By: Patricia Burch, County Extension Support Specialist, NC Cooperative Extension
I have had reports of several producers finding some dead cows in pastures recently. Several were from nitrate toxicity. Most nitrate toxicity issues occur when producers are feeding hay. So this week, I thought we would have a refresher on the particulars of nitrate poisoning of livestock.
Nitrate poisoning is caused when an animal consumes a feed source that is high in nitrates. In the animal’s stomach that nitrate is converted to nitrite. The nitrite is easily absorbed into the blood stream where it converts blood hemoglobin to methemoglobin, which cannot carry oxygen. The result is that the animal dies from a lack of oxygen. Symptoms of nitrate toxicity include labored breathing, muscle tremors, and a staggering gait after which the animal collapses, gasps for breath, and dies quickly. The membranes of the mouth are bluish from a lack of oxygen and the blood is chocolate-brown but turns brighter red when exposed to air.
What factors can cause nitrate accumulation? Basically, drought, reduced sunlight, excessive soil nitrogen, and young plants cause it. Drought and reduced sunlight cause nitrate accumulation due to the fact that the plant is not growing and utilizing the nitrogen it has absorbed. Mainly plants — such as sorghums — will take up excess levels of nitrogen if it is present. This is particularly true of young, immature plants.
The levels of nitrate to worry about vary according to the form of the forage. Research in Europe has shown that nitrate levels as high as 2%, or 20,000 parts per million (ppm), cause no serious problems while the forage is fresh. However, once a forage is dried down the potentially harmful nitrate levels change. Levels of 0% – 0.25%, 0 – 2,500 ppm, are generally considered safe for all classes of livestock. Levels of 0.26% to 0.5% should be used with caution and should be limited to one-half of the total ration of pregnant cattle and young animals. They should also not be fed with liquid feed or other non-protein nitrogen supplements. These levels can cause early term abortions and reduced breeding performance. If the levels from 0.6% – 1.5%, the forage should comprise no more than one-quarter of the ration. At these levels, we would also expect mid to late term abortions, weak calves, reduced milk yield, and decreased growth. Levels over 1.5% would give acute toxicity and death. This forage should only be used in a total mixed ration where the forage is limited to 15% of the total ration. These levels apply to cattle and goats. Those of you feeding horses will want to keep the nitrate level at or below 0.5% of the total dry matter diet. The "rule of thumb" is to select forage that has no more than 0.65% nitrate ion on a dry matter basis.
There are ways to manage around nitrate situations. First, no matter what the source, do not over-apply nitrogen. Apply at agronomic rates. Second, be aware that certain crops under adverse weather conditions are more susceptible to nitrate accumulation. Plan grazing and mowing schedules accordingly if at all possible. You may also consider planting forages with a relative lower risk of nitrate accumulation. Third, have your forage source analyzed for nitrate content. This is a free service from the NCDA&CS. You can then feed based on a known level of nitrate. It is also advisable to check the water source for nitrate levels. Cattle will adapt to higher levels of nitrate over time. Once acclimated, slightly higher levels can be fed safely.
You may have heard of producers losing animals to nitrate poisoning from hay that was "pumped on" or "had litter put on it". While it is possible, the use of these fertilizer sources does not automatically result in high nitrate levels. If used properly, these sources are no more likely to cause high levels than commercial sources. Additionally, if used in excess or at the wrong time, commercial sources are just as likely to cause a nitrate problem. Just remember to have the forage tested. Even at recommended rates, any nitrogen containing fertilizer can cause a nitrate situation.