumulate.
Some research indicates that substantial amounts of endogenous nitrate and nitrite may be produced in the animal. This may be recycled and may contribute to an accumulation of NO2 in the blood stream. Animals appear to vary considerably in the amount of endogenous nitrate and nitrite produced. Endotoxins present due to infection or stress enhance this endogenous production and may make animals more susceptible to toxicity. Animals usually are under more stress during hot, humid weather, at calving time, and during major ration changes (such as type of forage fed and levels of concentrate intake).
A lot of confusion or misunderstanding also results from differences in expressing nitrate levels. Some laboratories and researchers indicate nitrate levels as nitrate ion (NO3) while others use nitrate-nitrogen (NO3-N). Some do not indicate which unit is being used in analytical reports, research, or guidelines. This problem in itself has given rise to many conflicting recommendations and guidelines, as well as misinterpretation of test reports. Some laboratories report in parts per million, which results in a much larger figure than the percentage basis used by others. Furthermore, some people indicate levels in feeds on an as fed basis rather than the preferred dry matter basis as used in this article. The Nitrate Conversion worksheet in the Nitrate Calculator spreadsheet may be used to convert various expressions of nitrate concentration. Many analytical methods result in a nitrate content that includes the nitrite present. Since nitrite is more toxic than nitrate, nitrate content is not always a good indicator of potential toxicity due to possible variations in the proportion of nitrite present. Fortunately most nitrogen oxide is present as nitrate, not nitrite, in plant material and water.
Effects on Animals
Acute toxicity may result in serious illness or death due to a lack of oxygen in body tissues. Nitrate is reduced to nitrite in the rumen. Nitrite, which is absorbed into the bloodstream, combines with hemoglobin to form methemoglobin, which cannot carry oxygen to the tissues. Thus the animal may die from suffocation. When methemoglobin levels reach 50 to 65% of hemoglobin content, acute symptoms or death may occur. Symptoms include muscular weakness, incoordination, convulsions, accelerated heart rate, rapid breathing, and cyanosis or a discoloration of mucous membranes. Vaginal and other mucous membranes may turn from pink to a gray-brown at methemoglobin contents over 20%, while other symptoms may not result until methemoglobin reaches 50% or higher. Normal methemoglobin values range from 1 to 3%. Thus mucous membranes should be checked closely when feeding a ration suspected to contain elevated nitrate content.
Unfortunately, symptoms of prussic acid (cyanide) poisoning are similar to nitrate toxicity. When sorghum, sudangrass, or their hybrids, Johnson grass, or pasture containing access to cherry are involved in an acute problem, one must determine whether nitrate or prussic acid is involved. Freshly drawn blood from an animal afflicted with nitrate poisoning often is a dark chocolate-brown color, while that from prussic acid toxicity often may turn a brilliant cherry red upon exposure to air. Forages may be tested for nitrate and prussic acid (cyanide) to help with later confirmation.
Subclinical toxicity from nitrates may be reflected as reduced reproductive efficiency in adult cattle and lower weight gains with or without decreased feed intake in young stock. Research has shown that serum progesterone concentrations may be decreased greatly in open animals (50%), more moderately in early pregnancy (25%), and very little, if any, in mid- or later pregnancy. Late-term abortions rarely occur. Services per conception and first service conception rates may be most noticeably affected, resulting in more repeat breedings.
Fortunately acute nitrate toxicity rarely occurs under normal conditions. It is most apt to occur when cattle suddenly are allowed access to large amounts of high nitrate forage. (Generally this is an annual grown on highly fertilized fields, especially when the crop is weather-stressed.) Nitrate content of forages causing acute toxicity (on a dry matter basis) generally ranges from 1 to 3% or 10,000 to 30,000 ppm NO3; or 0.23 to 0.68% or 2,300 to 6,800 ppm NO3-N. Methylene blue may be used in treating animals for nitrate poisoning, while a sodium thiosulfate-sodium nitrite solution is used for prussic acid (cyanide) toxicity. Subclinical nitrate toxicity is more apt to occur than acute, especially when drought or otherwise stressed forages are fed. Even subclinical toxicity occurs infrequently in well-fed, well-managed herds. This results in part from a dilution effect when several forages and at least moderate amounts of grain or concentrate are fed, as is often practiced when feeding dairy cows.
