Extended periods of dry weather without rain can severely stress corn plants. When this occurs during tasselling and pollination, cob formation and grain yield can be significantly reduced. As the situation worsens, leaves turn brown and the plants can appear dry and “dead”. Some farmers attempt to salvage this corn by using it as forage for livestock, either as corn silage, green chop or pasture. Under certain environmental conditions, this corn can be high in nitrates (NO3) which can result in the fatal poisoning of livestock, and also the death of humans from silo gas. This is particularly high risk during the 5 – 7 days following a rain that ends a severe dry period.
Environmental Risks Causing High Nitrates
Nitrates accumulate in the corn plants when there is a large amount of readily available soil nitrates (heavy nitrogen or manure applications, legume plowdowns) and environmental factors interfere with the plants ability to metabolize it for plant growth. It is difficult to predict with certainty when high nitrates will occur. Nitrate accumulation is often greatest following a rain that ends an extended severe dry period. Following rainfall, the normal conversion of nitrates to plant protein resumes in a few days as the plant metabolism “catches up” to available soil nitrates, and plant nitrate levels return to more normal levels. The 5 to 7 days following a rainfall that ends a severe dry period would have the highest risk of excess nitrates, so avoid harvesting or grazing during this period. This period following the rain is much higher risk than the dry period itself.
Excessive nitrogen fertilization can be a contributing factor. Other weather events causing potentially high nitrates include hail, frost, and cloudy weather, although these are much less likely. Although corn is the most likely forage crop to cause nitrate poisoning, high nitrates can also occur in sorghums, sudangrass and cereals. Weeds such as lambsquarters, pigweed and others can also accumulate nitrates. Nitrate incidents with legumes are rare, but have been reported.
In the rumen, nitrates (NO3) are converted to nitrites (NO2), which are converted to ammonia and then amino acids. High levels of nitrites impare the ability of blood hemoglobin to carry oxygen. Symptoms of nitrate toxicity include a rapid breathing, fast and weak heartbeat, difficult breathing, muscle tremors, staggering and death. A blue mucous membrane may develop. The blood has a chocolate colour. Animals typically go down and die suddenly, within 3 hours of the initial symptoms. If you suspect nitrate poisoning, keep the animals quiet and comfortable and call your veterinarian immediately. Less affected animals may be listless and show more subtle symptoms. Sub-acute or chronic cases can cause poor appetite, reproductive problems (including abortion) and poor performance.
Fermentation Reduces Nitrates
Nitrates can be reduced 25 – 65 percent during a proper silage fermentation. Typical reductions are in the 40 percent range. Allow at least 3 – 5 weeks of fermentation before feeding. Corn silage that is harvested too wet or too dry will not ferment as well, resulting in the nitrates levels remaining higher than normal
The bottom third of the stalk contains 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. However, this will also further reduce yields in a year when the feed is badly needed. To maximize yield and manage nitrate risks, a good strategy would be to harvest at normal cutting heights, store as silage, analyze fermented silage samples for nitrates and then manage dietary levels through feeding management.
Graze or Green Chop with Caution
Green chopping or grazing corn stressed by dry weather can be an option for some producers facing feed shortages, but considerable caution should be used. It is difficult to predict nitrate levels. The risk of nitrate poisoning while green chopping or grazing this corn is significantly higher during the 5 – 7 day period after a rainfall than during the actual period of dry weather. Avoid grazing or green chopping during this period. Nitrate levels can fluctuate daily within the plant, so it is difficult to assess this dynamic risk. Concentrations are higher in the bottom of the plant compared to those found near the top. Green chopped corn that is not fed immediately undergoes respiration that converts nitrate to nitrite, so the risk is increased. Field sampling and laboratory analysis for nitrates can be useful, but keep in mind that it is difficult to get a representative sample and nitrate levels in these plants fluctuate. In a pasture situation, this high nitrate risk corn is likely their only source of feed. Turning hungry cattle into a field of stunted, cobless corn following a rain that ends a dry period is very high risk.
Nitrate Analysis and Feeding Management
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. Contact your feed laboratory for shipment protocols. Preventing nitrate loss in the sample is important, so freezing the sample and shipping overnight while still frozen is usually recommended. If high levels are reported, water and other feeds should also be tested to determine total nitrate intake. Water can contain significant amounts of nitrate.
There is some confusion over how laboratory nitrate levels are reported and used, so be sure you are interpreting the right method. The nitrate (NO3) level is 4.4 times the nitrate-nitrogen (NO3–N) level. Concentrations can also be reported as “ppm” or as a percentage.
As a general rule, NO3–N levels should be less than 1,000 ppm (NO3 levels <0.44%) to be without risk. Levels greater than 4,000 ppm NO3–N (>1.76 % NO3) are potentially toxic and should not be fed. Feeding forages with nitrate content between these levels is associated with risks relative to the amount fed and the type of livestock. The University of Wisconsin suggests total ration nitrate-nitrogen intake (including water) should be less than 1 gram per 45 kg (100 lbs) of body weight.
Dilution of high nitrate corn silage and careful management is required when formulating rations. Young, nursing and pregnant animals are higher risk. Healthy animals are at lower risk. Animals can adapt over time to higher nitrate levels, so introduce higher nitrate feeds slowly. Grains and concentrates are typically low in nitrates. Adequate non-structural carbohydrates (NSC) in the rumen assist the conversion of nitrate to ammonia, which reduces the potential for poisoning. Dividing daily intake into smaller feedings also reduces the risk.
The increased nitrate potential increases the risk of silo gas. Silo gases can be deadly! 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. Avoid storing high risk nitrate silage in tower silos. Delay harvest to lower nitrate risk, or use horizontal silos or bag silos to store this corn silage.
Use the silo gas precautions and procedures outlined in “Hazardous Gases” (OMAFRA Factsheet #04-087) at www.omafra.gov.on.ca/english/engineer/facts/04-087.pdf. Refer to the Ontario Farm Safety Association at http://www.farmsafety.ca/public/factsheets/silo_gas_dangers.pdf.