Soil physical quality under cattle grazing of a winter-fed brassica crop

This 2-year study investigated the effects of winter brassica forage crop grazing treatments on soil physical properties on a Fragic Pallic soil, susceptible to compaction, in South Otago, New Zealand. Soil physical measurements including bulk density, percentage of pores >300 mu m, macroporosity (air-filled porosity; percentage of pores > 30 mu m), total porosity, air permeability, and saturated hydraulic conductivity (K-sat) were taken 1.5 and 4 months after completion of winter grazing in year 1, and 2.5 months after grazing in year 2. Treatment main effects in year 1 for macroporosity were ungrazed crop (P < 0.05, 15.5%), established pasture (12.2%), crop on-off grazing (12.8%), crop grazed with back fence (9.6%), and current practice (strip grazing without a back-fence, 8.6%). During both winters the soil was generally more compact at 0.05-0.10 m depth than at 0.15-0.20 m, particularly for the current practice treatment, which may increase the risk of overland flow. Micro-topography differences between hump and hoof-hollow areas for the current practice treatment were also investigated during one sampling, with very low values of K-sat at 0-0.05 m in hoof-hollow areas (P < 0.05, 5 mm/h), compared with hump areas ( 129 mm/h). Overall, the crop on-off grazing treatment and, with some exceptions, the grazed with back fence treatment have some merit for reducing damage to soil physical properties compared with current practice.