Hail, drought, spray drift or frost can all disrupt the normal growth of plants, causing nitrate accumulations that can lead to nitrate poisoning. This year, depending where you’re from, had them all.
The number of animals affected by acute nitrate poisoning on the Prairies is usually low, but when losses occur, they occur suddenly and can be devastating. As long as the feeding program is managed correctly, high levels of nitrate need not be a problem, states “Nitrate Poisoning,” a Manitoba Agriculture fact sheet.
Sheep and cattle are more susceptible to poisoning than non-ruminant species because ruminant microbes favour the conversion of nitrate to nitrite, which is normally converted to ammonia. The ammonia then is converted to protein by bacteria in the rumen.
But if cattle rapidly ingest large quantities of plants that contain high nitrate levels, nitrite accumulates in the rumen, crosses the rumen wall and is absorbed into red blood cells where it combines with hemoglobin (an oxygen-carrying molecule) to form methemoglobin. Methemoglobin cannot transport oxygen as efficiently as hemoglobin and animals suffer from oxygen deprivation. Heart rate and respiration increase, the blood and tissues of the animal exhibit a blue to chocolate discolouration, muscle tremors often develop, animals become uncoordinated and eventually die from suffocation.
The majority of nitrate poisoning cases across the Prairies occur with drought and frost-stressed oats, corn and barley. The list of common plants known to accumulate nitrates is fairly extensive and includes wheat, sweet clover, flax, canola, rye, Sudan grass, sorghum-Sudan hybrids and millet. Common weeds known to accumulate nitrate include Canada thistle, dock, kochia, pigweed, nightshade, Russian thistle and wild sunflower.
Fertilized plants have higher nitrate levels than plants that aren’t fertilized. The abnormal accumulation of nitrate can also be influenced by moisture and soil conditions.
Producers should test stored feed for the presence and amount of nitrate. Frequent intake of small amounts of a high-nitrate feed increases the tolerance to and total amount of nitrate that can be safely consumed, states a Saskatchewan Agriculture factsheet titled “Nitrate Toxicity.” Cattle in good condition may be able to maintain normal growth while consuming feeds with nitrate levels of one per cent or higher if rations are balanced and the transition to higher-nitrate feeds is gradual.
But animals should not be allowed to consume feeds containing more than 0.5 per cent nitrate if they have not been previously exposed. To help animals safely make the transition to high-nitrate forages, mix them with low-nitrate forages so that the overall nitrate level remains less than 0.5 per cent.
Remember that the feed must be physically mixed. This is easier to do when grain is fed and forages are chopped and mixed, such as in feedlot rations. Saskatchewan Agriculture warns producers not to offer one bale of high-nitrate feed beside one or more bales of low-nitrate feed, as some animals may only eat from the high-nitrate bale. Introduce questionable feed over a period of one to two weeks and avoid an on-and-off pattern of feeding high-nitrate feed. If mixing is not possible, feed low-nitrate feed first.
Balanced rations lower the risk of nitrate poisoning. Feeding adequate levels of energy, vitamins (A and E) and trace minerals helps prevent toxicity, Saskatchewan Agriculture notes. Producers can also feed grain along with high-nitrate feeds, as energy from the grain seems to complete the conversion of nitrate to bacterial protein in the rumen. Make sure livestock always have access to clean water.
Nitrates in both the feed and water must be considered because they are cumulative. Nitrate toxicity is unlikely to occur from water containing less than 443-ppm nitrate (NO3).
Ensiling tends to reduce the nitrate content of forages. Forages high in nitrate can lose from 40 to 60 per cent of their nitrate content during fermentation, notes Manitoba Agriculture. However, ensiling doesn’t guarantee that excessive nitrate will drop to safe levels. Harvest forages suitable for silage at the stage of optimal quality and quantity. Then test the feed if high nitrate levels are a possibility. Harvesting other crops, such as oats, closer to maturity may be a consideration if nitrate levels are high.
If hay dries quickly, it loses very little nitrate. However, as Saskatchewan Agriculture points out, bad weather has destroyed more hay than nitrates. Cut when the weather is favourable, test after baling and manage accordingly. Nitrate concentration in dry hay bales doesn’t change much over time.
Saskatchewan Agriculture also warns producers to avoid feeding damp hay, straw or fodder that is also high in nitrate. This feed is especially toxic because some of the nitrate has already been converted to nitrite. High-nitrate feeds piled in mounds and allowed to heat before feeding are also very dangerous to livestock for the same reason.
Saskatchewan Agriculture also offers advice to producers facing nitrate poisoning. Remove the suspect feed and call a vet immediately, as nitrate poisoning turns fatal quickly. Veterinarians treat nitrate poisoning by administering methylene blue solution intravenously. Provide a high-energy feed to help reduce the nitrate’s effect. If you have to handle the affected cattle, do so as quietly as possible, and handle as little as possible, to avoid exacerbating the effects of oxygen deprivation.
Proper sampling and feed testing gives producers an opportunity to develop a safe strategy with nitrate-contaminated feed. Your nutritionist and veterinarian need to be in the formula.