Dairy farm characteristics and managed flows of phosphorus

Nonpoint sources of P pollution have been implicated in the declining water quality of many lakes in the northeastern U.S. Most of the agricultural nonpoint P contribution to surface waters comes from field runoff. Water quality may be improved by better understanding the movement, or flow, of P through a farm so that reasons for buildup of high soil P levels can be identified and remedies explored. In this study, the managed flows of P (P in imported and exported products) were estimated based on 1hour farmer interviews on 45 Vermont farms and 1 New York farm. Farm P inflow/outflow budgets were developed using information from the interviews. It was estimated that an average of 57% of the P brought onto the farms was not exported. Phosphorus imported in feed and minerals averaged 65% of the total P imports, while purchased fertilizer contributed to an average of 35% of the total farm P imports. Phosphorus was often fed in excess of the cow's nutritional requirements recommended by the National Research Council. Soil test P levels on two pairs of farms with similar animal densities and soil types reflected the large differences in the estimated net P accumulation. For all 46 farms, there was a significant relationship between net P accumulation and animal density (r2 = 0.59). Farms grouped by management operation type (confinement, pasture - based [non-organic], and pasture-based [organic]) were different in average farm size, animal density, P imports, net P accumulation, milk production, and predominant crop. Feeding of excess P results from the high P levels recommended by feed salesmen and nutritionists, who typically take into account the available home-grown forages and provide the suggested needs for purchases of concentrates and minerals. In a survey of seven Vermont dairy feed consultants and salespersons, rations were designed to feed cows as much as 50% more P than research has indicated is necessary.