With the increase in dairy herd sizes across Wisconsin, the question of whether larger farms accumulate more phosphorus than smaller farms is being asked. To address this question and others, 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 (Outagamie and Brown Counties) over a two year period to determine the per-acre rate of nitrogen, phosphorus and potassium loading on farms in the watershed. Incoming sources of these nutrients were quantified from feed, fertilizer and livestock purchases, as well as natural sources of nitrogen(rainfall, legume credits). Economic nutrient export was calculated, as well as an estimate of environmental phosphorus losses.

The mass balance showed an average of 87 lb/ac nitrogen accumulation, 15 lb/ac phosphorus accumulation and 80 lb/ac potassium accumulation on dairy farms. Cash grain accumulation rates were –9, 3, and 23 lb/ac, respectively. Differences in cow populations did not change per acre 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.