Too much N can do cattle in
Nitrogen is crucial for plant growth. However, too much plant nitrogen—specifically too much plant nitrogen in the wrong form— can be deadly to cattle.
As spring arrives, producers should be particularly vigilant about potential issues. Plants naturally take up nitrogen from the soil and use it for photosynthesis. Young, growing plants are most likely to accumulate nitrates, especially if subjected to stress, such as a late frost.
Nitrate poisoning can result if livestock consume forages with excessive nitrate levels. Here’s what happens. Rumen activity breaks down the nitrate (NO3) into nitrite (NO2), which has one less oxygen molecule. Nitrite is the real cause of “nitrate” poisoning. Nitrite enters the bloodstream and alters the oxygen-carrying process, resulting in reduced oxygen supply to the body. Nitrites also dilate blood vessels, further complicating oxygen transport.
Symptoms of nitrate poisoning in animals include rapid or difficult breathing, dark-colored blood, muscle tremors, drooling, abortions, lower production/growth, frequent urination, poor appetite and diarrhea. It can even lead to death.
If animals consume low levels of nitrates, they can usually handle it with no ill effects. The trouble arises when they are fed elevated levels of nitrates. Understanding nitrate’s role in the plant and when levels may be high can help avoid this problem.
Grass pastures respond well to nitrogen applications. Nitrate, the form of nitrogen that plants prefer, is water soluble and absorbed mainly through the roots. In the presence of sunlight, the plant metabolizes nitrogen into amino acids and proteins. Disrupting the normal cycle can cause excessive nitrate because uptake from the soil will be faster than the plant’s metabolism.
Nutrient management plans for livestock operations often use manure applications to return nitrogen to the soil. Equipment should be calibrated so you know how much manure is being applied, taking care to avoid over-application. If manure was applied in the fall, perform a nitrate test the next spring to evaluate how much nitrogen is available for the coming crop. If you sidedress during the growing season, a nitrate test prior to application can determine how much N is needed.
While important, the amount of nitrate in the soil is not the only indicator of how much nitrate will be in the harvested forage. Plants that are stressed during the growing season, whether by insects, diseases, weather or other factors, could have higher nitrates when harvested. If you can minimize plant stress during the growing season, you should have fewer problems with excess nitrate.
When nitrates are a concern, harvest on bright sunny days, and avoid harvesting during long cloudy stretches. The lower stalk tends to have higher levels of nitrates. Raising the cutter bar can help keep the stems out of harvested material.
Likewise, don’t harvest three to five days after a drought-breaking rain. In this situation, the crop may gladly take up excess nitrogen, but if harvested too soon, the forage will not have had time to convert the nitrates. Similarly, frost can trap nitrates in the plant, so avoid harvesting immediately afterward.
If you are concerned that the forage may be high in nitrates, consider ensiling it, which can decrease nitrate levels by 33% to 50%. The wetter the forage that is ensiled and the more extensive the fermentation, the greater the reduction in nitrate levels.
Just like you wouldn’t harvest immediately after a frost or drought-breaking rain, avoid grazing the crop in those conditions. If that timing cannot be avoided, offer a safe feed to your animals first so they do not overconsume the high-nitrate forage.
When grazing, cattle will tend to not eat the lower stalk of the plant, which, again, is where nitrates accumulate. Having animals consume 50% to 65% of the forage will minimize the risk. Forcing animals to clean up all the forage can be trouble, especially if they don’t all eat at once. The first cows eat the leaves, increasing the percentage of stalks. A cow late to the party gets a higher nitrate load.
If excess nitrogen is suspected, it is important to do a forage test. Nitrate levels should be considered based on the total diet, including water, which can contain a significant amount of nitrates.
A nitrate value of less than 2,500 parts per million (ppm) is typically considered safe. If forages have over 4,500 ppm of nitrates, caution should be taken, especially when feeding young or pregnant animals. Monitor animals for signs and symptoms of nitrate poisoning.
Nitrogen is both a need for forage production and a risk to cattle, so careful management is important. Visit with your MFA livestock and agronomy specialists for more information on how to keep both plants and animals healthy and thriving this spring.
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