Nutrient Loss In Snowmelt Runoff From Cattle Winter Bale-Grazing In SW Saskatchewan.

Winter cattle bale-grazing on annual cropland or on pastures has become common practice on the Canadian Prairies, replacing winter corral feeding.  However, because most water movement on the Prairies occurs during snowmelt, this practice has potentially serious environmental impacts.  A three-year study was started in Swift Current Saskatchewan to determine nutrient forms and concentrations in dissolved and particulate runoff samples from winter bale-grazing sites.  In fall 2008, a 10-year old Russian wild rye pasture was split in half, with each replicate containing three microwatersheds (MW); each MW was 0.4 ha with approximate slope of 5%.  In each replicate, the east pastures and MWs were used as controls. In the west pastures, cattle bale-grazing occurred from November to February (1900 cow d ha^-1).  This experiment was repeated in fall 2009, but the grazed MWs were on an adjacent crested wheat grass pasture, and more controls spread with manure in the fall were added.  Runoff samples were collected during snowmelt events in March, and filtered within 24-h of collection.  Particulates were analyzed for total P, C and N.  Filtrate was analyzed for dissolved C, ammonium, nitrate, total N, total P and molybdate-reactive P (MRP).  Samples were also analyzed by ³¹P NMR to characterize dissolved and particulate P forms, along with samples of manure mixed with hay and plant litter from the feeding site.  Concentrations of dissolved C, particulate organic C, total N, dissolved ammonium, total P, dissolved total P and MRP were significantly higher in runoff from bale-grazed sites relative to ungrazed controls.  Dissolved P from bale-grazed sites was almost entirely orthophosphate, while dissolved P form control sites included both organic and inorganic P forms.  There were no significant differences in particulate P concentrations from grazed sites and controls, which contained both inorganic and organic P forms.