PRECISION CATTLE MANURE MANAGEMENT: IMPACTS ON SURFACE SOIL AND RUN-OFF WATER QUALITY

Nutrient losses from agricultural soils, especially phosphorus (P), contribute to the eutrophication of surface waters. Nutrients accumulated at the soil surface or released from plant residue are susceptible to movement in runoff during snowmelt. The objective of this study was to evaluate the performance of precision variable rate application compared to traditional constant rate applications of cattle manure on nutrient loss and coliform export in surface snowmelt runoff at the Livestock Forage Center of Excellence Research Facility near Clavet, SK. The release of soluble residual nutrients to simulated snowmelt from the surface-active layer (AL) of soils and plant residues in post-harvest barley growth was also evaluated. For the field study, three zones within the field were set up, with three replicate watersheds identified in each zone. The zones were precision (variable-rate with set-backs) manure application zone, traditional (constant rate) manure application zone, and a commercial fertilizer application zone (control) to which only commercial inorganic fertilizer was applied. All treatments were applied in the spring of 2019 after snowmelt and before seeding. A second study assessed each surface component's potential: active layer, residue, and combined sample (residue and active layer) to release nutrients under controlled conditions by simulating a snowmelt event. Samples of plant residue and surface soil active layer (AL) were collected from all three zones of the treated watersheds in the fall of 2019. Upper and mid-slope landscape positions with and without volunteer barley regrowth were sampled. The samples were analyzed for total P and N; the active layer sample was further analyzed for nitrate (NO3--N) and ammonium (NH4+-N). Concentrations of dissolved reactive and unreactive (organic) P, NH4+-N and NO3--N and most probable numbers of coliforms were measured in the snowmelt run-off collected in treatment watershed catchment basins in the field in spring of 2018 and 2019 (background pre-manure application) and spring of 2020 (post-manure application), and in the simulated snowmelt runoff in samples from fall of 2019 (post-manure application). In both studies, the concentrations of soluble P in collected snowmelt water and apparent export in runoff were higher in watersheds with a constant manure application rate compared to precision variable rate and the commercial fertilizer only zones, which were not significantly different. The coliform content of water was not affected by treatment. Residue samples from areas of high volunteer barley growth and traditional constant rate manure application watersheds also released more N and P to simulated snowmelt runoff. Variable-rate application of manure with a set-back from basin centers may be considered a suitable management practice to help reduce concentrations of nutrients in snowmelt run-off water and their export off-site