Why test for nitrates?

Corn that has been stressed all summer due to drought may have few if any ears, and usually will have an energy value 85 to 100 percent of normal corn silage. The worst problems occur when the drought is severe enough to produce dead leaves. Although nitrate levels in drought-stricken corn may be high, 30 to 60 days after ensiling more than half the nitrates are converted to ammonia, which can be utilized by the rumen bacteria. For this reason, nitrate toxicity rarely occurs when feeding ensiled drought corn.

However, if the drought damage was extreme and high levels of nitrogen were applied to the soil, a nitrate test on the silage should be conducted. This test can be done during ensiling to get an idea of how high the nitrate level is. Dividing the results by half puts you in the ballpark of what nitrate levels are. Retesting after the 30-60 day period gives you a better estimate of nitrate levels.

The following precautions will reduce nitrate levels in plants and reduce feeding problems in rations.

1. Do not harvest suspected crops for three to five days after an appreciable rain or long cloudy spell.
2. Harvest as close to usual maturities as possible.
3. Cut the crop somewhat higher above the ground than usual because nitrates often accumulate in stalks.
4. Utilize suspected material for silage rather than greenchop because ensiling breaks down some of the nitrates.
5. Test suspected forages for nitrate content, preferably before feeding them.
6. Feed another forage prior to feeding suspected or high nitrate forage to help limit meal size.
7. Gradually introduce suspected forage into the ration over a period of one to two weeks to allow for adaptation and to reduce risks.
8. Feed forages and total mixed rations more frequently to reduce meal size when suspected or high nitrate forage is used or silo gas is present.
9. Feed at least three to five pounds of concentrate per head daily to cattle to reduce possible toxic effects when suspected forages are fed.
10. Limit dry matter intake per single meal if stored forage contains 1,000 ppm N0₃-N or more on a dry matter basis. Allow a delay of two to three hours after completion of a meal before feeding a high nitrate forage again.
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11. Retest suspected and high nitrate forage periodically due to large variations which often occur in forage.
12. Test all forage and water for nitrates if one stored forage contains over 1,000 ppm N0₃-N.
13. Consider blending high nitrate silage or haylage with those containing lower amounts before feeding to provide less than 1,000 ppm N0₃-N in the blend dry matter and thus enable free-choice feeding.
14. Restrict the N0₃-N content of the total ration dry matter, including contribution from water to not over 400 ppm to 900 ppm when using stored forage in addition to meeting any maximum forage dry matter intake for a single meal. Rate of nitrate intake is the most critical factor influencing possible toxicity. This is affected by rate of forage dry matter intake in a given period of time and its nitrate content. Forages containing under 1,000 ppm nitrate nitrogen (N0₃-N) on a dry matter basis may be fed free-choice or with no restriction on meal size, provided the total level of N0₃-N in the total ration dry matter, including that from water, is kept at a safe or low-risk level. Stored forages containing higher levels generally require limiting meal size to avoid elevated methemoglobin levels in the blood and other toxic effects.
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Guideline for Corn Silage

Because of the high nutrient and mineral variability in drought stressed corn, be sure to run frequent forage tests on these feeds.