RESULTS OF A PROGRAM TO CONTROL PHOSPHORUS DISCHARGES FROM DAIRY OPERATIONS IN SOUTH-CENTRAL FLORIDA, USA

During 1987-1992, a mandatory program to control phosphorus discharges was implemented at dairy operations located to the north of Lake Okee chobee, Florida, USA. Thirty of 48 dairies participated in this progra m and implemented best management practices (BMPs), which included the construction of intensive animal waste management systems. Eighteen d airies closed their milk-producing operations under a government-funde d buyout program. In this paper, we compare trends in runoff total pho sphorus (TP) concentrations among the dairies that remained active and implemented BMPs. A central feature of the dairy waste management sys tem is the high intensity area (HIA), defined as the milking barn and adjacent vegetation-free land, encircled by a drainage ditch and dike. Animal waste from the HIA is diverted into anaerobic lagoons and stor age ponds, from which water is periodically removed and used for irrig ation of field crops. The impacts of BMP construction on runoff TP con centrations were immediate and, in most cases, dramatic. Average TP co ncentrations declined significantly (P < 0.001), from 9.0 to 1.2 mg TP liter(-1) at dairies in one basin (Lower Kissimmee River), and from 2 .6 to 1.0 mg TP liter(-1) in another (Taylor Creek/Nubbin Slough). Som e sites experienced greater declines in TP than others. To elucidate p ossible causes for the difference in response, a multivariate statisti cal model was utilized. Independent variables included soil pH, soil d rainage characteristics, spodic horizon depth, and the areas of differ ent BMP components (pasture, HIA, spray fields). The analysis signific antly separated dairies with the highest and lowest runoff TP concentr ations. Lowest TP occurred at dairies having particular soil character istic (shallow spodic horizon) and certain BMP features (large HIA and small heard pastures).