The effects of treading by dairy cows on soil properties and herbage production for three white clover-based grazing systems on a clay loam soil P. Phelan*†, B. Keogh†, I. A. Casey†, M. Necpalova*† and J. Humphreys* *Animal and Grassland Research and Innovation Centre, Teagasc, Moorepark, Fermoy, Co. Cork, Ireland, †Department of Chemical and Life Sciences, Waterford Institute of Technology, Waterford, Ireland Abstract White clover can reduce fertilizer-N requirements, improve sward nutritive value and increase environmen- tal sustainability of grazed grasslands. Results of previous experiments in glasshouse conditions and on mown plots have suggested that white clover may be more suscepti- ble than perennial ryegrass to treading damage on wet soils. However, this phenomenon has not been investi- gated under actual grazing conditions. This experiment examined the effects of treading on clover content, herb- age production and soil properties within three clover- based grazing systems on a wet soil in Ireland for 1 year. Treading resulted in soil compaction, as evidenced by increased soil bulk density (P < 0001) and reductions in the proportion of large (air-filled) soil pores (P < 0001). Treading reduced annual herbage production of both grass and white clover by similar amounts 059 and 045 t ha 1 respectively (P < 0001). Treading reduced the sward clover content in June (P < 001) but had no effect on annual clover content, clover stolon mass or clover content at the end of the experiment. Therefore, there was little evidence that white clover is more susceptible to treading damage than perennial ryegrass under grazing conditions on wet soils. Keywords: treading, white clover, perennial ryegrass, soil, grazing, dairy cows Introduction White clover (Trifolium repens) is an important legume for grazed grassland (Frame et al., 1998; Abberton and Marshall, 2005; Peyraud et al., 2009). Clover plays a key role in facilitating biological N fixation (BNF) and can thereby reduce the requirements of fertilizer N to support sward herbage production and forage nutritive value (Rochon et al., 2004). Therefore, white clover is essential for organic grazing systems (Baars, 2002) but is also increasingly important for conventional (non- organic) systems because of the increasing price of fer- tilizer N, which has tripled in the last decade (World Bank, 1990–2010). Grazing systems based on mixed white clover and perennial ryegrass swards (hereafter grass-clover swards) can result in higher farm-level profitability than perennial ryegrass-only swards (Doyle and Bevan, 1996; Andrews et al., 2007; Fal- coner et al., 2011; Humphreys et al., 2012). Replacing fertilizer N with a greater use of clover also has impor- tant environmental benefits, such as increased biodi- versity (Power and Stout, 2011), reduced nitrate leaching (Andrews et al., 2007) and reduced green- house gas emissions (Li et al., 2011). However, the results of previous experiments conducted on small mown plots (Menneer et al., 2005b) or in pots in glasshouses (Grant et al., 1991) have suggested that white clover is more susceptible to treading damage than perennial ryegrass. This greater susceptibility may compromise clover’s suitability on farms that have poorly drained soils. This, however, has not been investigated under actual grazing conditions. Treading (trampling) damage to plants and soils occurs under the hooves of grazing animals and can reduce subsequent herbage production from grassland (Drewry et al., 2008; McDowell, 2008). This reduction is a result of the dislodgement, damage and burial of plants, and the compaction or consolidation of soil particles (Greenwood and McKenzie, 2001). Soil mois- ture reduces soil resistance to deformation and increases susceptibility to treading (Mapfumo and Chanasyk, 1998). However, intensive food production from grazed grassland is often situated in regions that have high rainfall (Smit et al., 2008). Ireland, in Correspondence to: P. Phelan, Animal and Grassland Research and Innovation Centre, Teagasc, Moorepark, Co. Cork, Ireland. E-mail: paul.phelan@teagasc.ie Received 23 January 2012; Revised 12 July 2012 doi: 10.1111/gfs.12014 © 2012 John Wiley & Sons Ltd. Grass and Forage Science, 68, 548–563 548 Grass and Forage Science The Journal of the British Grassland Society The Official Journal of the European Grassland Federation