– Rory Lewandowski, Extension Educator, Ag/NR Athens County
During periods of drought such as we have experienced this growing season, many of our forages may accumulate nitrates in high concentrations. Feeding forages high in nitrates to livestock can cause health problems, and, in severe cases, result in animal death.
Most forage plants take up nitrogen from the soil in the nitrate form and under normal growing conditions quickly convert it in a progression to nitrite, ammonia and finally plant proteins and other nitrogenous compounds. However, when normal plant growth is slowed down or stopped, as we see in drought conditions, the plant will continue to uptake nitrate but the conversion to other forms does not take place and so nitrates accumulate in the plant.
Plants do not accumulate nitrates at equal levels. There are certain plants that are more likely to accumulate nitrates. Some of these plants include: sorghum, corn, oats, sudangrass, millet, turnips, and even orchardgrass and fescue. Among these species, we generally associate the annual forages with accumulating higher nitrate levels than the perennial grasses. Certain weed species such as pigweed and lambsquarter will also accumulate nitrates in high levels. The degree to which even these listed plants accumulate nitrates is influenced by nitrogen fertilization prior to a drought. In other words, pastures or fields fertilized with nitrogen are more likely to see nitrate accumulations at high levels compared to non-fertilized situations.
Be aware that the first week or two following significant rains that break a drought is a time when plants can accumulate high levels of nitrate. This is because wet soils have an increase in microbe activity that leads to a flush of soil nitrate being available. Plants recovering from a drought take up large nitrate amounts, in excess of what they are able to process into nitrites, amino acids and proteins. It can take one to two weeks before nitrates are fully processed and a balance is reached in the plant. Generally nitrates will accumulate in the highest concentrations in the stalks and stems of plants and are lowest in new leaf growth.
It is actually not the nitrate in forages that causes the potential poisoning in livestock but rather the conversion of the nitrate to nitrite. The nitrite ion oxidizes ferrous iron in blood hemoglobin to the ferric state, forming methemoglobin, which is unable to act as an oxygen carrier. If enough of the hemoglobin is changed to methemoglobin then tissues begin to die from lack of oxygen. At sub-lethal levels the animal experiences abdominal pain, diarrhea, muscle weakness and in coordination. As severity and toxicity increases symptoms will include bluish discoloration to mucous membranes and unpigmented areas, torpor, coma and finally, death of the animal.
There are some animal factors that determine to what extent nitrate concentrations in forages can affect animal health. It has been found that animals in good body condition on diets with good energy levels can tolerate up to 50% of their hemoglobin being converted to methemoglobin without serious effects, while this level may be lethal to animals in poor condition and/or on low energy diets. Fasting can also increase the potential for nitrate poisoning. Animals held off feed before having access to forages that have accumulated nitrates are more likely to have symptoms of nitrate poisoning compared to animals that have been maintained with adequate dry matter intake. Research has also demonstrated that animals have the ability to adapt to nitrate levels. Animals that have been consumed diets with gradually increasing nitrate levels can tolerate a higher end point of nitrates in the diet as compared to animals exposed to a high level suddenly.
What are the best ways to handle forages that may have accumulated high levels of nitrates? First assess the risk. Is the forage one that is a known nitrate accumulator? Was nitrogen fertilizer applied? If the answer to these questions is yes, then it may be a good idea to get the forage tested for nitrates before feeding to know what kind of potential toxicity exists. Results may be reported as nitrate-nitrogen or as nitrate. At nitrate-nitrogen levels up to 1000 ppm and nitrate levels of 4400 ppm the forage is safe to feed at up to 100% of the ration. Above these levels, the forages may still be used but at a percentage of the total ration. Generally when nitrate-nitrogen reaches 4000 ppm or nitrate level is above 17000 ppm the forage should not be fed at any percentage in the ration. Other factors that determine at what level and to what extent a forage with nitrates can be fed is whether or not the animal is pregnant, what condition the animal is in and the energy level of the ration.
Forages that have accumulated nitrates but that have been ensiled can be safer to feed than that same forage cut and made as dry hay. Once forage has been cut for hay and dried, the nitrate levels will not change. Forage that goes through the ensiling process will convert some of the nitrates into a gas and end up with a lower nitrate concentration after the fermentation process is complete, generally at least 21 days. Feeding green chop or grazing forages high in nitrates is risky. Again, if a nitrate problem is suspected, test the forage before feeding.
For more information about nitrates in forages, assessing risk or interpreting nitrate test results, review any of these internet sites:
http://cherokee.agecon.clemson.edu/mmm373.htm
http://virtual.clemson.edu/groups/psapublishing/DISASTER/drought/Drout12.htm