Feed and Forage--Toxicity

NITRATE POISONING

The recent droughts, their damage to corn and grain sorghum crops, and the use of these crops for livestock feed have brought up many questions concerning nitrate poisoning (methaemoglobinaemia). Nitrate poisoning is not uncommon, as most annual plants may accumulate nitrates under stress. Symptoms are seldom obvious in livestock. Generally, they are first noticed when coma or death occurs.

Some farmers test a field for nitrates by turning in a "sacrifice animal" to see if it shows any abnormal behavior or dies. A word of warning is in order here. When one animal is turned into a field, it will lightly graze the new leaves, which contain the lowest concentration of nitrates in the plant. Thus a sacrifice animal may not detect a high nitrate problem that will show up when the herd is turned in and heavier grazing occurs.

The Problem

The stresses that slow plant growth and cause nitrate to accumulate to toxic levels are normally drought or frost. However, small grains actively growing in the fall or spring have been shown to accumulate nitrates at toxic levels during several cloudy or overcast days. Additionally, spraying with 2,4-D or 2,4-DB may cause increased nitrate accumulations in some plants.

Certain plants are more likely to accumulate toxic levels of nitrate than others. Pigweed, lambsquarter, and oats are known to be accumulators. These plants may contain toxic levels of nitrate when other plants growing beside them will not. For instance, a fertilized and drought-stressed bermudagrass pasture may not result in any animal health problems, but the same pasture with pigweed would cause animal deaths if the pigweed were eaten. In a Missouri study, sudangrass, orchardgrass, and tall fescue accumulated nitrate at high levels; smooth bromegrass and ladino clover were intermediate; and alfalfa and wheat had only low levels.

Nitrate levels will be highest in the stalks or stems and lowest in the new leaf growth. Nitrate content can change dramatically (either higher or lower) from day to day. Once cut for hay, nitrate levels do not change appreciably. However, ensiling high nitrate material will cause a portion of the nitrates to be converted to a brown gas and given off. Thus, silage becomes safer to feed, but the gas is poisonous and care must be taken when near the silo while forage is fermenting.

Animal Response

Sublethal symptoms of nitrate poisoning are seldom very apparent. Signs of abdominal pain and diarrhea are seen due to the irritant action of nitrate ion. This is accompanied by muscular weakness and incoordination, convulsions, and accelerated heart rate. Abortion may occur in pregnant animals. In severe cases, progressive cyanois occurs, which is first visible as a bluish discoloration of the mucous membranes and unpigmented areas of the body and leads to torpor, coma, and death. Dead animals will have discolored, dark, tarry blood (chocolate blood).

Nitrate in the diet (feed or water) at moderate levels will cause reduced milk production, lowered rates of gain, and reproductive difficulties. At higher levels, death will result. The exact level at which a given response occurs depends on the condition of the animal and the energy level of the diet. Cattle in good condition can tolerate 50 percent of the hemoglobin being changed to methemoglobin without ill effects, while this could be lethal to cattle in poor condition. High levels of energy in the ration speed up the reduction of nitrate in the rumen to ammonia and detoxify it.

Animal fasting increases the susceptibility to nitrate poisoning. Cattle feeders do not "fast" their cattle on purpose, but drought frequently leaves the cattle with little to eat. This commonly results in cattle that are in poor condition, that have low energy in their diet, and that will gorge themselves if given the opportunity. All these will increase the severity of a given level of nitrate.

Additionally, there appears to be some degree of adaptation to high levels of nitrate. Animals that have gradually increased levels of nitrate in the diet tend to be able to tolerate higher levels of nitrate than animals first exposed to the higher levels.

The variable effects of nitrate make it difficult to make an absolute statement as to the actual toxic dose of nitrate. Table 1 gives a range that reflects varying animal condition and dietary energy level as well as other factors.

Table 1. Lower Range of Lethal Dietary Concentrations of Nitrate Causing Animal Death

How to Handle the Problem

Before feeding forages that may be high in nitrate, first take a sample and send it to the Clemson Agricultural Service Laboratory for nitrate analysis. The most critical factor in obtaining accurate and reliable information regarding nitrate levels in the forage is proper and careful sampling of the forage. For hay or silages, take several representative samples, mix well, and send to the laboratory. Sample silage after fermentation is completed because nitrate levels may be lowered from that of the freshly chopped sample. In pastures, it is important to sample the entire portion of the plant that the animal will be eating. Remember that young leaves on top of the plant will tend to have a lower nitrate level than other parts of the plant. The sample may be sent in for analysis in a paper bag. Drying has little effect on nitrate levels of the sample.

The results from the Clemson Agricultural Service Laboratory will be reported as nitrate-nitrogen (NO3-N) in ppm. Results from some laboratories may be reported in other units and must be converted to nitrate-nitrogen before comparison to Table 1 can be made. The conversion factors are presented in Table 2.

Table 2. Conversions of Nitrate-Nitrogen from Other Forms of Expression

Nitrite (NO 2)                                             0.30
Nitrate (NO 3)                                             0.23
Sodium nitrate (NaNO 3)                         0.16
Potassium nitrate (KNO 3)                       0.14

When a feedstuff has been determined to have a potentially toxic level of nitrate, the forage may be fed if appropriate cautions are taken. The following options exist:

1. Consider the class and condition of animals to receive the forage. Animals such as milking cows and stockers, from whom a high level of productivity is expected, may show reduced performance at lower levels than indicated in the chart for animal death. Animals in poor condition will be affected by lower levels of nitrate than animals in good to excellent condition.

2. Do not allow animals to gorge themselves on the high-nitrate feedstuff. Be sure that the animals have not been fasted so that they will not eat excessive amounts.

3. Dilute the forage with low-nitrate forage. This can be done by feeding less high nitrate silage or hay or by limiting the time animals are allowed to graze, and meeting the remainder of the animal requirements with a low-nitrate feedstuff. Be sure to feed the forages at different times so that all animals eat some of each. If a big bale of high-nitrate hay is simply put alongside a low-nitrate hay, some animals will eat only one type and those eating the high-nitrate hay may show symptoms of nitrate poisoning. Remember that nitrate converts to nitrite in hay that has become wet, and this makes the hay much more toxic.

4. Feed an energy supplement. An energy source, such as corn grain, will increase the rate of nitrate metabolism in the rumen, thereby detoxifying it.

   Standard treatment for animals suffering from acute nitrate toxicity is an intravenous injection of a reducing agent, generally methylene blue solution. The reducing agent converts methemoglobin to oxyhemoglobin and reverses the effect of nitrite. Treatment should be repeated in severe cases.