title: Phosphorus and sediment loss in a catchment with winter forage grazing of cropland by dairy cattle.
abstract: The loss of phosphorus and sediment to surface waters can impair their quality. It was hypothesized that the practice of winter grazing dairy cattle on cropland of moderate slope (5-20%) would exacerbate the loss of P and suspended sediment (SS) from land to water. In a small (4.3 ha) catchment two flumes were installed, upstream and downstream of one field (about 2 ha) that had been cropped for 2 yr and grazed in winter (June-July) by dairy cattle. Flow proportional samples were taken and measured for dissolved reactive phosphorus (DRP), particulate phosphorus (PP), total phosphorus (TP), and SS. During the 2002 hydrologic year (March-February) loads of SS increased per hectare downstream (1449 kg ha(-1)) compared to upstream (880 kg ha(-1)). The same increase from upstream (873 kg ha(-1)) to downstream (969 kg ha(-1)) happened in 2003. However, while in 2003 TP increased downstream by 1.64 kg ha(-1) compared to upstream (0.24 kg ha(-1)), in 2002 an increase of only 0.006 kg ha(-1) at the downstream flume occurred compared to upstream (0.98 kg ha(-1)). Investigation of P transport pathways suggested that overland flow contributed <0.1 kg P ha(-1) to stream flow, 10 and 5% of TP load in 2002 and 2003, with the greater load in 2002 reflecting more rainfall in that year. The contribution to stream flow by subsurface flow was estimated at 0.3 kg P ha(-1). Stream bed sediments showed an increase in total P concentration in summer when no flow occurred due to the admission by the farmer of 10 cattle upstream of the cropped paddock in summer 2001-2002 and 20 cattle between the two flumes in 2003 to graze stream banks. This action was calculated to contribute via dung at least, the remaining P lost: about 0.5 kg P in 2002 and 1.0 kg P in 2003. Clearly, not allowing animals to "clear-up" stream banks is a priority if good surface water quality is to be achieved. Furthermore, compared to stock access the impact of winter grazing cropland on P losses was minimal, but SS load was increased by an average of 75%.
authors.affiliation: AgResearch Ltd., Invermay Agricultural Centre, Private Bag 50034 Mosgiel, Otago, New Zealand. richard.mcdowell@agresearch.co.nz
authors.forename: R W
authors.initials: RW
authors.lastname: McDowell
mesh_terms: D000822:Animal Husbandry; D000818:Animals; D001937:Brassica; D002417:Cattle; D018556:Crops, Agricultural; D004926:Escherichia coli / Q000302:isolation & purification; D019015:Geologic Sediments; D009520:New Zealand; D010758:Phosphorus / Q000032:analysis*; D045483:Rivers / Q000737:chemistry / Q000382:microbiology; D012621:Seasons; D014871:Water Microbiology; D014872:Water Movements; D014873:Water Pollutants / Q000032:analysis; D014881:Water Supply
chemical_list: D014873:Water Pollutants; D010758:Phosphorus