by: Stephen B. Blezinger
Ph.D, PAS
Part 1
From time to time we hear about a producer that runs into a problem with nitrate toxicity in his herd. This year, in many parts of the country, conditions are particularly good for the accumulation of nitrates in certain forages. Additionally, with forages in short supply and producers struggling to meet their herd forage needs, attempting to graze or even purchasing affected forages or hays tends to become more common. Additionally it is not uncommon to find elevated nitrate levels in some water sources as well. Combined, nitrates from forages and water can create significant problems for the animal and the producer. So the following discussion will elaborate on some of the particulars.
Understanding Nitrate Toxicity
Nitrate poisoning is a condition which may affect ruminants consuming certain forages and/or water that contain an excessive amount of nitrate. Nitrate is not a foreign compound. It is not uncommon for it to be found at low levels in any number of forages and at low levels it is not an issue.
Under normal conditions, nitrate ingested by ruminant animals such as cattle, sheep and goats is converted to ammonia and then bacterial protein in the rumen by the bacteria population. The steps of conversion in this process include:
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Nitrate is converted to nitrite faster than nitrite is converted to ammonia. Consequently, when higher than normal amounts of nitrate are consumed, an accumulation of nitrite may occur in the rumen. Nitrite then will be absorbed into the bloodstream and will convert hemoglobin to methemoglobin, which is unable to transport oxygen. Thus, when an animal dies from nitrate poisoning, it is due to a lack of oxygen. In other words the animal essentially suffocates.
The occurrence of nitrate poisoning is difficult to predict because nitrate levels can change rapidly in plants and the toxicity of nitrate varies greatly among livestock due to age, health status, and the exact diet the animal is on.
Particular concern should be applied when:
1) Plant growth has been less than half of normal
2) Nitrogen application more than twice recommended.
Nitrate Levels in Plants
Plants normally take up nitrogen from the soil in the form of nitrate, regardless of the form of nitrogen fertilizer (including manure) applied. However little nitrate accumulates in plants, when growth is normal, because plant stems and leaves rapidly convert nitrate to plant amino acids and protein. This conversion of nitrates from the soil to protein in the plant is part of the normal plant growth process. Under certain conditions, however, this balance can be disrupted so that the roots will take up nitrate faster than the plant can convert the nitrate to protein.
The nitrate-to-protein cycle in a plant is dependent on three factors:
1) Adequate water
2) Energy from sunlight
3) A temperature conducive to rapid chemical reactions.
If any one of these factors is inadequate, the root continues to absorb nitrate at the same rate while storing it unchanged in the stalk and lower parts of the leaves. When this situation develops, nitrate accumulates. Nitrates may also accumulate in plants from excessive nitrogen fertilization, for example on fields where a large amount of manure have been applied. Or even if excessive amounts of commercial fertilizer is applied. This is another reason why it is important to soil test so the actual N requirements are known.
Some plants are more likely to accumulate nitrate than others. Crops capable of high levels of nitrate accumulation under adverse conditions include corn, small grains (oats, rye, ryegrass, wheat), sudangrass, and sorghum. Weeds capable of nitrate accumulation include pigweed, lambsquarter, sunflower, bindweed and many others. Vegetables capable of accumulating large amounts of nitrate that are most frequently grazed include sugar beets, lettuce, cabbage, potatoes and carrots.
Nitrates in Water
Nitrates and nitrites are water soluble. They move in and with the water and can accumulate under the right conditions. Any nitrate added to, or produced within, the soil may be leached or washed away by moving water, either by surface run-off or ground water percolation. Again, this can originate from either commercial fertilizers or from manure applied to the land.
Nitrates are more concentrated below or near the area of waste accumulation or disposal such as manure piles, feedlots, septic tank disposal fields, cesspools, privies, etc. Excess nitrates also are more apt to be found in ground water under low areas and waterways that collect or convey water samples from shallow, dug, bored and driven wells more frequently contain excess nitrates than water from deeper, drilled wells. Nitrate levels generally are highest following wet (excessively rainy) periods and lowest, even down to zero nitrates, during dry periods which may cause a false sense of security. Preferably, a well should be tested immediately following a wet period.
Degrees of Toxicity can Vary
Ruminant animals can tolerate a wide range of nitrate levels, depending on several factors. Factors reducing nitrate toxicity include:
� The animal can become conditioned to eat larger amounts of feed with high nitrate content if the increase is gradual.
� Healthy animals are less likely to be adversely affected than animals in poor health.
