STRATHMORE, Alta. — Nitrogen makes plants grow, but nitrates in forage can kill livestock.
“Typically, your first symptom of nitrate toxicity is a dead animal,” Barry Yaremcio of Alberta Agriculture told a forage seminar in Strathmore.
Cattle are at the highest risk to die from nitrate poisoning; sheep and horses are more tolerant.
Large applications of nitrogen fertilizer or manure increase soil nitrate levels and the amount of nitrate available to the plant.
“The application of fertilizer will predispose these plants to accumulate nitrate,” said Yaremcio.
Excess nitrates accumulate in stressed plants after drought, hot winds, hail and frost. Dying weeds sprayed with chemicals also accumulate nitrates.
A frost of – 6 C for five to six hours kills the entire plant. Roots are unable to transport water and nutrients into the plant, and nitrates do not accumulate. The crop is safe for harvest any time.
A light frost is more dangerous; it may take two weeks for nitrogen levels to lower.
“If you have a light frost, get out there and cut it immediately. Don’t wait.”
The highest concentrations occur four to five days after the injury. Plants take 12 to 14 days to recover and the same amount of time is needed for nitrates to return to normal levels.
The highest concentration usually occurs in the bottom six inches of the stem. Nitrates do not accumulate in the grain.
Cut as soon as possible after frost or a hailstorm.
“If you wait 14 days after a frost, there will be a lot of leaf drop and a stemmier product,” Yaremcio said. “The product quality is going to go down.”
Raise the head or cutter bar so four to six inches of stem are left behind.
Research from the United States suggests nitrate levels are reduced if the damaged crop is made into silage. Nitrate content falls during the initial stage of silage fermentation until the pH is below five. The pit needs to be covered and cared for properly.
High nitrate levels in forage can be reduced by making sure green feed is dry when it is put up. The problem can worsen if it is baled at 18 to 20 percent moisture and then heats.
Microbial action that causes heating may convert nitrates in feed to nitrites, which are 10 times more toxic in feed than nitrates.
Forage, silage and water should be tested. Nitrates in the water will add to what is in the forage.
Yaremcio recommended mixing a handful of silage from each load in a plastic bread bag. Send it to the lab at the beginning of the week so it does not have to wait.
Otherwise, the contents will start to ferment in the bag and change the nitrate level.
An animal’s ability to withstand nitrate poisoning depends on body condition score, stage of production, type of feed and amount of time to adapt to the ration.
Nitrates in feed are converted into nitrites in the rumen. Under normal conditions, the ammonia goes to the blood stream and the kidneys, where it is excreted as urea in the urine.
Excess nitrates change the shape of red blood cells, which carry oxygen from the lungs to body tissues and carbon dioxide from the tissues back to the lungs. Death is caused by lack of oxygen.
Accumulation in the body is possible, and symptoms may not be present for the first four or five days of the feeding program.
Conventional wisdom suggests that nitrate levels higher than half a percent are dangerous, but Yaremcio said that may not be the full picture.
This level is based on research conducted in 1964 when nitrates were injected into the blood stream. It may take two to three days in the digestive process, so the effects are much slower.
Feed containing one percent nitrate can be blended with good feed at 25 percent of the ration. Cattle will take four to five days to adapt to a high nitrate feed.
Adding soluble carbohydrates from grain helps reduce the impacts and metabolic conversions.
Plants that accumulate nitrates
Annual forage crops accumulate greater amounts of nitrates than perennials. These crops are usually planted into well fertilized, manured or recently plowed grassland or pasture. Annual crops are also harvested at an early stage of development (milk to dough), when nitrate content is highest. Some plant species can become nitrate accumulators if the right conditions exist. Problem crops include:
- barley greenfeed
- wheat greenfeed
- oat greenfeed
- rye greenfeed
- canola plants
- beet tops
- sugar beet tops
Source: Alberta Agriculture