Nitrate Toxicity and
Prussic Acid Poisoning
In Dairy Cattle

James W. Smith & Larry D. Guthrie, Extension Dairy Scientists

Forages are an important part of a profitable dairy operation. Usually forages can be safely fed or grazed without concern. However, under certain conditions, they can accumulate toxic levels of nitrate or prussic acid. When these forages are consumed by cattle, the result is often costly losses. This publication provides guidelines on the prevention and control of nitrate toxicity and prussic acid poisoning in dairy cattle.

Nitrate Toxicity

Nitrate Accumulation by Forage Plants

All plants contain some nitrates. Nitrates normally are taken up by plant roots and metabolized by the plant. Nitrate accumulation in plants can occur when the soil contains large amounts of nitrate or when some factor or factors interfere with normal plant growth.

Weather conditions during plant growth often contribute to nitrate accumulation. Nitrates cannot be used by the plant whenever anything interferes with normal photosynthesis and protein formation. Drought conditions often cause nitrate to accumulate in plants but not all drought conditions cause high concentrations of nitrate in plants. Some soil moisture is required for absorption and accumulation of nitrates. Normal plant growth resumes following appreciable rainfall and nitrate levels will decline. Drought stricken forages may have the highest nitrate levels for three to seven days after rainfall. Delay harvest during this period. Other weather factors such as frost, hail, low temperatures or cloudy overcast skies that alter normal plant growth can cause nitrates to accumulate.

Plant species vary in the amount of nitrate that accumulates. Certain weeds such as pigweed, Johnsongrass, nightshade, bindweed, ragweed and lambsquarter tend to accumulate nitrate and levels can increase in these plants even under normal growing conditions. Silage from drought-stricken corn is often associated with nitrate problems. High levels of nitrates can also occur in annuals including small grains, sorghum and sudangrass as well as perennials such as coastal bermudagrass. High nitrate levels in forages is often associated with excessive application of nitrogen fertilizer or poultry manure.

Herbicides such as 2, 4-D tend to disrupt normal growth processes in a plant and may cause a temporary increase in nitrate level. The nitrate content of certain harvested forages may be lowered by herbicide spraying since nitrate accumulating weeds are eliminated.

Nitrate concentration is usually the highest in young plants and tends to decline as the plant matures. However, mature plants can be high in nitrates depending upon soil nitrate levels and other conditions.

Plant parts closest to the ground contain the most nitrate. Nitrates are normally utilized in the leaves of plants. Unfavorable conditions cause nitrate accumulation in the stalks and stems of plants. Table 1 illustrates the location of nitrate concentration within a plant. Nitrate concentration in forage may be reduced by varying the height of cut at harvest as shown in Table 2.

Table 1. Nitrates (ppm) by Part of Corn Plant

Table 2. Nitrates and Yield by Height of Cut of Corn

Nitrates in Water

Water can supply nitrates to the diet or the more toxic, nitrite. Toxicity from water alone is unlikely especially if the source is well water. Contamination of water with nitrates is usually from surface sources. High levels of nitrate can leach through soils that have received heavy applications of manure. Water from farm ponds, road ditches or other surface impressions which collect drainage from poultry houses, feedlots, heavily fertilized fields, septic tanks or manure lagoons may contain high concentrations of nitrate. A nitrate content in water of less than 100 ppm NO₃ is recommended

Why Nitrate Is a Problem

Nitrate is not always toxic to animals. Most forages contain some nitrate. Nitrates are converted to ammonia in the rumen by microbes as shown by the following reaction.

Nitrate toxicity is a function of the amount and rate at which nitrate is consumed. Under certain conditions, the rate at which nitrite is converted to ammonia becomes limiting and nitrite accumulates. Nitrite is absorbed from the rumen and combines with hemoglobin in the red blood cells to form methemoglobin. Methemoglobin cannot transport oxygen efficiently to the animal tissues; this results in symptoms of nitrate toxicity.

Some minerals such as molybdenum, copper, iron, magnesium and manganese are involved in the conversion of nitrate to ammonia which prevents nitrite accumulation. Some rations especially those high in forages can lack some of these minerals important for normal rumen metabolism.

The lack of adequate dietary energy in a ration can contribute to nitrate toxicity. High energy concentrates such as corn grain are usually low in nitrate content. Concentrates fed with forage will dilute the higher nitrate content of a forage. Microorganisms in the rumen require energy in order to metabolize nitrates. Energy in the form of concentrate is readily available to rumen microbes. If adequate energy is available, the amount of ammonia that passes into the blood is reduced.

