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Phosphorus Loading per Acre vs. Cow Populations in a Dairy Watershed in Northeast Wisconsin

Published by the American Society of Agricultural and Biological Engineers, St. Joseph, Michigan

Citation:  Pp. 566-572 in Total Maximum Daily Load (TMDL) Environmental Regulations: Proceedings of the March 11-13, 2002 Conference, (Fort Worth, Texas, USA)  701P0102.(doi:10.13031/2013.7612)
Authors:   Kevin A. Erb
Keywords:   Phosphorus, Dairy Manure, Mass Balance

The Lower Fox River (Lake Michigan basin) in Wisconsin is well known for point source (PCB) as well as non-point source (TSS, phosphorus) pollutants entering the Bay of Green Bay. This basin is also one of the fastest growing regions in Wisconsin for not only expanding dairy farms, but urban development as well. The natural conflict has raised serious concern among stakeholder groups about the long-term impacts of these changes on surface water quality.

A mass balance of 13 dairy farms, ranging in herd size from 50 to 500 head and 4 cash grain operations was conducted in the Lower Fox River Basin over a two year period to determine the per-hectare rate of nitrogen, phosphorus and potassium loading on farms in the basin. Incoming sources of these nutrients were quantified from feed, fertilizer and livestock purchases, as well as natural sources of nitrogen to examine if loading was higher for larger farms. Economic nutrient export was calculated, as well as an estimate of environmental phosphorus losses.

The mass balance showed an average of 98 kg/ha nitrogen accumulation, 17 kg/ha phosphorus accumulation and 90 kg/ha potassium accumulation on dairy farms. Cash grain accumulation rates were 10, 3, and 26 kg/ha, respectively. The mass balance showed differences in cow populations did not change per hectare phosphorus accumulations. Potassium accumulations increased as herd size and acreage increased.

Eleven of the 13 dairy farms had already implemented nitrogen based nutrient management plans. The study indicates phosphorus accumulations could be reduced by more than 90% by implementing a number of additional management practices, including switching to lower phosphorus protein supplements, growing rather than purchasing protein sources, reducing the amount of phosphorus in the dairy ration and reallocating manure across the farm to fields with the greatest phosphorus need.

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