High Nitrate Potential in Corn Silage

Crop conditions have turned from dry to wet in many areas and our corn silage harvest is continuing. This has raised some concerns about the potential for nitrates in corn silage. Nitrates are a possibility under these conditions and are often hard to predict when then will appear.

Frequently, nitrates are most severe on drought stunted, heavily manured fields for 3–4 days following a drought ending rain event. We will likely have some fields that fall in that category. Higher nitrate silage can also pose a higher risk for the development of silo gas.

To reduce nitrate levels in drought-stressed plants, harvest crops in the afternoon on a warm sunny day; be sure to wait 3 to 5 days after an appreciable rain or long cloudy spell. Since nitrates accumulate in the stalks, consider a higher cut height. If high nitrate levels are suspected, use forage as silage rather than green-chop. Ensiling reduces nitrates by 50 to 60 percent. Ideally, allow the forage to ferment for 3 to 4 weeks before feeding to allow the fermentation process to complete.

Any suspect feed should be tested for nitrate levels before feeding. The silage can also be tested at harvest to determine if nitrates are a cause for concern. Because nitrate levels decline during ensiling, regular forage tests for formulating rations, taken as the silage is fed, should also include a nitrate analysis. Often as agronomists we get questions about this problem but rarely do we get any feedback because once the crop is harvested then it becomes a feed issue. Testing silage at harvest for nitrate is a way you can get some feedback on how the crop nitrate level responds to the farmer’s management and the specific conditions of the growing season.

The most critical factor influencing possible toxicity is rate of nitrogen intake, which is affected by forage dry matter intake over a given time period. Feeding practices that regulate dry matter intake can be used to manage high nitrate forages. When stored forages contain more than 1,000 ppm NO3-N, intakes generally must be managed to avoid elevated methemoglobin levels in the blood and other toxic effects. See table on back side of this page for guidelines on feeding forages with high nitrate levels to dairy cattle.

More details on feeding forages with high nitrates can be found in our on-line publication From Harvest to Feed: Understanding Silage Management.

Guidelines for feeding forages with high nitrate levels to dairy cattle.¹
Nitrate Nitrogen (NO3-N)
(ppm dry matter basis)
Source: Adams, et al. 1992. Prevention and Control of Nitrate Toxicity in Cattle. Penn State Extension Fact Sheet.
¹Taken from: From Harvest to Feed: Understanding Silage Management, by C. M. Jones, A.J. Heinrichs, G.W. Roth, and V.A. Ishler, Departments of Animal Science and Plant Science, Penn State University.
< 1,000 Safe to feed under most conditions
1,000-1,700 Gradually introduce to ration. Feed some concentrate. Test all feeds and water. Dilute to 900 ppm NO3-N in total ration dry matter. Restrict single meal size.
1,700-2,300 Possible acute toxicity. Feed in a balanced ration with concentrate included. Dilute to 900 ppm NO3-N in total ration dry matter. Restrict single meal size.

Prepared by Greg Roth, Grain Production Specialist and Doug Beegle, Soil Fertility Specialist