Source: Manitoba Agriculture, Food and Rural Initiatives
Toxic levels of nitrates in forages are always a possibility whenever the normal growth of plants is disrupted by hail, drought, spray drift or frost. The percentage of animals affected by acute nitrate poisoning in Manitoba is usually low. However, when death losses do occur, they occur suddenly and can be devastating. High levels of nitrates need not be a problem as long as the feeding program is managed correctly.
What are Nitrates?
When growing conditions are favorable, plants take up nitrogen largely in the form of nitrate. The nitrate is rapidly converted to ammonia which is incorporated into plant protein.
Unfavorable growing conditions can interfere with nitrate use and cause it to accumulate in the plant.
Under normal conditions, cattle convert the nitrates in forage to nitrite. This is then converted to ammonia and used by rumen microbes to make protein. The problem arises when nitrate converts to nitrite faster than nitrite converts to ammonia.
When this occurs, nitrite accumulates and is absorbed into the bloodstream where it binds to hemoglobin thus reducing the oxygen carrying capacity of the blood. Animals can die by suffocation.
Factors That Influence Nitrate Accumulation
Nitrates accumulate in some plants more than in others. Cereal crops and weeds are more likely to accumulate nitrates than legumes and grasses. While it has been known to happen, a very small percentage of legume and grass hays will contain appreciable amounts of nitrates.
Level of Fertilization
High levels of soil nitrates as a result of nitrogen fertilization, manure application, legume plowdown or summerfallow can result in high levels of nitrates in forage. For example, redroot pigweed from a hog run was found to contain well over 3% nitrates, seven times that contained by the same weed harvested in an adjoining field.
Stage of Forage Development
Early cut forages (boot to milk stages) commonly contain higher nitrate levels than the same forage when almost mature (dough stage).
Light Intensity and Temperature
Poor light intensity (shade) and high temperatures lead to high nitrate levels in plants. Nitrates accumulate during the night and dissipate rapidly on bright sunny days with moderate temperatures.
Frost Damaged Crops
Nitrate levels will be highest just before sunrise. If an early frost comes, it usually occurs just before sunrise, when the nitrate content of the plant is at its peak. Any surviving leaves will likely be shaded by the frost damaged leaves above so the potential for nitrate reduction is limited.
Hail Damaged Crops
Immediately after a hailstorm, the nitrate content can be expected to be quite low because hailstorms usually occur after a hot, bright, sunny day. However, nitrates can then accumulate overnight and would remain high because the loss of leaves decreases the potential for nitrate conversion to protein. The degree of leaf loss determines the extent to which nitrate levels will decrease.
If conditions improve and the plant starts actively growing, some of the accumulated nitrates may be used up in a few days. This usually occurs only in the top leaves which are exposed to sunlight. The bottom leaves, which are shaded, may still contain high levels of nitrates.
Will Nitrates Decrease After Harvesting?
If hay in the windrow remains damp for several days or is rained on, nitrate levels will decrease. Hay dried quickly will lose very little nitrate. Nitrate concentration in hay bales does not change appreciably over time.
Ensiling will result in a 40-60% reduction in nitrate level.
How to Test Your Feed for Nitrates
Nitrates in forages can be detected only by chemical analysis. If you suspect a problem take a representative sample of the feed to your local agricultural representative for a spot test. This will determine if nitrates are present. Feed testing laboratories can determine the actual level of nitrate.
Feeding Forages High in Nitrates
Animals should not be allowed to consume feed containing more than 0.5% nitrate (0.8% potassium nitrate). High nitrate forages should be physically mixed with low nitrate forages so
the overall nitrate level is less than 0.5%. Putting out one bale of high nitrate feed along with one or more bales of low nitrate feed is not acceptable. Feeding adequate levels of energy and vitamin A reduces the risk of toxicity. Change to the questionable feed over a period of 1-2 weeks and do not continuously move cattle on and off high nitrate feeds.
Symptoms of Poisoning
Symptoms of lethal nitrate poisoning include labored breathing, frothing at the mouth, rapid pulse, weakness, diarrhea, frequent urination, incoordination and convulsions. Death may occur in three to four hours. Post-mortem examination reveals dark, chocolate-colored blood. Methylene blue is the treatment for nitrate poisoning.
Sublethal doses may result in loss of appetite, lowered milk production, slow growth and abortions.