Managing Nitrates in Annual Forages
Dry conditions can create high nitrates in annual forages and weeds. Understanding management practices that can mitigate the negative impact on beef cattle can help producers avoid a wreck.
Feeds that contain nitrates can be successfully fed to cattle. When feeds containing nitrates are consumed by ruminants, nitrates are changed in the rumen to ammonia and are rendered non-toxic to the animal. Also, the rumen bacteria can use ammonia.
Nitrite and nitrate poisoning
Nitrite is one of the intermediate products in the breakdown of nitrate in the rumen and is the cause of nitrate poisoning.
Nitrite can be absorbed into the bloodstream. Nitrite in the bloodstream changes hemoglobin to methemoglobin. Hemoglobin carries oxygen from the lungs to other tissues in the body, but methemoglobin is incapable of carrying oxygen. Thus, nitrates become a problem when enough methemoglobin is produced that the oxygen carrying capacity of blood is reduced to a critical level.
Nitrite in the blood causes a brownish discoloration of the blood, due to the presence of methemoglobin, and is a sign of nitrate poisoning. Besides the chocolate-colored blood, other physical signs of nitrate poisoning include
- difficult and rapid breathing,
- muscle tremors,
- low tolerance to exercise,
- frequent urination,
- collapse, and
Nitrates in blood may also cause blood vessels to dilate and are responsible for peripheral circulatory failure.
Lack of oxygen to the fetus probably causes abortions that sometimes occur following nitrate poisoning. Abortion due to nitrate is accompanied or preceded by some evidence of nitrate problems in the adult animal, including chocolate-colored blood and bluish discoloration of unpigmented (around the eyes) areas of the skin or mucous membranes.
Safely feed forages with high nitrate levels
Forages that contain high nitrate levels can be diluted in the diet with grains or with other forages low in nitrates and then can be fed safely. This can be accomplished easily in feedlot rations where grain is fed and forages are chopped and mixed as a complete ration.
Feeding grain in combination with high nitrate feeds helps reduce the effect of the nitrate content. Energy from the grain apparently helps complete the conversion of nitrate to bacterial protein in the rumen.
Frequent intake of small amounts of a high nitrate feed increases the total amount of nitrate that can be consumed daily by livestock without adverse effects, and helps livestock adjust to high nitrate feeds.
Cattle losses to nitrate toxicity usually occur in hungry cattle that have not had time for some adjustment to feeds with potentially toxic levels of nitrates. For example, cattle that go without feed for a day or longer during snow storms often rapidly eat a large amount when they finally gain access to feed. If the feed they receive is high in nitrates, cattle losses can, and usually do, occur. If cattle are allowed to adjust to feeds that have boarder-line toxic levels of nitrates, they will develop microbes in the rumen that convert nitrates to a non-toxic form.
Feed long stem forages such as wheat, oat, and cane hay that contain high amounts of nitrate in limited amounts several times daily rather than feeding large amounts once or twice daily. In addition, long stem hays suspected of nitrates can be fed in combination with hay low in nitrate to dilute the nitrate intake with little risk of nitrate problems. Grinding and mixing high nitrate forage with a forage low in nitrates is the most common and effective management practice to avoid nitrate toxicity.
Livestock should have access to clean water at all times.
Grazing pastures with high nitrate levels
Allowing livestock to graze pastures suspected of having high nitrate levels is not without risk. Implementing one or more of the following management practices will reduce the risk of livestock losses to nitrate toxicity.
- Don't overstock suspected pastures.
- Don't strip-graze suspected pastures.
- Provide other feeds that contain little or no nitrate during grazing.
- Graze suspected pasture during the day and remove at night the first week to reduce the amount of pasture consumed and to acclimate cattle.
- If possible, don't graze suspected pasture until one week after a killing frost.
Feeding drought-damaged corn
Corn plants grown in drought conditions can potentially contain nitrates. The majority of the nitrates will be in the lower eight inches of the stalk. Raising the chopper height to 6 to 8 inches will reduce the amount of nitrates in the silage. Yield will be reduced, but so will nitrate level.
Ensiling drought-damaged corn can reduce nitrates in the silage 40 to 60 percent. Before feeding drought-damaged corn silage, allow it to go through at least a 21 to 28 day fermentation period before feeding. Shorter fermentation times may cause some of the nitrates to still be in the dangerous nitrite form, just like heated green chop.
Don't let cattle losses due to nitrates be a problem for your operation. Sample feeds, especially summer annuals grown under drought conditions, and tests them for nitrates. Use management strategies to dilute nitrates in high nitrate feeds and manage grazing situation that may expose cows to forages high in nitrates. Remember that total nitrate intake is the sum of the nitrates that come from the feed, but also the nitrates that may be in the water.
Dr. Rick Rasby, Professor of Animal Science
Animal Science, University of Nebraska - Lincoln, Lincoln, NE