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Does Grazing Contribute To Groundwater Contamination?

Managed rotational grazing on deep silt-loam soils does not appear to contribute to groundwater contamination, say researchers from the U.S. Dairy Forage Research Center.
“However, farmers should be careful on sandy soils or on shallow soils over fractured limestone, sand, or gravel,” said Michael Russelle, soil scientist with the USDA Agricultural Research Service.

Russelle and Rao Kanneganti, an agronomist at the center on the University of Wisconsin-Madison campus, studied nitrogen cycling and managed rotational grazing on natural cool-season grass and legume pastures in Prairie du Sac, Wis., from 1994 to 1996.
The researchers buried 12 plastic tubes, called lysimeters, randomly in each paddock. The lysimeters captured moisture and dissolved nutrients seeping through the soil, including nitrate.
Nitrate leaching losses averaged only 1 to 3 pounds per acre. However, directly under urine spots the amount of nitrate in the lysimeters increased to 20 pounds per acre.

The total amount of nitrate leached from the pastures corresponds to 3 parts per million in water. The standard for nitrate in drinking water is 10 parts per million. “Since nitrate concentrations are not additive, this amount does on contribute to groundwater contamination,” said Russelle. In contrast, water seeping directly from urine spots contained 58 parts per million nitrate.
“We don”t have estimates for nitrate leaching near the water tank or in the shade,” said Russelle. “But from our results, these high-traffic areas would be expected to leach nitrate at significant levels and possibly contribute to groundwater contamination,” he said.

The total amount of nitrate leached from the pastures corresponds to 3 parts per million in water. The standard for nitrate in drinking water is 10 parts per million. “Since nitrate concentrations are not additive, this amount does on contribute to groundwater contamination,” said Russelle. In contrast, water seeping directly from urine spots contained 58 parts per million nitrate.
The pastures were fertilized with 30 pounds of nitrogen in mid-June and in mid- to late July. Stocking density averaged 1.5 cows per acre during the grazing season. Cows were not overwintered on the pasture.

“We would probably see more leaching if we increased nitrogen fertilization, or increased stocking density,” said Kanneganti. According to the researchers, overwintering cows on pasture may also increase nitrogen leaching. Pasture plants are dormant in the winter and do not use nitrogen deposited by the herd, allowing more nitrate to seep through the soil or run off during snowmelt.
Since nitrogen output in urine is sensitive to dietary changes, researchers also varied the diet of the herd. Cows were fed no supplementation, supplementation at 33 percent of the diet, and supplementation at 66 percent of the diet.

“Supplementing energy, or grain in the diet, helps microorganisms in the cow”s rumen convert nitrogen into protein for milk production, leaving less nitrogen to be excreted in urine,” said Russelle.

The researchers expected to find the least nitrate leaching in pastures where cows were fed 33 percent supplement. Pastures where cows were fed too much grain, 66 percent of their diet, and no grain were expected to leach higher amounts of nitrate from urine. The researchers did not find any differences in nitrate leaching between pastures. However, milk production and nitrogen secretion in milk increased with supplementation.

“Nearly all of the work on nitrate leaching and diet supplementation has been done in marine climates in Europe, New Zealand and Australia,” said Russelle. Most Wisconsin pasture plants have much deeper root systems than the ryegrass and clover grown in those climates. “This may be the reason we did not measure differences between pastures,” he said.

Russelle advises farmers to move water tanks, feed supplemental energy, allow enough time for vegetative regrowth so plants can absorb nitrogen, and not to overwinter in sensitive areas. “Problems can arise when there is mismanagement in any system, and grazing is no exception,” he said.

The pasture research is part of an ongoing study. Kanneganti is also using the data to test pasture analysis components of DAFOSYM, the dairy farm analysis computer program developed by the USDA Agricultural Research Service. “The program will allow farmers to analyze their farm, simulating different environmental conditions and management techniques such as stocking density, feed supplementation, and grass management,” said Kanneganti.