The effects of hillslope forage crop grazing in winter on soil erosion

Farming intensification has led to a greater use of forage crops to feed cows over winter. This farming practice is now commonly taking place on rolling to steep land in areas where stock are wintered off-farm, such as Southland and South Otago. While the impacts of forage crop grazing on soil physical properties such as compaction, and overland flow of nutrients, sediment and pathogens have been studied, there is a lack of understanding of how this practice influences soil transport. This study used a novel technique to quantify the rate soil is pushed downslope by the hooves of cows while grazing kale, and found a linear relationship between soil transport and slope for slope gradients of less than 0.25. The steep slope of the linear relationship, which defines the soil transport coefficient, indicates rapid downslope soil movement relative to slope gradient. As a consequence, soil erosion on convex sites occurs at rates that likely exceed soil production rates. The formation of stock tracks on slopes greater than 0.25 caused greater uncertainty in soil transport rates, and more research is required to determine the effect of track formation on soil movement. Keywords: winter cropping, brassica, forage crops, erosion, soil creep