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Fertilizing, not cows, is the problem on pastures
Although
cattle add some nitrogen to pastures via their feces and urine, it isn’t enough
to warrant removing them from a pasture, 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. The
EPA guidelines for drinking water stipulate 10 parts per million (ppm) nitrate-nitrogen
as the maximum allowable safe level.
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 to 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 caneventually
turn them in to 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 awhile. Letting cattle graze saves the time and labor of bailing
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.
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