Nitrogen (N) and phosphorus (P) loss from agriculture persists as a water quality issue. For dairy farms, nutrients can be lost from cropland, pastures, barnyards, and outdoor cattle lots. We monitored nutrient runoff for 3.5years from plots representing cattle lots of differing stocking densities and corn silage cropland. About 510% of annual precipitation became runoff for cattle lot and corn silage treatment plots. Sediment loss was low for cattle plots, with loss proportional to stocking density, and was greatest for corn silage plots. Runoff NO-N was consistent over time, with o80% of samples less than 5mgLp#. Runoff NH-N and particulate N were also consistent through time, but high concentrations occurred soon after manure application. Sediment P loss was related to sediment loss, while dissolved P loss was more influenced by manure. Soil P and runoff dissolved P increased in cattle plots over time in proportion to stocking density. There were no similar P increases in corn silage plots because P inputs and outputs were well balanced. High dissolved P concentrations occurred soon after manure application, but decreased again as a function of cumulative precipitation. Cattle lots can be significant sources of P in runoff, but may constitute only about 3% of total annual P loss from a dairy farm where they represent 15% of total farm area.
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