� Adequate amounts of available carbohydrates (grain) allow the animal to consume more nitrate because carbohydrates enhance the conversion process from nitrate to microbial protein.
Factors increasing nitrate toxicity includes:
� Rapid diet changes can trigger nitrate poisoning.
� Parasitism or other conditions causing anemia will increase susceptibility.
� Nitrate in more than one diet component (e.g. water and forage).
Symptoms of Nitrate Problems
Many general symptoms such as poor appetite, weak calves, lambs or kids, abortions, poor growth, and general unthrifty conditions are frequently blamed on nitrate. These and other general problems can also be caused by a number of disease, nutritional or managerial problems. Therefore, it is essential to evaluate the situation thoroughly before concluding that nitrate is the problem.
Simple chronic nitrate toxicity is rare. More likely, high nitrate is only one of several factors resulting in poor performance. For example, poor performance on a ration that is low in energy or lacking in essential minerals is apt to be worse if nitrate is also present. When good feeding and management practices are followed, it is very difficult to produce chronic nitrate problems.
Many factors have been suggested to explain the different results obtained in research trials studying the nitrate problem. Age, condition, and species of the animal, other chemical compounds and nutrients in the ration, and the types of nitrogen compounds in the feed or water are some of the factors that must be considered.
The most dramatic nitrate toxicity problems have occurred when hungry cattle have been put on corn stalks, oat straw or weedy pasture so care needs to be used when considering allowing grazing of these plants. Under these conditions the highest nitrate forages are fed as the total ration, and the feeding of well-balanced rations and adaptation by the animal are ignored. Sudden change to high nitrate corn silage as the main feed can cause problems. Milking cows and other animals receiving large amounts of grain are not as likely to have nitrate toxicity problems as dry cows, heifers and other animals because the milking cows are on a higher energy ration and because the high nitrate feedstuff is likely to be a smaller proportion of the total diet.
Grains and other concentrates are low in nitrate. Forages (leaves and stems) will accumulate more nitrate than grains. Because forage comprises a larger percentage of ruminants diets, high nitrate in feed is more likely to be of concern in feeding ruminants than non-ruminants. However, nitrate from feed or water can cause problems for all animals and to humans.
Accurate Sample for Analysis
It is always best to be safe than sorry. If fresh-chopped forages or silage is suspected of being high in nitrates, it should be tested. Many labs can effectively test nitrate levels at a reasonable cost.
The two key steps to getting accurate nitrate analysis include:
1) Make sure the sample is representative of the feed or water that is being analyzed.
2) Prevent losses of the nitrate between sampling and laboratory analysis.
Consider the following:
Silage
� Take samples during the unloading process. (If indoors, make sure silo room is well ventilated.)
� Take at least 5-10 separate samples while unloading.
� Mix well and remove about 0.5 lb for testing.
Green-chopped Forage
� Collect several handfuls from different loads or different parts of a load.
� Mix well and remove about 0.5 for testing
Standing Corn or Sorghum
� Cut at least 15 whole plants taken at random.
� Cut plants at same height as field chopper.
� Chop plants into one-half inch lengths and mix well.
� Remove about 0.5 lb for testing.
Preventing Loss of Nitrates in Sample
It is important to preserve the sample as well as possible in order to get an accurate analysis. Preventing nitrate loss in the sample is critical to getting accurate results. This is difficult in silages and fresh forages therefore it is best to take fresh silages and samples directly to the laboratory if at all possible. If samples must be held or shipped, samples should be frozen or dried before shipment. Freeze samples in airtight, plastic bags for at least 24 hours and ship in insulated containers to reach the laboratory while still frozen. DO NOT ship samples late in the week risking processing and analyzation delayed over the weekend. Alternatively, dry samples by spreading in a thin layer on clean paper to dry. Artificial heat is desirable, but the sample should not get above 160° F. Fresh forages will ferment in airtight bags and for this reason are usually dried. Silage samples are usually frozen because it is difficult to dry without losing volatile materials.
Conclusions
High nitrate levels can create problems for animal health and performance. Recognizing the potential problems in advance is important to avoiding any losses in performance or animal life. The previous guidelines can help the producer recognize some of these issues. In Part 2 of this series we will review interpretation of the analyses as well as discuss some steps that can be taken if high nitrate levels are found in forages.
Dr. Steve Blezinger is a management and nutritional consultant with an office in Sulphur Springs, TX. He can be reached at 667 CR 4711, Sulphur Springs, TX 75482, by phone at (903) 352-3475 or by e-mail at [email protected]. For more information you can visit follow us on Facebook at www.facebook/reveillelivestockconcepts.
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