Rumen microbes are capable of adapting to higher nitrate levels over time. The rate at which high nitrate feeds are consumed is very important. Animals grazing continuously are often at less risk for nitrate toxicity compared to those receiving hay or silage since intake of dry matter is more uniform throughout the day.

The type of system used to feed animals may be a factor affecting nitrate toxicity. Total mixed rations (TMR) where forages and other feed ingredients are blended pose less risk at a given nitrate level compared to free choice feeding of forage. The TMR system provides a more uniform intake of nitrate during the day compared to several peak intakes with many other feeding systems.

Symptoms

Toxicity may result in serious illness or death of the animal due to a lack of oxygen in body tissues. Nitrite formed in the rumen is absorbed into the bloodstream and combines with hemoglobin in the red blood cells to form methemoglobin. Methemoglobin cannot transport oxygen so death may result due to suffocation.

Toxic symptoms may occur one to two hours after consumption of feed or water containing high levels of nitrate. Symptoms of toxicity may not occur for several days after intake of moderate levels of nitrate.

Brown, chocolate-colored blood resulting from the presence of methemoglobin indicates severe nitrate toxicity. Breathing becomes progressively more difficult until marked respiratory distress and violent respiratory movements are observed. Other physical symptoms include frothing at the month, rapid weak heart beat, muscle spasms, incoordination, convulsions, diarrhea and frequent urination.

A brownish or bluish discoloration may be noted around white areas of the skin and mucous membranes at methemoglobin levels of over 20 percent. These areas should be checked frequently when feeding a ration suspected to be high in nitrate content. Other symptoms may not result until the methemoglobin level reaches 30 to 50 percent with death occurring at a level of 70 to 80 percent.

Reduced reproductive efficiency including abortions and lower weight gains in young stock may result when nitrate is consumed at lower levels where physical symptoms are not obvious. Research with lactating dairy cattle indicates that a ration containing approximately 1600 ppm of nitrate in the total diet dry matter caused open and early pregnant cows to have lower levels of blood progesterone than mid pregnant cows or cows on a low nitrate diet.

Progesterone is a hormone essential for maintaining pregnancy. Lack of oxygen to the fetus may also cause early abortions although late term abortions due to nitrate intake are unlikely. The number of services required per conception and the first service conception rate may be affected which results in repeat breedings.

Reporting Nitrate Levels

Nitrate levels are expressed in several forms. Some laboratories report nitrate levels as nitrate ion (N0₃) while others use nitrate nitrogen (NO₃-N). Table 3 gives conversion factors. Results may be reported in parts per million (ppm) which gives a larger value than expression on a percentage basis. Nitrate levels in feeds may be given on either an as fed or dry matter basis. In this publication, nitrate refers to the nitrate ion (NO₃). Recommendations will specify either parts per million or percentage. All references to forage or ration levels assume a 100 percent dry matter basis.

To convert a percent to parts per million, move the decimal point four places to the right or vice versa. (Example - .26% = 2600 ppm; 3500 ppm = .35%).

Table 3. Factors for Converting Different Expressions of Nitrate Content¹

Harvest Procedures Affect Nitrate Levels

Delaying the harvesting of drought-damaged crops as long as practical may reduce nitrate content. If the plant is actively growing, the concentration of nitrate declines until maturity. If nitrates are suspected, forage should be chemically analyzed for nitrates prior to harvest to aid in making harvest and feeding decisions.

Ensiling tends to reduce the nitrate content of forages. Nitrate content of corn silage may be reduced by up to 50 percent (Table 4). Recommended ensiling practices including chopping at the proper stage of maturity and packing well should be followed to improve the fermentation process. Silage must be allowed to undergo a complete fermentation for at least 21 days.

Harvesting high nitrate forages as green chop can be dangerous since nitrate levels will not be lowered by fermentation. Green chop allowed to heat may cause increased conversion of nitrates to nitrites making it even more dangerous.

Table 4. Effect of Ensiling on Nitrate Concentration in Corn Silage

Formulating Rations with High Nitrate Forages

Table 5 gives recommended guidelines for feeding nitrate containing forage to various classes of livestock. Following these recommendations should prevent chronic toxicity symptoms such as reduced reproductive efficiency and depressed milk production in lactating cows.

To calculate the amount of nitrate in the total diet, multiply the nitrate level (ppm/lb) in each feed source by the amount of each feed consumed (dry matter basis). Total the amount of nitrate intake and divide by the total pounds of dry matter consumed. Table 6 illustrates an example calculation.

