What are nitrates?

Nitrates are naturally occurring in many plants, but can be found in higher concentrations in corn and some weeds such as lambsquarters and pigweed. Nitrates accumulate in the corn plants when there is a large amount of soil nitrates and a lack of moisture that interferes with normal plant growth. Nitrate accumulation is often greatest following a rain that ends a long dry period.

What levels are considered dangerous?

Nitrate concentrations as measured by parts per million (ppm) should be less than 1,000 ppm in order to be fed to beef cattle without risk. Levels over 4,000 ppm should not be fed. Forage with nitrates in between these two levels is associated with risks relative to the amount fed and type of livestock. Careful management is required.

How can I find out if my corn is okay?

Nitrate levels can fluctuate daily within the plant, with concentrations higher in the bottom of the plant compared to those found near the top. Corn plants typically contain significantly higher levels of nitrate immediately after a rainfall event that follows a prolonged period of dry weather. The risk of nitrate poisoning while green chopping or grazing this corn is much higher for the 5 - 7 day period after a rainfall than during the actual period of dry weather.

Testing at harvest will provide a general idea of the relative nitrate levels, but not the concentration of nitrates in the silage being fed. The best time to test for nitrate concentration is after fermentation is complete. Obtain a representative sample. Keep the samples refrigerated and send to the lab as quickly as possible. If high levels are reported, water and other feeds should also be tested.

What is silo gas?

The increased nitrate potential increases the risk of silo gas. Nitrogen dioxide, NO2, is a dangerous chemical asphyxiant and is produced almost immediately after plant material is placed into a silo. Even short-term human exposure can result in severely injured lung tissue and sudden death. It has a characteristic bleach-like odour and may be visible as a reddish-brown haze. It is heavier than air, therefore it will tend to be located just above the silage surface. It may also flow down silo chutes and into feed rooms. Use the silo gas precautions and procedures outlined in OMAFRA Factsheet Hazardous Gases on Agricultural Operations, Order No. 14-017. Refer to the Ontario Farm Safety Association Factsheet Silo Gas Dangers.

For more information on nitrate poisoning and other adverse weather related topics, consult OMAFRA's adverse weather page at or call the Agricultural Information Contact Centre at 1-877-424-1300.