Risks of Nitrate Poisoning in Pastures

Mark Johns and Barry Yaremcio, Ag – Info Centre, Alberta Agriculture & Rural Development
(Previously published on Alberta.ca – Agriculture and Forestry: February 26, 2018)

This past weekend I had a question from a sheep producer asking why he was loosing several ewes unexpectedly. Further into the conversation, he also mentioned that he figures on losing a dozen ewes during this time (fall) each year. My response to this was “has there been any instances of frost over the course of time that you have been loosing ewes and what types of forages are in your pastures?” Of course without visually seeing these animals and not having any lab work or even a field necropsy performed, it is hard to say what the exact cause of each case may have been. However, as we begin to move into colder temperatures with periods of frost and with producers potentially spreading manure prior to the winter months, it is important consider how these scenarios can affect plant species in your pastures. With this being said, the scenario listed above could have been the result of nitrate poisoning. To learn more about this issue with grazing livestock, check out this Q&A session provided by Mark Johns and Barry Yatemcio.

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 (see list below) 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.

Common crops and weed species problems:

Agricultural crops
Barley greenfeed Beet tops Bull thistle Canada thistle
Wheat greenfeed Flax Fire weed Kochia
Oat greenfeed Sugar beet tops Lambs quarters Mustards
Rye greenfeed Sorghum Nightshade Pigweed
Canola plants Russian thistle Millet
White ragweed Smartweed

Yaremcio, 1991

A selection of common plants know to accumulate nitrates:

Bermudagrass Rescuegrass Cudweed Lambsquarter
Pearl millit Rye Dock Nightshade
Crabgrass Ryegrass Horsenettle Pigweed
Corn Sudangrass Jimsonweed
Johnsongrass Sorghum x Sudan
Tall fescue Wheat

Hancock, 2013

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 10-14 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 [livestock]?
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.

For more information regarding nitrate poisoning and feeding high nitrate feeds to livestock, I encourage you to check out the links listed below!