Plant Species and Agronomic Factors
The majority of nitrate poisoning cases in North Dakota occur with drought-stressed corn and small grains. However, a number of other plants also can accumulate nitrate, including sudangrass, sorghum-sudan hybrids and millet. Table 1 lists common plants known to accumulate nitrate if conditions are favorable.
Plants that have been fertilized typically have higher nitrate levels than nonfertilized plants. The abnormal accumulation of nitrate in plants is influenced by various factors, such as moisture conditions, soil conditions and type of plant.
Plant stressors, such as drought, are associated with increased levels of nitrate in plants. Soils high in nitrogen readily supply nitrate to plants. Additionally, factors such as acidic soils, sulfur or phosphorus deficiencies, low molybdenum and low temperatures are known to increase nitrate uptake by plants.
Table 1. Common plants known to accumulate nitrate
Crops |
Weeds |
Barley |
Canada thistle |
Corn |
Dock |
Flax |
Jimsonweed |
Millet |
Johnson grass |
Oats |
Kochia |
Radishes |
Lambsquarters |
Rye |
Pigweed |
Soybeans |
Russian thistle |
Sorghum |
Smartweed |
Sudangrass |
Wild sunflower |
Sugar beets |
|
Sweetclover |
|
Turnips |
|
Wheat |
|
Plant Parts
Plant parts closest to the ground (stalks) contain the highest concentrations of nitrates. Leaves contain less than stalks or stems, and the seed (grain) and flower usually contain little or no nitrate. Most of the plant nitrate is in the bottom third of the stalk.
Research from Oklahoma has shown that the lower 6 inches of the stem in pearl millet contains three times more nitrate than the top part of the plant. Raising the cutter bar above 6 inches can reduce nitrate content of forages harvested as hay or silage, although this may be difficult to do with drought-stressed forages.
Stage of Plant Growth
Nitrate decreases as plants mature. Young plants have higher nitrate concentrations than mature plants. However, mature plants still can have excessive nitrate concentrations if environmental and soil conditions are favorable.
Weather Conditions That Favor Nitrate Accumulation by Plants
Not all drought conditions cause high nitrate levels in plants. Some moisture must be present in the soil for the plant to absorb and accumulate nitrate. If the major supply of nitrates for the plant is in the dry surface soil, very little nitrate will be absorbed by the roots. In plants that survive drought conditions, nitrates are often high for several days following the first rain (as the plant regrows following drought).
Frost, hail and low temperatures all interfere with normal plant growth and can cause nitrates to accumulate in the plant. Frost and hail may damage, reduce or destroy the leaf area of the plant. A decrease in leaf area limits the photosynthetic activity of the plant, so nitrates absorbed by the roots are not converted to plant proteins but are accumulated in the stem or stalk instead.
Most plants require temperatures above 55 F for active growth and photosynthesis. Nitrates can be absorbed quickly by plants when temperatures are low, but conversion to amino acids and protein occurs very slowly in plants during periods of cool weather. This allows nitrate to accumulate in the plant.