The effects of winter forage crop grazing of hillslopes on soil erosion in South Otago

Abstract

Agricultural development has lead to deforestation, intensification and increased erosion worldwide. In New Zealand, increasing cow numbers has led to greater demand for forage crops to feed stock off-farm in winter. Expansion of dairying on flat land has pushed wintering systems on to rolling to steep land, particularly in the Southland and South Otago regions. While the impacts of forage crop grazing on soil compaction and overland flow of sediment and nutrients has been studied, there has been no previous work done on the direct influence of this farming practice on soil transport. This study used a novel technique to quantify the volume of soil transported downslope beneath the hooves of cows that were grazing kale over the 2015 winter period. Steel ball bearings were buried in the soil prior to grazing, and the distance they had moved was determined after winter, and used to infer soil transport. A linear relationship was found between soil transport flux and slope gradient of up to 0.25 m m-1, with stock track formation on steeper slopes causing greater spatial variability of soil transport rates and non systematic dependence of soil transport hillslope gradient; further research is required to describe this relationship. The steep slope of the relationship for gradients <0.25 indicates that rapid downslope transport occurs relative to gradient under forage crop grazing. This soil transport results in erosion on convex sites, at rates that exceed soil production rates, leading to unsustainable soil loss in these areas. Soil transport under conventional cultivation was also determined in this study, using the same methodology. No linear relationship was found between transport rates and gradient. However, despite the lack of relationship, downslope soil transport rates under cultivation exceeded those under cow grazing, indicating that significant soil transport results from this practice. The combination of soil transport under grazing and cultivation allows the impact of the forage crop grazing system as a whole to be understood.