Crops that have late applications of fertilizers or experience physical damage or drought conditions can contain high levels of nitrates, which can have detrimental effects on feeding and silage quality. In addition, crops with high nitrates can create dangerous gases during ensiling and adversely affect biogas reactors.
Signs of High Nitrates
There are few visible signs of high nitrate levels in the growing or freshly harvested plant.
During ensiling, nitrates can be converted into nitrogen oxide — even in some cases into nitric acid, which can cause a bright yellow color or result in “bleached” looking silage. Nitric oxide converts to nitrogen dioxide (NO2) when it contacts oxygen in the air. Nitrogen dioxide is poisonous and can injure and kill people as well as animals. The lethal gas is yellowish-brown and can smell like laundry bleach. For important information on this silage gas, visit our safety section.
Nitrogen dioxide gas is yellowish-brown and highly toxic
When fed to animals, silage with high nitrate levels can cause signs such as:
- Reduced appetite
- Weight loss
- Runny eyes
Acute toxicity in animals can result in labored breathing, muscle tremors, collapse and even death. Toxicity is related to the total amount of forage consumed and how quickly it is consumed. In general, forages containing more than 6,000 ppm nitrate should be considered toxic.
High nitrate levels also reduce silage quality. Increased levels of nitrates in the growing crop use more plant sugars, which reduces the sugar available to produce lactic acid during ensiling. In addition, high nitrate levels raise the buffering capacity of the forage — requiring more lactic acid to achieve the same pH drop. As a result, forages ensiled with higher levels of nitrates will have a slower fermentation, lower digestibility and increased dry matter (DM) losses.
Risks for High Nitrates
Nearly all plants contain nitrate, but some species are more prone to accumulate nitrate than others. Crops such as:
- Cereal grains like wheat and oats
- Pearl millet
- Under extreme stress, alfalfa
These plant species — combined with stressors like drought, frost, hail or disease — can lead to high levels of nitrates in the crop. Plant roots continually absorb nitrates, but environmental factors can limit the conversion of nitrates into amino acids. This leads to accumulation in the plant.
Applying high amounts of fertilizer late in the season can boost concentrations as well. Split nitrogen applications can provide better nutrient distribution and reduce the risk of toxicity.
Preventing High Nitrate Silage
Nitrates accumulate in the bottom portion of the plant. Therefore, raising the cutter bar to leave about the bottom one-third of the plant in the field can be effective in reducing nitrate levels in the resulting silage.
Proper fertilization, weed, insect and disease control reduces the chances of high nitrate levels in the forage crop.
Determining Excess Nitrate
Plant nitrate levels can be tested in the field using nitrate test strips. The test takes less than a minute. Fresh grass samples can also be sent to a laboratory for more exact nitrate levels. Nitrate levels should be less than 0.1% (1,000 ppm) before cutting to ensure fermentation is not adversely affected.
Check out these resources written by silage experts for more information on high nitrates in silage: