Drought raises possibility of nitrate toxicity this year

Drought-stressed crops can significantly accumulate nitrates, which can cause problems if fed to cattle this winter. | Mary MacArthur photo

Nitrate poisoning in cattle is a potential issue to be aware of in years with significant drought.

There will probably be significant amounts of drought-stressed cereal crops that are fed to beef cattle this winter in parts of Western Canada and we know that drought-stressed small grains and corn can accumulate nitrates to a significant degree.

Hail or frost damage can also cause plants to accumulate nitrate as well.

Other crops that can accumulate nitrate include canola, flax, beet tops and sorghum. There are also a wide variety of weeds such as Canada thistle and kochia that could be classified as nitrate accumulators.

The nitrates are not directly toxic to the animal. We need to consider that we are actually feeding the microbes that live in the rumen as well. Some of these rumen bacteria are able to convert nitrates to nitrites and then finally convert it to ammonia. Ammonia is absorbed into the bloodstream and is eventually excreted in the urine as urea. If nitrites accumulate in the blood, this is when we see signs of toxicity.

When the nitrate levels are excessively high in the feed and the rumen bacteria don’t have time to adapt, this nitrate cycle can be overwhelmed. The bacteria can’t convert all of the nitrites to ammonia and the nitrites start to accumulate in the blood. Some of the nitrates that can’t be converted by the bacteria also get recycled through the bloodstream and into the saliva, which can eventually increase the levels of nitrates and eventually the levels of toxic nitrites even further.

The reason nitrites are toxic is because of their ability to bind with hemoglobin in the blood. Hemoglobin is the molecule that allows the red blood cells to carry oxygen to the tissues. Nitrites convert hemoglobin molecules to methemoglobin. Methemoglobin can’t carry oxygen and as a result these high levels of nitrites can eventually create an oxygen starvation for the tissues of the animal.

The clinical signs that we see in cattle affected with nitrate toxicity range from sudden death to signs of oxygen starvation, such as respiratory distress, an increased heart rate, weakness, muscle tremors and difficulty walking or animals that are down and can’t get up.

If you examine the mucous membranes, those tissues that are usually pink such as the inside of the mouth, under the eyelids or the vulvar area of cows, these tissues will be blue-grey instead of pink because of the lack of oxygen in the tissues. At postmortem, if the carcass is examined while relatively fresh, the blood will have a chocolate-brown colour, which is characteristic of nitrate toxicity. There is some evidence that lower levels of nitrate toxicity can result in reduced productivity and can also cause abortion or stillbirth in pregnant cows.

If affected animals are identified early, your veterinarian can provide treatment with a slow IV injection of a methylene blue solution. However, animals must be handled extremely carefully to minimize the stress of handling when their tissues are so oxygen deprived.

Feed testing is the best way to identify feeds that could potentially be high in nitrates. This can be done most accurately at a feed testing laboratory. There is also a quick qualitative diagnostic test that can be done on the cut stem of plants, but it is probably best to take a representative feed sample and send it to a diagnostic or feed testing laboratory to get more accurate results.

Your veterinarian, nutritionist or local livestock extension specialist can help you interpret the results.

An overall nitrate level of greater than 0.5 percent should not be fed to animals unless precautions are taken.

It is possible to gradually adapt the rumen bacteria to higher levels of nitrates in the feed. We can gradually increase the levels of nitrate and if we allow time for the rumen bacteria to be adapted, we can eventually feed higher levels of nitrates safely.

Other strategies to control nitrate toxicity in some situations might include feeding stored feeds twice daily rather than feeding large amounts once daily. High nitrate feeds can be diluted with other lower-nitrate feed sources to adjust the overall nitrate levels.

If animals are grazing high-nitrate feeds, you may want to lower the stocking density or manage the grazing so that animals are less likely to graze the lower stem material of the plant where nitrates are usually highest. Making sure that the overall diet is well balanced and meeting all of the cattle’s energy, protein and trace mineral needs is also important in preventing nitrate toxicity. Adequate energy helps with the conversion of nitrate by the bacteria in the rumen and will allow higher levels of nitrate to be fed.

This is definitely a year where many parts of Western Canada could potentially have higher nitrate levels in cereal crops and if you are planning on using these for winter feeds, it would be wise to have your feed tested to avoid potential problems with nitrate toxicity.

John Campbell is a professor in the department of Large Animal Clinical Sciences at the University of Saskatchewan’s Western College of Veterinary Medicine.

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