Avoid nitrate poisoning Dr. Jason McGillivray | Back to Vet Advice
As the winter feeding period fast approaches, many ranchers are busy trying to secure sufficient quantity and quality of feed for the winter at the least cost. Our area has been extremely dry and feed supplies are tight. With conditions such as we have experienced this year, nitrate levels in feeds can be a problem. The key to avoiding a "wreck" with nitrates is remembering the risk factors and planning ahead.
With feed containing normal levels of nitrates, the nitrate in the forage is converted to nitrite in the cow's rumen by the rumen bacteria. This nitrite is then converted to ammonia. Excess ammonia is absorbed into the bloodstream and excreted in the urine as urea. Nitrate poisoning occurs when nitrate in the feed produces more nitrite in the cow's rumen than the bacteria can convert. The excess is absorbed through the rumen wall into the bloodstream where the nitrite binds to the hemoglobin, forming methemeglobin. Hemeglobin carries oxygen in the blood to the body's tissues. Methemeglobin cannot carry oxygen. If enough methemeglobin is produced the cow suffers from oxygen shortage. The clinical signs of acute nitrate poisoning include laboured breathing, excessive salivation, blue-gray mucus membranes, staggery gait and occasional convulsions. Animals can die within the first one to eight hours of eating the feed; this is often the first sign of a problem.
In pregnant cows, abortions may occur. But late-gestation cows may have premature, weak calves that often die within 24 hours of birth. In a case we looked into where abortions had occurred without cows dying we found out the rancher had suddenly increased the use of this toxic feed from 30 to 60 per cent of the ration. The cows become tolerant to the nitrates at that level, but the fetuses were deprived of oxygen and died
Thin cattle or cattle with pre-existing respiratory problems are more likely to be affected than healthy cows in good body condition. Also, if a mixture of feeds is being fed at the same time, these thin cows may get pushed out and consume more of the high nitrate feeds. Once-a-day feeding versus twice-a-day feeding can exacerbate the problem as the mount of nitrate consumed at one time would be higher and pecking order is more of an issue.
A diagnosis can be made based on a history of events leading up to the symptoms, combined with an examination of the animals or carcasses. Blood from affected cows is dark chocolate brown in colour. A feed analysis is essential to confirm the diagnosis.
The prescribed treatment for nitrate poisoning is intravenous administration of methylene blue. However, because of the number of cattle often involved and the difficulty in obtaining large volumes of methylene blue, treatment is often not practical. Most of the damage has already been done. The stress of handling these oxygen-limited cattle often does more harm than the treatment does good. Minimal handling and rapid removal of the feed is often the best course of action.
Oats, barley, wheat or rye green feed, canola plants, sorghum or corn are more likely to accumulate nitrates, along with weeds like millet, kochia, mustards, pigweed, Russian thistle, fireweed and lamb's quarter. I investigated a case of nitrate poisoning involving first cut grass/alfalfa hay that was harvested in July. The field was a new seeding and had a lot of millet and pigweed. The hay was fed out on winter pasture to bunch the cows up so they could be gathered and moved to winter feed grounds. The cows were hungry and rapidly consumed this feed. Four per cent of the cows died and approximately 15 per cent of the herd aborted within 10 days.
A variety of conditions can cause these susceptible plants and weeds to accumulate nitrates. Frost is the most common; however, drought and heat stress, cool cloudy weather, herbicide application and plant disease can limit the use of nitrogen drawn up by the roots and create a toxic plant. High nitrogen levels in the soil are another contributing factor. Nitrate levels can decrease in plants three to five days after a period of stress if growing conditions return to normal.
After a heavy frost, crops should be cut as soon as possible to prevent further uptake of nitrates as return to normal growth is unlikely. Once the crop is cut, nitrate levels will not change over time. Ensiling can decrease the level of nitrates in feed. However, heating of feed baled too moist can actually increase the toxicity by converting the nitrate to the more toxic nitrite.
Feed analysis should be done whenever the above risk factors apply before these feed types are purchased or fed. Feed analysis for nitrates alone is approximately $10/sample or approximately $40 to $50/sample for a routine feed analysis including nitrate levels.
Nitrate levels may be reported in three different ways depending on the analytical procedure used by the lab. When reported as nitrate (NO3) a level of 0.4 per cent, dry matter basis, or nitrate nitrogen (NO3-N) a level of 0.12 per cent, or potassium nitrate (KNO3) a level of 0.8 per cent are generally considered safe.
Consult your local veterinarian for proper feed sampling techniques and interpretation of the results.
