Stress Increases Nitrate Levels in Forages

EDMONTON - Aug 26/04 - SNS -- This year's growing conditions were more stressful than usual in many parts of the growing area in western Canada, raising some concern nitrate levels could be higher than normal in forages.

This is a problem for livestock feeders because nigh nitrate levels are unhealthy for animals. As a consequence, if becomes an issue for forage producers, because nitrates lower the value of their crops.

The implication of nitrate levels on animal feeding practices is summarized by Alberta Agriculture in the following table:

     Forage Nitrate Levels and Implications
               (percent)

  NO3   NO3-N    KNO3 Remarks

 <0.5   <0.12   <0.81 Generally safe for beef cattle and sheep

 <1.0   <0.23   <1.63 Caution - some subclinical symptoms may appear
                      in pregnant horses, sheep and beef cattle

  1.0    0.23    1.63 High nitrate problems - death losses and
                      abortions can occur in beef cattle and sheep

<1.23   <0.28   <2.00 Maximum safe level for horses. Do not feed
                      high nitrate forages to pregnant mares.

Nitrate is one of the forms 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.

"Other factors that can affect plant nitrate level include large applications of nitrogen fertilizer or manure increase soil nitrate and thus the nitrate available to the plant," says Mark Johns, forage specialist with Alberta Agriculture, Food and Rural Development, Stettler. "Herbicides that disrupt or interfere with normal plant function may also result in 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.

"Annual forage crops tend to accumulate greater amounts of nitrates than perennial forages," adds Johns. "Oats and millet 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 on the crop."

Nitrates accumulate over time in an injured or damaged crop. Typically, the highest accumulations of nitrates will occur two to three days after the injury or stress. It is best to cut or harvest the crop within one 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 reduce nitrate levels.

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," cautions Barry Yaremcio, beef/forage specialist with Alberta Agriculture, Stettler. "In fact, if round bale greenfeed, is baled too moist (18 to 20 per cent moisture) and heats up, 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."

Nitrate levels may be reported in three different ways depending on the analytical procedure used. The results may be reported as nitrate (NO3), nitrate nitrogen (NO3-N) or potassium nitrate (KNO3). Be sure you know which method was used before trying to interpret the results. Refer to the following table.