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Continual Fertilizing, Not Cows, Is the Problem on Sensitive Pastures

By Don Comis
June 8, 2004

Although cattle add some nitrogen to pastures via their feces and urine, it isn't enough to warrant removing them from a pasture, according to an Agricultural Research Service scientist, even if the pasture is above groundwater contaminated by high levels of nitrate-nitrogen.

A study by Lloyd Owens, a soil scientist at the ARS North Appalachian Experimental Watershed Laboratory in Coshocton, Ohio, has shown that it doesn't make any difference in groundwater nitrate levels whether cattle are on the pasture or not.

What does make a difference is fertilization. Pastures with high nitrate levels can't be fertilized for at least a few years, until the levels drop sufficiently. U.S. Environmental Protection Agency guidelines for drinking water stipulate 10 parts per million (ppm) nitrate-nitrogen as the maximum allowable safe level for drinking water.

Owens studied problem pastures with groundwater nitrate-nitrogen levels of 13 to 26 ppm, caused by heavy experimental fertilization for 11 years before the study. He stopped fertilizing for a seven-year study to see if that would bring nitrate levels down to safe levels. For comparison, he let cattle graze on two pastures, and fenced them out and made hay from two other pastures.

In the groundwater underneath three pastures, the nitrate-nitrogen levels dropped below 10 ppm within three years; after five years, the levels below all four pastures fell to 2 to 4 ppm.

Because of soil conditions, some fields are more prone to high nitrate levels. Fertilizing every year can eventually turn them into problem fields. The finding is good news for farmers because they don't have to remove cattle from these problem fields, as long as they stop fertilizing for a while. Letting cattle graze saves the time and labor of baling hay for feed, which is what was done on the two test pastures where cattle couldn't graze.

The withholding of fertilizer caused only a slight decrease in grass growth, so it doesn't seem to be a serious disadvantage to farmers, especially compared to the environmental benefit.

More information about nitrate research can be found in the current issue of Agricultural Research magazine.

ARS is the U.S. Department of Agriculture's chief scientific research agency.