Forage crops can accumulate toxic amounts of nitrate (NO3).
By Elin Kittelmann,
Fallon/Carter County Extension Agent
Forage crops can accumulate toxic amounts of nitrate (NO3). High nitrate has been reported in cereal grains (oats, rye, wheat, barley, triticale, etc.), bromegrass, orchardgrass, fescue, sorghum, sudangrass, millet, corn, sweetclover and alfalfa. Several weeds, such as kochia, lambs- quarter, pigweed, quackgrass and Russian thistle, can also have high nitrate levels, especially when growing under adverse conditions.
Nitrogen from the soil is taken up by plant roots in the form of nitrate. Plants convert nitrate (NO3) to nitrite (NO2) which in turn is converted to ammonia and then to amino acids, the building blocks of protein. Nitrate levels cycle during a 24-hour period; during the night, nitrate accumulates when photosynthesis is inactive, then during the day, nitrate is quickly converted to protein. Under normal growth conditions, there is little nitrate buildup, even though plant roots are absorbing large amounts of nitrate, because protein conversion keeps pace with root absorption. However, in certain conditions, this balance can be disrupted so that the roots will accumulate nitrate faster than the plant can convert the nitrate to protein. Abnormal growing conditions such as drought, frost, unseasonable or prolonged cool temperatures, hail, shade, disease, high levels of soil nitrogen, soil mineral deficiencies or herbicide damage can cause high nitrate accumulation in forages.
All plants contain nitrate, but nitrate levels toxic to livestock are mostly associated with forages (hay, fodder, silage, pasture or weeds). Crops grown under “stress” conditions or on soils that have received high applications of manure or nitrogen fertilizer are suspect. Avoid cutting when nitrate concentrations are at peak levels. For example, toxic levels of nitrate can accumulate in forages immediately after a drought-ending rain or irrigation. Since peak nitrate levels occur in the morning, delay haying or grazing until the afternoon of a sunny day. Nitrate toxicity is most likely to occur when livestock are pastured or fed green-chop, followed by hay. Silage is the least hazardous feed. Ensiling forage usually lowers the nitrate level 10 to 60 percent. The nitrate level in hay will usually remain constant or decline slightly in storage.
Producers should never assume their forage levels are safe if they know a crop was exposed to any adverse growing conditions that increase nitrate accumulation. Higher nitrate levels are usually present in immature plants (vegetative to the boot stage in small grains) and decrease as plants mature (milk to dough stage in small grains). Nitrate concentration is highest in the stems, especially in the lower third of the stem, and at the nodes. An intermediate level usually exists in leaves, and very little is found in the grain.
Take precautionary measures if you suspect high nitrate concentrations prior to harvesting or feeding forage. Even under ideal conditions, nitrate accumulation is unpredictable. Nitrate concentration can vary in areas of a single field, haystack or silo. Therefore, nitrate testing is advised in many situations. The Fallon/Carter County Extension Office offer a free nitrate test for producers. Samples maybe dropped off at the Extension Office or arranged by contacting Elin at 406-852-3693. Prior to harvest, sample standing crops such as oats or barley. Collect 20 stems randomly by traversing in a zigzag pattern across an entire field. Clip the plants at ground level and bring to the Extension Office. If nitrate is detected using the qualitative test and growing conditions are normal, delay harvest for several days, which will usually reduce nitrate levels rapidly. Periodic testing may be necessary to assure that the nitrate level has declined. It is recommended to test grain forages again before feeding. A free quantitative test is available at the Fallon/Carter Extension office or we can help send your sample to a lab for future analysis.
If you have any questions contact the Fallon/Carter County Extension Office at 406-778-7110.