By Harry Brook
Everyone is aware that plants hit by frost can accumulate nitrates, but did you know that any stress can cause the same thing to happen? In fact, drought is probably the worst situation for nitrate to concentrate in forage plants.
Under drought conditions, a lot of annual crops are salvaged and used for feed. These are the feed sources that should be checked for high-nitrate levels. Usually, annual crops are heavily fertilized, expecting good yields. With drought conditions, crops are stressed and much of the fertilizer is not able to be used for plant growth. Severe drought impairs the crop’s ability to convert nitrogen into protein and it can accumulate in all parts of the stem and leaves.
What happens when a high-nitrate feed is consumed by an animal? In cows, the nitrate is converted to nitrite and then broken down to ammonia by rumen bacteria. If levels get too high, the bacteria cannot break down the nitrite fast enough and nitrite passes through the rumen wall into the blood stream. Hemoglobin in the blood stream carries oxygen to keep the animal alive. Nitrite in the blood stream attaches to the hemoglobin and prevents it from carrying oxygen, thus depriving the animal of oxygen and, eventually the animal will suffocate.
Safe levels of nitrate in feed can vary from individual to individual but the rule of thumb is usually that it is safe to use feeds with 0.5% nitrate or less, on a dry matter basis. However, when changing feed types, it is always a good idea to introduce new feeds gradually to allow the rumen bacteria to adapt to the new feed. Given time to adjust, some cattle can safely consume more than the 0.5% nitrate containing feeds.
Some of the worst cases of nitrate poisoning in cattle have come from cattle coming off a very short pasture and being given dry feed, free choice. In those cases, the hungry animals eat a lot of dry feed with elevated nitrate levels and it magnifies the problem due to sudden influx of nitrate where the rumen biota have not had time to adapt.
Cattle on swath grazing with higher levels of nitrate than 0.5% can do quite well when given time to adjust to the feed. There also are cases of chronic nitrate toxicity where the clinical signs of poisoning are not seen. It can result in reduced weight gains, lower milk production, depressed appetites and possibly, more infections. These sub-clinical signs of toxicity can occur with nitrate levels of 0.5 to 1.0% on a dry matter basis.
Most people know after a hail storm or frost there is a period of a few days where the upper part of the plant is damaged but the roots are unaffected. The roots will continue to collect nitrate and move it up to the upper part of the plant. If the upper part of the plant is damaged, due to any cause, there can be an accumulation of nitrate in the tissues. If the upper part of the plant still has some green leaf material, it can still convert the nitrate to protein which is why nitrate levels in a growing plant my go down over time. However, drought is a case where damage to normal plant function is not that obvious.
Cattle are most susceptible to nitrate poisoning so it is important to be aware of nitrate levels. If you are feeding annual crop taken for feed or unfamiliar salvaged feeds, in a drought, get it feed tested for nitrate levels. If nitrates are a problem, the feed can still be used but you will have to limit the consumption to reduce the dosage. Feed tests are cheaper than dead cattle.