Farmers who find excessive levels of nitrates in groundwater can keep on grazing their cattle without fear of further contamination from the nitrogen in cattle waste—as long as they reduce or eliminate nitrogen fertiliser use for at least a few years.
This particular wisdom comes from a study at the Agriculture Research Service North Appalachian Experimental Watershed Laboratory in Coshocton, Ohio. For several years, Lloyd Owens, a soil scientist at Coshocton, used a herd of 30 beef cows to rotationally graze four 3.2 ha fields on a hillside. Owens sampled nitrate levels in the groundwater below each field. US Environmental Protection Agency guidelines for human drinking water stipulate 10 parts per million (ppm) as the maximum allowable safe level for nitrate-nitrogen. There is no danger to cattle from grazing on pastures with high nitrate levels.
Scientists have gathered data on the watersheds in this rolling countryside for more than 60 years. The fields had been used as pasture for about 30 of those years. During the 11 years before the latest study began, Owens tested heavy nitrogen fertilisation - 170 kg per ha each year - to see whether it would produce more and better grass for the cattle to graze without doing environmental harm. Unfortunately, it caused too much nitrogen to leach into the groundwater under these experimental pastures. Levels reached 13 to 26 ppm.
Some fields are more likely to have high nitrate levels in the groundwater beneath them, and annual fertilisation can eventually turn them into problem fields. So Owens began a study to see whether he could lower nitrate levels by eliminating fertiliser for seven years and either grazing cattle or harvesting hay from the fields.
“It’s a nice finding, because it doesn’t force farmers to remove cattle from problem fields, as long as they stop fertilising. And it saves the time and labour of baling hay for feed,” Owens says.
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