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 wasteas long as they reduce or eliminate nitrogen
fertilizer use for at least a few years.
That particular wisdom comes from a study at the ARS
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 8-acre fields on a
hillside. Owens sampled nitrate levels in the groundwater below each
field. U.S. 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 fertilization150 pounds per acre each
yearto 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 fertilization can eventually turn them into
problem fields. So Owens began a study to see whether he could lower
nitrate levels by eliminating fertilizer for 7 years and either grazing
cattle or harvesting hay from the fields.
He compared two pastures where cattle were allowed to graze with two
pastures that were fenced to keep the animals out. In the "no cattle"
pastures, the grass was cut and baled for hay twice a year.
"When you harvest the hay, you remove some nitrogen from the soil,"
Owens says. "And when cattle graze, they remove some of the nitrogen."
Owens found that the nitrate-nitrogen in groundwater was brought down
to about the same level (2 to 4 ppm) under both management practices,
and the lack of fertilizer caused only a slight decrease in grass growth.
But, most importantly, it didn't make any difference whether cattle
were on the land or not.
"It's a nice finding, because it doesn't force farmers to remove
cattle from problem fields, as long as they stop fertilizing. And it
saves the time and labor of baling hay for feed," Owens says.By
Agricultural Research Service Information Staff.
This research is part of Water Quality and Management, an ARS National
Program (#201) described on the World Wide Web at www.nps.ars.usda.gov.
Lloyd B. Owens
is with the USDA-ARS North Appalachian
Experimental Watershed Laboratory, State Rte. 621, Coshocton, OH
43812; phone (740) 545-6349, fax (740) 545-5125.
"High Nitrate in Groundwater? Stop FertilizingBut Let
the Cows Graze!" was published in the June
2004 issue of Agricultural Research magazine.