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Extension Responds: Drought
Nitrates in Forages
By Keith Kelling, Extension Soil Scientist, UW-Madison/Extension
August 26, 2003
Once again we are confronted with the problem of high nitrate levels in drought-affected forage. The droughts of 1970 and 1976 did indeed result in very high levels of nitrate in many instances.
High levels of nitrates in the diet of animals, including man, can lead to methemo-globinemia, a disease in which some of the blood hemoglobin is converted to methemoglobin. In the latter compound, the oxygen is bound so firmly that the methemoglobin cannot function as an oxygen carrier, as does hemoglobin.
Nitrates in Plants
Nitrates taken up by plants are reduced to organic compounds in the leaves. The first step in the process involves the enzyme nitrate reductase, just as in the rumen. High temperatures and drought combine to reduce the activity of the enzyme; consequently, nitrate accumulates in the plant. Other factors favoring nitrate accumulation are excessive soil N, low temperature, nutrient deficiencies other than N, shading, cloudiness, frost and hail damage, and certain hormonal herbicides. The problem appears to be particularly acute when the drought (or frost) is severe enough to produce dead leaves. In that case, there is insufficient nitrate reductase produced to reduce the nitrates to organic N. Nitrates tend to be particularly high the first few days after a drought-breaking rain. Forage should not be fed green-chopped at this time. If the forage survives the drought, the nitrate level should drop to normal after about 2 weeks.
General Information on Nitrate in Feedstuffs
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Nitrate toxicity is greatest when the total ration lacks energy, minerals, and vitamins.
- Usual dry matter intake of dry cows and heifers is 2 pounds per 100 pounds of body weight and 3 to 4 pounds per 100 pounds weight for milking cows. Use the intake and guidelines in the following chart to determine the amount of this feed that can be safely fed.
- Grains and legume forages are low in nitrate. The stalks and leaves of corn, sorghums, small grains, and sudan grass grown on soils high in nitrogen are apt to be high in nitrate. Use low-nitrate feeds to dilute the nitrate in high-nitrate feeds. High nitrate in water adds to the total nitrate intake. Access to runoff water from barnyards or well-fertilized fields should be avoided, especially if feed is high in nitrate.
- Nitrate is highest in the bottom of the stalk and is less in the top of the stalk and in the leaves. Chopping the total plant decreases the chance of animals eating only high-nitrate stalks. Pasturing is hazardous when animals might eat only lower stalks.
- Cutting high to leave the bottom of the stalk in the field lowers nitrate.
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Ensiling the crop decreases the nitrate. Some nitrate is lost as silo gas. This gas is hazardous to people and animals; do not enter the silo without thorough ventilation and ventilate the silo chute to the outside. In 30 to 60 days after ensiling, the nitrate level is decreased to half of the original amount.
Guidelines to Use of Feeds with Known Nitrate Content
The total amount of nitrate consumed rather than the concentration in the tissue is the critical consideration. Professionals from the Department of Dairy Science give the following guidelines:
Nitrate-N content Guidelines for feeding (dry matter basis)
Below 1000 ppm Safe
1000 to 2000 ppm Limit this feed to half of total ration
2000 to 3000 ppm Limit this feed to one-third of total ration
3000 to 4000 ppm Limit this feed to one-fourth of total ration
Over 4000 ppm Special caution needed; ensile to reduce nitrate content
Testing for Nitrates
If fresh-chopped forage or silage is suspected of being high in nitrates, it should be tested. Samples can be analyzed by some commercial forage testing laboratories as well as by the Soil & Forage Analysis Lab (8396 Yellowstone Dr., Marshfield, WI, 54449) and the Soil & Plant Analysis Lab (5711 Mineral Point Rd., Madison, WI, 53705).
Sampling for Nitrate Analysis
Silage
• Take samples during the unloading process. If indoors, make sure the silo room is well ventilated.
• Take at least 5 separate samples while unloading.
• Mix well and remove about 0.5 lb for testing.
Green-chopped Forage
• Collect several handfuls from different loads or different parts of a load.
• Mix well and remove about 0.5 lb for testing.
Standing Corn or Sorghum
• Cut at least 15 whole plants taken at random.
• Cut plants at same height as field chopper.
• Chop plants into one-half inch lengths and mix well.
• Remove about 0.5 lb for testing.
Packaging and Mailing Samples
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