Grazing strategies for reducing contaminant losses to water from forage crop fields grazed by cattle during winter

Forage brassica crops are widely used for over-wintering cattle in many parts of New Zealand where relatively cool winters limit pasture growth rates. However, relatively little is known about the potential impacts of this activity on water quality. Here we report findings from a paired catchment study that quantified the concentrations and fluxes of nitrogen (N), phosphorus (P), sediment and the faecal indicator bacteria Escherichia coli (E. coli) in overland flow and subsurface drainage derived from fields where cattle were wintered on swede and kale crops from 2011 to 2013, then returned to pasture and grazed by sheep during 2014. The effectiveness of a strategic grazing approach that aimed to protect Critical Source Areas (CSAs) was also evaluated during 2012 and 2013. Automated sampling and monitoring of flows indicated that short-duration, high-energy overland flow events mostly occurred in the 6 to 8 week period following grazing of the forage crop, presumably as a consequence of the reduced soil infiltration caused by cow treading damage incurred whilst grazing the wet soils. Flow-weighted concentrations of P, sediment and E. coli varied widely between events and between years and were high compared to values reported in the literature for other land uses, and compared to guideline values for the protection of water quality. Estimated annual fluxes of P, sediment and E. coli in this overland flow were correspondingly high, ranging up to 3.8 and 3,740 kg ha-1 for P and sediment, respectively, and up to 4.3 x 1010 MPN ha-1 for E. coli. The implementation of a strategic grazing method whereby cows strip-grazed from the top of the catchment and moved downslope with restricted access to the gullies where flow convergence occurred, considerably reduced overland flow volumes and associated fluxes of sediment and P, to levels observed when the catchments were grazed by sheep in 2014. These findings indicate that simple and low cost techniques that take into account paddock soil type, topography, drainage and stock management can significantly reduce overland flow and contaminant losses from grazed winter forage crop paddocks.