Corn silage with high levels of nitrates can be toxic to animals and humans. Nitrates oxidize iron atoms in hemoglobin (in red blood cells), making it unable to carry oxygen.
Along with the potential for increased nitrates comes an increased risk of silo gas, which can be fatal. Nitrogen dioxide (NO2) is a dangerous chemical asphyxiant produced almost immediately after plant material is put into a silo. “Even short-term human exposure can result in severely injured lung tissue and sudden death,” said Joel Bagg, forage specialist with the Ontario Ministry of Agriculture and Food.
Nitrates: good and bad
Nitrates are a part of most farmers’ fertilizer plans, being an essential nutrient especially important to getting high yields.
An extended period of dry weather during tasselling and pollination will hinder corn cob formation and grain yield by reducing the plant’s ability to metabolize nitrates and use them for growth.
“If the situation worsens, leaves may turn brown and plants can appear to be dying,” said Bagg. “As this happens, some farmers attempt to salvage this corn by using it as forage for livestock, as corn silage, green chop, or pasture.”
It is critical to be aware that, under certain conditions, this corn can be high in nitrates (NO3), which can lead to death in the livestock consuming it.
“The five to seven days following a rainfall after a severe dry period would have the highest risk of excess nitrates, so avoid harvesting or grazing during this period,” heeded Bagg. “This period following the rain is much higher risk than the dry period itself.”
Although corn is the most likely forage crop to cause nitrate poisoning, high nitrates can also crop up in cereals.
Nitrate poisoning symptoms include rapid or laboured breathing, fast and weak heartbeat, muscle tremors, staggering, and death. Less severe symptoms may include listlessness and other, more subtle symptoms. Chronic cases can produce symptoms, such as poor appetite, reproductive problems (including abortion) and poor performance.
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There are ways to reduce nitrates in corn, so that it can be used and not wasted. For one, fermentation reduces nitrates by 25 to 65 per cent during proper silage fermentation. Bagg recommended allowing at least three to five weeks of fermentation before feeding. Corn silage harvested either too wet or too dry will not ferment as well, which can result in nitrates levels remaining higher than normal.
Secondly, the bottom third of the corn stalk has a much higher level of nitrates. “If high nitrates are a concern, the cutter bar could be raised to leave more of the stalk in the field,” suggested Bagg.
“To maximize yield and manage nitrate risks, a good strategy is to harvest at normal cutting heights, store as silage, analyze fermented silage samples for nitrates, and then manage dietary levels through feeding management.”
Bagg recommended caution when it comes to grazing or green chopping. “These can be stressed by dry weather, which may be an option for some producers facing feed shortages.”
Bagg said, “It’s difficult to predict nitrate levels. The risk of nitrate poisoning while green chopping or grazing this corn is significantly higher during that five to seven day period after a rainfall than it is during the actual period of dry weather.”
Bagg recommended avoiding grazing or green chopping during this period, as nitrate levels can fluctuate daily within the plant, making it hard to assess risk.
“Green chopped corn that is not fed immediately undergoes respiration that converts nitrate to nitrite, so the risk is increased,” said Bagg.
Field sampling and lab analysis for nitrates can be helpful, but it is hard to get a representative sample as nitrate levels fluctuate.
“In a pasture situation, this high nitrate risk corn is likely their only source of feed,” said Bagg. “Turning hungry cattle to a field of stunted, cobless corn following a rain that ends a dry period is very high risk.”
The best time to test for nitrate concentration is after the fermentation is complete.
As a general rule, NO3-N levels should be less than 1,000 parms per million (NO3 levels less than 0.44 per cent) to be without risk. Levels greater than 4,000 ppm NO3-N (more than 1.76 per cent NO3) are potentially toxic and should not be fed.
Grains and concentrates are typically low in nitrates. Adequate non-structural carbohydrates (NSC) in rumen assist the conversion of nitrate to ammonia, reducing poisoning potential.
According to Morgan Cott, agronomist with the Manitoba Corn Growers, silage acres have remained fairly steady in the province and do not fluctuate a great deal. “They reflect our beef and dairy herds, which don’t tend to fluctuate,” said Cott.
“Nitrate toxicity has been well-known for a long time and since most producers use silage piles, the risk of toxicity is low. The biggest risk would probably be for new producers, less educated in silaging and/with beef/dairy production.”
In 2012, Manitoba had 273,000 acres of grain corn and 75,000 acres of silage corn. In 2013, the acreage increased to about 342,000 acres of grain corn and 86,000 acres of silage corn. †