Nitrate Risk in Forage Crops - Frequently Asked Questions

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 How does nitrate get into the forage?
Nitrate is the form of nitrogen that plant roots take up from the soil. It is transported to the leaves where it is eventually converted into protein. When plants are stressed or injured this process is interrupted and excess nitrates accumulate. Drought, hot dry winds, hail or frost can result in high nitrate levels. Even cool, cloudy weather can cause the problem.

What other factors will affect plant nitrate level?
Large applications of nitrogen fertilizer or manure increase soil nitrate and thus the nitrate available to the plant. Herbicides that disrupt or interfere with normal plant function may also result in nitrate accumulation.

Is there a stage of plant growth that is more prone to nitrate accumulation?
Immature plants will usually have higher nitrate levels. In cereal forage crops, nitrate levels can start to decline from the milk stage onward. However, never assume that a crop will be safe. Oats can still have relatively high nitrate levels even at the milk stage. Always test to be sure.

Are some plants more prone to accumulate nitrates?
Annual forage crops tend to accumulate greater amounts of nitrates than perennial forages. Oats and millet and can be particularly troublesome. Several weedy species will also accumulate nitrate if appropriate conditions exist. Never assume a particular crop will be safe. If there has been a stress and soil nitrate is expected to be high, have a nitrate test conducted by a lab.

When is the best time to cut injured or damaged crops?
Nitrates accumulate over time in an injured or damaged crop. Typically, the highest accumulations will occur 2-3 days after the injury or stress. It is best to cut or harvest the crop within 1 day of the damage. Nitrate levels will gradually decline 10 to 14 days after the injury as the plant resumes growth and repairs itself. Plants killed by the injury or stress will not be able to decrease their nitrate levels.

Does baling or ensiling reduce nitrate levels in feeds?
Ensiling may reduce nitrate concentrations under some conditions. However, this cannot be relied upon to always ensure lower nitrate levels. Crops ensiled with a high soluble sugar content (e.g. cereal grains) have a rapid fermentation process. This rapid fermentation does not promote degradation of nitrate during the ensiling. Checking silage nitrate levels when the pit is being filled usually provides an accurate indication of what the nitrate level will be later on.

Curing and baling will not reduce nitrate levels. In fact, if round bale greenfeed, is baled too moist (18-20% moisture) and heats the problem can become worse. The nitrate present in the feed may be converted to nitrites by the microbial action that causes heating. Nitrites in a feed are ten times more toxic than nitrates.

What levels of nitrate are safe to feed to cattle?
Nitrate levels may be reported in three different ways depending on the analytical procedure used. The results may be reported as nitrate (N03), nitrate nitrogen (N03-N) or potassium nitrate (KN03). Be sure you know which method was used before trying to interpret the results. Refer to the following table.

% NO3
% KNO3
Generally safe for beef cattle and sheep
0.5 - 1.0
0.12 - 0.23
0.81 - 1.63
Caution - some subclinical symptoms may appear in pregnant horses, sheep and beef cattle
High nitrate problems - death losses and abortions can occur in beef cattle and sheep
Maximum safe level for horses. Do not feed high nitrate forages to pregnant mares.
*The values quoted above are on a dry (moisture free) basis.

Adapted from Agdex 400/60-1

Nitrates at
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Nitrate Toxicity of Montana Forages

Prepared by Mark Johns and Barry Yaremcio, Ag - Info Centre, Alberta Agriculture & Rural Development
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For more information about the content of this document, contact Barry Yaremcio.
This document is maintained by Brenda McLellan.
This information published to the web on August 4, 2004.
Last Reviewed/Revised on February 26, 2018.