The information and precautions presented earlier must be taken into account when using any guidelines on safe levels of nitrate content in the diet. It is important to use caution in feeding forages that are suspect in regard to possibly high nitrate or prussic acid levels.
Weather stress and fertilization practices must be considered. Before feeding, test suspected forages for nitrates using representative samples obtained as you would for other nutritional evaluation. If such tests indicate a nitrate level for a single forage in excess of 0.44% NO3 or 1,000 ppm NO3-N on a dry matter basis, test all forages, water, and possibly concentrates for nitrate content. Pending test reports on other feeds, use the information in Table 2 as a rough guide for using high nitrate forage(s). It is important, however, to limit meal size for any forage containing 0.50% NO3 or 1,100 ppm NO3-N or more (criteria for limiting meal size may be found in the Nitrate Calculator spreadsheet ). These maximums on forage dry matter intake in a single meal are those that research has shown maintain methemoglobin levels at a normal level of 3% or less.
Table 2. Guide to Using Stored Forages with Various Nitrate Contents.a
|Forage nitrate content, dry matter basis||Comments|
|as Nitrate Ion (NO3)||as Nitrate-Nitrogen (NO3-N)|
|aIf one forage contains over 0.44% NO3 or 1000 ppm NO3-N test all forages, water, and possibly concentrates. Include nitrate intake from water as part of dietary intake. Use the Nitrate Calculator spreadsheet and information in the Guidelines section to determine and adjust nitrate intake and meal size. Somewhat higher levels of nitrate may be tolerated when pasture is the sole source of forage. See Table 8. |
bSee the Nitrate Calculator spreadsheet to determine safe intake of forage dry matter with a given nitrate content in a single meal.
|0.0 to 0.44||0 to 4,400||0 to 0.10||0 to 1,000||Safe under most conditions|
|0.45 to 0.75||4,500 to 7500||0.10 to 0.17||1,000 to 1,700||Gradually introduce to ration|
Feed some concentrate
Test all feeds and water
Dilute to 0.40% NO3 or 0.09% NO3-N or less in total ration dry matter
Restrict single meal sizeb
|0.76 to 1.00+||7,600 to 10,000+||0.17 to 0.23||1,700 to 2,300+||Possible acute toxicity |
Feed in a balanced ration with concentrate included
Dilute to 0.40% NO3 or 0.09% NO3-N or less in total ration dry matter
Restrict single meal sizeb
Upon completion of testing other forages and water, determine the NO3-N content of the total ration dry matter for the average animal in the herd or group. If necessary, alter forage intakes to keep the NO3-N content at not over 0.04 to 0.09% of the total ration dry matter, including NO3-N contribution from all feeds and drinking water. The lower value should prevent all appreciable adverse effects, provided maximum single meal intakes are not exceeded. The upper value should prevent acute and chronic toxicity with the exception of a moderate increase in reproductive problems. The Nitrate Calculator spreadsheet can be used to calculate adjustments to forage intakes that will keep the overall diet within the recommended range for nitrates.
Table values may be useful when calculating the nitrate concentration of the overall diet pending tests of non-suspected feeds and water. Expected nitrate contents for various feeds and water under usual conditions are given in Table 3. Information to estimate water intakes may be found in the Nitrate Calculator spreadsheet on the Water Intake worksheet.
Table 3. Expected Nitrate Content (on a dry matter basis) of Feeds Grown Under Normal Weather Conditions.
|NO3 range, ppm||NO3 mean, ppm||NO3-N mean, ppm|
|aMixed mainly grass. |
bMixed mainly legume.
cEnsiled forage may contain about 30% less nitrate than hay or fresh forage of the same nitrate content at harvest.
|Water||0 to 44||34||8|
|Dairy grain mix||111 to 400||209||48|
|Corn grain||139 to 188||164||37|
|Protein concentrate||105 to 120||113||26|
|Corn silage||440 to 2,200||1,365||310|
|Grass, MMGa forage||46 to 1,600c||1,003c||228|
|Lequme, MMLb forage||490 to 4,100c||2,025c||460|
While it is prudent to maintain a NO3-N content in the total ration dry matter including contributions of drinking water at not over 0.04% for reproducing animals, a lower or higher level may be chosen based on information presented in Table 4. For example, if no reproductive impairment is desired, a level somewhat under 0.04% or 400 ppm NO3-N may be used. If one is willing to accept a slight to moderate rise in infertility, then a level of NO3-N as high as 0.09 to 0.13% NO3-N might be considered in order to use more high-nitrate forage. This assumes that amounts of dry matter intake per single meal from high-nitrate forage will not exceed the recommended limits.
