b'5The importance of timing of scheduled ploughing and reseeding operations for fine sediment generation from hydrologically - isolated grassland fields in the South West UKPulley S1 and Collins AL11Sustainable Agriculture Sciences, Rothamsted Research, North Wyke, Okehampton, EX20 2SB UKGrassland represents the largest crop by area in the UK and, under intensive farming, is a major potential diffuse source of fine sediment. Scheduled ploughing and reseeding operations are used to increase pasture yields and quality in lowland landscapes that are dominated by intensive ruminant grazing. The timing of ploughing and harrowing operations strongly affects erosion risk, as soils are of greatest risk of erosion when a seedbed has been prepared but crop cover has not yet developed. Additionally, factors such as rainfall intensity and soil moisture have been shown to affect erosion response to ploughing. Recent work has determined the magnitude of the erosion response of ten grassland field-scale catchments during ploughing and reseeding operations and the factors controlling that response. Sediment loads originating fromhydrologically-isolated grassland fields on the North Wyke Farm Platform were compared after12 reseeding events to determine the effect of rainfall, soil moisture, soil type and fieldtopography.All fields experienced an increase in suspended sediment concentration for a given flow rateafter ploughing; however, the magnitude of the increase was highly variable. Post-ploughsediment yields varied from 0.20 to 4.73 t. ha yr-1 and rainfall from 3061121 mm. Thepost-plough periods account for a very high proportion of the total sediment fluxes despite only covering an average of 10.9% of the 5.5 years of monitoring. When rainfall and/or soil moisture were low during summer ploughing operations, as little as 15% of the total sediment flux occurred after ploughing. However, when scheduled ploughing was undertaken in October in two fields, an average of 56% of the total 5.5 year sediment flux took place in the seven month post plough period. When compared to soil moisture and rainfall, catchment characteristics such as slope or soil type show negligible impact on sediment loads. Overall, the findings underscore the high risk associated with undertaking scheduled ploughing regardless of soil moisture status.49'