Table 7 shows examples of feeding forages containing varying levels of nitrate to different classes of livestock. The table lists the maximum percent and pounds of dry matter from forage and the maximum amount of silage or hay that can be safely fed. For example, assume that 30 percent dry matter corn silage containing 4000 ppm nitrate is available to feed a group of non-pregnant lactating cows. Assuming the cows consume 45 pounds of dry matter, they could receive up to 38 percent or 17.1 pounds of dry matter from the nitrate containing forage. This equals 57 pounds of silage. (17.1 pounds DM ÷ (30/100) = 57 pounds).

Table 6. Calculation of Nitrates in the Total Diet when Feeding More than One Forage Containing Nitrates

Table 7. Feeding Schedules for Forages Containing Varying Levels of Nitrate Fed to Different Classes of Livestock

¹DM = dry matter
²Refers to the nitrate containing forage

Management Guidelines

Prussic Acid Poisoning

Prussic acid poisoning may occur when livestock consume certain forages or wild plants. Certain plants of the sorghum species contain a non-toxic cyanogenic glucoside called dhurrin (or durrin). Under certain conditions hydrocyanic acid (HCN) or prussic acid is released from dhurrin as the result of enzymatic action. This process is called cyanogeneses.

HCN or prussic acid can be released in a relatively short period of time if the plant tissue is injured or damaged. Enymes can more easily free larger quantities of HCN if the plant is damaged. Wilting, chewing, freezing, cutting and trampling can cause HCN release. Rumen microbes by enzymatic action can cause the breakdown of dhurrin and release of HCN.

Potential HCN poisoning is more likely to occur with forage sorghums or sudangrass. Sorghums and sorghum-sudangrass hybrids generally contain more HCN than sudangrass. Some varieties of sudangrass tend to be lower in HCN than others. Other plants which can release toxic levels of HCN are Johnsongrass, choke cherry, black cherry and velvetgrass.

Symptoms

Following consumption of forage, HCN is released in the rumen and absorbed into the bloodstream. HCN combines with hemoglobin in the red blood cells to form cyanoglobin. HCN prevents the transfer of oxygen from hemoglobin to the tissues. Symptoms of HCN toxicity include increased respiration rate, increased pulse rate, muscular tremors, foaming from the mouth and a blue coloration of lining of the mouth. Clinical signs of HCN poisoning are seldom observed since death often occurs within 15 to 20 minutes following intake of toxic forage. A characteristic symptom is the bright red color of the blood.

Factors Causing HCN Formation

The HCN content of sorghum or sudangrass is highest during the early stages of growth. Leaf blades contain higher levels of HCN than stems. Tillers have higher levels than older plant parts since they consist mostly of leaves without much stalk material. As plants mature, the stalk or stem portion increases in relation to the leaves causing the HCN content of the total plant to decline. However, the top most leaves may still contain dangerous levels of HCN. Cattle tend to selecti- vely graze the young, new growth which are higher in HCN content. Potential poisoning may not decline much with plant maturity for grazing animals.

Increased nitrogen levels in the soil are associated with higher HCN content. An imbalance of nitrogen and phosphorous in the soil increases HCN potential. An adequate level of phosphorous in the soil tends to decrease HCN level.

Any condition that alters normal plant growth including drought and frost will usually cause HCN release. HCN is released very quickly from frozen leaves. Frosted sudangrass is extremely dangerous until thoroughly dried. Drought- stricken susceptible plants may be high in HCN. During a drought, plants remain at a small stage where they tend to be higher in HCN. Drought also reduces the availability of phosphorus for the plant.

Management Practices

Hay - Properly conditioned and cured hay is usually not dangerous. Field conditioning of hay causes HCN to be released. Field curing or drying of sudangrass hay releases 50-75% of the HCN.

Silage - Susceptible plants should be conditioned and wilted prior to ensiling. Ensiling does not remove the HCN. The gas escapes during the moving and feeding process. Silage is generally safe for feeding after a three to four week ensiling period.

Pasture - Grazing may be the most dangerous method of feeding susceptible forage. Cattle prefer to graze leaves and young shoots which tend to be high in HCN. Following a frost, cattle tend to graze new shoots which are high in HCN rather than drier frost-damaged leaves.

Green Chop - Feeding the same material as green chop is usually safer than grazing since it cannot be selectively consumed. Plant stems help dilute the high HCN content of the leaves. Young plants and tillers should not be fed as green chop.

Recommendations

The University of Georgia and Ft. Valley State College, the U.S. Department of Agriculture and counties of the state cooperating. The Cooperative Extension Service offers educational programs, assistance and materials to all people without regard to race, color, national origin, age, sex or disability. An Equal Opportunity/affirmative action organization committed to a diverse work force

Issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, The University of Georgia College of Agricultural and Enviromental Sciences and the U.S. Department of Agriculture cooperating.

Gale A. Buchanan, Dean and Director