Nitrate is found in most all forages and when consumed by ruminants, nitrate is normally reduced to ammonia which is then absorbed or exacted as urea. The problem occurs when forages with excessively high Concentrations of nitrate is fed to livestock. High nitrate feeds, when digested, create an intermediate product nitrite, which is the cause of nitrate poisoning.
Nitrate poisoning occurs when nitrite is absorbed into the blood where it changes the red-colored blood pigment, called hemoglobin, to methemoglobin. Hemoglobin carries oxygen from the lungs to the other tissues, but methemoglobin cannot carry oxygen. Poisoning occurs when the methemoglobin concentration in the blood stream is so high that the oxygen carrying capacity of the blood is reduced to a critical level. Blood changes in color, as this occurs, to a dark chocolate brown. Death, abortions, and reduced milk production are results of nitrate poisoning. Animals will vary in tolerance to Nitrate. Usually young, pregnant, weak, parasitized, or ill livestock are most susceptible.
Nitrate in feeds is reported usually in parts per million(ppm) nitrate-nitrogen(N03-N). See Table 1 for animal response to various concentrations of N03-N in forages.
Table 1 Animal Response to Nitrate Nitrogen Concentration
|N03-N ppm||Animal Response|
|Less than 1400||Safe|
|1400-2100||Marginal, use caution when feeding. Can cause reduced milk production, abortions, and low rate of gain. It would be best to limit daily use to1/2 of the total dry matter intake.|
|2100-3000||Feeds in this range should be limited to 1/3 of the total daily dry matter intake.|
|3000 - 4000||Feeds in this range should be limited to 1/4 of the total daily dry matter intake.|
|4000 - 5000||Feeds in this range should only be 10-15% of the total daily dry matter intake.|
|Greater than 5000||Do not feed - Death may occur.|
A simple algebra equation will help you decide how much high nitrate feed can be mixed with a low nitrate feed to obtain a safe level.
N = [(1400-L)/(H-L)]xl00
H = ppm N03-N of high nitrate feed
L = ppm NO3-N of low nitrate feed
N = percent high nitrate feed that can be mixed with low nitrate feed.
Potential forage nitrate concentration can be reduced at harvest time. Harvest only mature plants. Raising the cutter bar avoids harvesting the nitrate that concentrates in the bottom third of the stalk. Ensiling forages often reduces nitrate content by 40 to 60% during fermentation.
Avoiding nitrate poisoning once the crop is harvested is possible through various management techniques. The first step is to always have forages tested for nitrate. Once the exact concentration is known, high nitrate forages can be mixed with low nitrate forages to dilute the total nitrate concentration to a safe level. High energy rations will help livestock tolerate a higher nitrate content. Adding either vitamin A or iodized salt to the feed or fed free choice will also help livestock tolerate elevated nitrate levels.
Feeding livestock small quantities of feed frequently allows a higher nitrate feed to be consumed without toxic effects. Gradually introducing a higher nitrate feed allows animals to adjust to the change. Balanced rations, particularly high carbohydrates rations, tend to reduce potential nitrate problems.
Any feed known or suspected to be high in nitrate should not be fed damp as the dampness seems to increase toxicity. Clean water should be available at all times, frequent intake of water appears to increase the total amount of nitrate that can be consumed daily without harmful effects. Nitrate toxicity rarely occurs from water alone. Water high in nitrate can contribute to nitrate poisoning.
Causes of High Nitrate Feeds
Nitrate is taken up from the soil by plant roots and convened into amino acids, proteins, and other nitrogenous compounds. This conversion takes place in actively growing leaves with the highest concentration of nitrate found in the stalks, before conversion takes place. Excessive amount of nitrate may occur when any stress causing an abrupt decrease in plant activity occurs. Stress conditions include. 1) Shading or low light intensity; 2)Weather such as drought, frost, hail, and temperatures below 55 degrees F; 3) Herbicides, particularly 2,4-D; 4) Plant diseases.