Caution! Silo gas and nitrate poisoning

Rain events across the province have interrupted a dry spring. While the water is welcome, the 5-7 days after a rain that ends a severe dry period increases the risk of nitrates in forage crops! Nitrates are of concern because they can increase silo gas production and cause nitrate poisoning in livestock.

Silo Gas

Silo gas (nitrogen dioxide, N2O) is produced almost immediately after filling a silo. It has a bleach-like odour and may be visible as a reddish-brown haze. However, it is not always visible. Nitrogen dioxide is heavier than air, therefore it tends to be located just above the silage surface and on the ground around the silo. It may flow down silo chutes and into feed rooms. Tower silos are at greater risk because the silo gas is contained at the silage surface level, and operators often enter the silo after filling to level silage and set up the unloader. It is difficult to predict when silo gas will be produced, so always take precautions following harvest.

People exposed to silo gas are at risk of severe respiratory distress, permanent damage to lungs, and even sudden death. When inhaled, nitrogen dioxide mixes with body moisture to form nitric acid which causes severe burning of the lungs and the rest of the respiratory system. Pulmonary edema results. Victims often collapse. Other people attempting a rescue can also be overcome. People exposed to silo gas should get immediate medical attention.

Silo Safety (Workplace Safety and Prevention Services)

Hazardous Gases on Agricultural Operations (OMAFRA factsheet)


Nitrate Poisoning

Nitrates (NO3-N) in forage are converted to nitrites (NO2-N) in the rumen. Normally, the nitrites are quickly converted to ammonia (NH3-N) by rumen bacteria and is absorbed into the blood stream to be excreted with urine. When there are high levels of nitrates in the feed, the rumen microbes cannot keep up with nitrite production. The nitrites form methemoglobin in the blood, which reduces oxygen-carrying capacity. Signs of acute nitrate poisoning in animals include staggering, vomiting, laboured breathing, blue-grey mucous membranes, and death (typically within three hours). Chronic nitrate poisoning often appears as reduced weight gain, early-stage abortions, and premature births.

Testing the forage is the only way to know whether the level of nitrates may pose a problem. Most laboratories that conduct feed and forage analysis offer a nitrates test. Be sure the sample is representative of the feed, and it should be frozen to keep the nitrate levels from changing between the farm and the lab. The test results may report nitrates in a few different ways: as nitrate (NO3) or as nitrate-nitrogen (NO3-N). These measurements may be expressed as a percentage or in parts per million (ppm).

Table 2. Guidelines for forage nitrate levels on a dry matter basis in livestock rations




Potassium Nitrate


% ppm % ppm % ppm
Generally safe 0.15 <1,500 <0.04 <350 <0.81 <8,100
Generally safe for non-pregnant livestock. Limit to 50% of total ration for bred animals. 0.15-0.5 1,500-5,000 0.04-0.11 350-1130
Limit to 25%-50% of ration for non-pregnant livestock. Do not feed to pregnant animals. 0.5-1.0 5,000-10,000 0.11-0.23 1130-2260 0.81-1.63 8,100-16,300
Do not feed >1.0 >10,000 >0.23 >2260 >1.63 >16,300
Sources: Glunk et al 2015; Saskatchewan Ministry of Agriculture, Natural Resources and Industry

To reduce the risk of acute nitrate poisoning, feed animals several meals a day, rather than one large one. Livestock fed once a day tend to eat a very large meal when the feed arrives; if the ration is high in nitrates, there is a large spike in their methemoglobin levels about eight hours later. Feeding twice a day results in ruminants eating smaller meals, and a smaller methemoglobin spike four hours after each meal.

Another option to manage nitrates is dilution. Blending high-nitrate forages with low-nitrate feedstuff may bring the amount of nitrate in the ration down to safe levels. Talk to a nutritionist about diluting nitrate concentrations in forages.