Table 4. Possible Effects of Various Contents of NO3-N in the Total Ration Dry Matter.a
|Possible toxicity||Stored forage and concentrateb, ppmd||Pasture alonec, ppmd|
|aIncludes NO3-N from water. Nitrate intake per meal also must be limited by restricted intake of forage dry matter to make a ration safe or low risk.|
bRefers to rations containing 60% or less concentrate dry matter and those containing a combination of stored forage and pasture or green chop as well as concentrate.
cNo appreciable amount of concentrates or other forage being consumed.
dPercent NO3-N equals ppm ÷ 10,000, or move decimal point four places to the left.
eRefers to a slight to moderate increase in services required per conception and repeat breedings. Abortions generally do not occur until content of NO3-N in the ration dry matter reaches 1,400 ppm, or more commonly 2,300 ppm, when feeding stored forages. No aborted fetuses have occurred in some research at even higher levels of nitrate intake when meals have been spaced, such as in feeding a TMR.
fA change in color of mucous membranes of the vagina, mouth, or eye (from pink to a grayish-brown) may be noted at levels of intake near this level. Respiratory and other distress symptoms or death may occur at high levels, particularly if dry matter intakes per meal exceed recommendations for high nitrate forages.
|None||up to 400||up to 800|
|Impaired fertilitye||401 to 1,300||801 to 2,600|
|Reduced gains||1,301 to 1,700||2,601 to 3,400|
|Clinical symptomsf||over 1,700||over 3,400|
- Test suspected forages for nitrate content, preferably before feeding them.
- Introduce suspected or high nitrate forages gradually into the ration over a period of two to three weeks to allow for adaptation and reduce risks.
- Feed another forage prior to feeding suspected or high nitrate forage to help limit meal size.
- Feed forages and TMR more frequently to reduce meal size when suspected or high nitrate forage is used or silo gas is present.
- Feed at least three to five pounds of grain per head daily to cattle fed suspected forages or high nitrate forages to reduce possible toxic effects.
- Limit dry matter intake per single meal if stored forage contains 1,100 ppm NO3-N or more on a dry matter basis. Allow a delay of two to three hours after completion of a meal before feeding high nitrate forage(s) again.
- Re-test suspected and high nitrate forages periodically, due to the large variations that often occur in forages.
- Test all forages and water for nitrates if one stored forage contains over 1,000 ppm NO3-N.
- Consider blending high nitrate silage or haylage with those containing lower amounts before feeding to provide less than 1,100 ppm NO3-N in the blended dry matter and thus enabling free-choice feeding.
- Restrict the NO3-N content of the total ration dry matter, including contribution from water, to not over 0.04 to 0.09% when using stored forage in addition to meeting any maximum forage dry matter intake for a single meal. Use the Nitrate Calculator spreadsheet to adjust the ration.
- Observe animals closely for symptoms of toxicity. Check the color of mucous membranes in the vagina, mouth, or eyes two hours following the start of a meal consisting of a suspected or high nitrate forage (over 1,100 ppm NO3-N). Membranes will turn from pink to a grayish-brown at a methemoglobin content of 20% or higher. This is the earliest sign of a possible toxicity occurring with slight to acute symptoms. Acute symptoms may include rapid breathing, incoordination or staggering, and signs of suffocation.
Usually, stored forages with less than 1,100 ppm NO3-N may be fed free choice to cattle. Alternative levels are indicated in Table 4 and the text. Use precautions against silo gas poisoning with suspected or known high nitrate forages, including the possible development of nitrogen oxide gases in silage or total mixed rations that heat appreciably in the bunk or manger. Cows are very sensitive to silo gas. Do not fill silage carts or TMR mixers and feeders appreciably in advance of feeding. Aerate and feed gas-producing silages in a well-ventilated place, preferably outside. When silage is not well preserved, nitrogen oxide gases may continue to be produced for considerable periods after ensiling, rather than for a few weeks after harvest.
Written by Richard S. Adams, Department of Dairy and Animal Science, Thomas R. McCarty, Penn State Extension in Cumberland County, and Lawrence J. Hutchinson, Department of Veterinary Science