Management options to decrease phosphorus and sediment losses from irrigated cropland grazed by cattle and sheep R. W. Mc D owell & D. J. H oulbrooke AgResearch Ltd, Invermay Agricultural Centre, Private Bag 50034, Mosgiel, New Zealand Abstract In southern New Zealand, grazing of forage crops is common practice to satisfy feed requirements of animals in winter when pasture growth is limited. This practice has been shown to cause soil physical damage and increased loss of surface water contaminants sediment and phosphorus (P) to water bodies. Strategies to mitigate the loss of sediment and P were trialled on a Pallic soil type (Aeric Frag- iaquept) in the North Otago Rolling Downlands of New Zealand. All sites were irrigated and meas- urements were made of losses in overland and sub-surface flow from intensive cattle or sheep grazed, winter forage crops, and sheep grazed pasture. Two mitigations (restricted grazing of crop to three hours and the application of aluminium sulphate) were assessed for their potential to decrease contam- inant loss from cropland. Volumes of surface runoff and loss of total P, filterable reactive P and sediment showed significant differences (P < 0.05) between the control treatments (i.e. no mitigation) with cattle crop (88 mm surface runoff) > sheep crop (67 mm) > sheep pasture (33 mm). The contrib- ution of irrigation water to overland flow water, as a result of saturation-excess conditions, varied between treatments with more loss under cattle crop (20% of total) compared with sheep crop (15%) and sheep pasture (11%). These differences are probably an effect of soil physical condition and highlight the importance of accurate irrigation scheduling to keep soil moisture below field capacity. Restricted winter grazing and alum application after grazing significantly (P < 0.05) decreased P losses in surface runoff under cattle (from 1.4 to 0.9 kg P ⁄ ha) and sheep (from 1.0 to 0.7 kg ⁄ P ⁄ ha) grazed crop plots by about 30%. In cattle grazed plots, restricted grazing also decreased suspended sediments (SS) by 60%. The use of restricted grazing is suggested as a means of decreasing P and SS loss from grazed winter forage crops. The use of alum shows some promise for decreasing P losses, but requires further work to determine its long-term effectiveness and use in other soils and manage- ment regimes. Keywords: Phosphorus, runoff, treading, particulate phosphorus, grazing, pasture, cropland Introduction The loss of phosphorus (P) and suspended sediment (SS) from land to surface water is associated with poor water quality (Miller et al., 2004). Losses via surface runoff (hereaf- ter referred to as runoff) are enhanced by soil treading dam- age from grazing animals that decreases soil infiltration rate, porosity, plant growth, and increases soil bulk density (Sin- gleton & Addison, 1999; Drewry et al., 2000; Bilotta et al., 2007). While these changes may increase the potential for runoff and with it P and SS loss, they may also decrease the quantity of drainage and with it P loss. In cooler areas of New Zealand, pasture growth is slow during winter. A common solution for the provision of out- door feed supply is for animals to graze a field of forage crops such as Brassicas for 2–3 weeks. Normally, a farm will put approximately 10% of its effective area into winter for- age crops that are grazed field by field until pasture growth on the remaining 90% of the farm area improves in spring. Depending on conditions such as soil moisture, vegetation cover, topography, stocking density and animal type this can lead to much P and SS loss (McDowell, 2006a). Previous work at a site in southern New Zealand showed that grazing of forage cropped plots in winter, and associated treading Correspondence: R. W. McDowell. E-mail: richard.mcdowell@ agresearch.co.nz Received February 2009; accepted after revision May 2009 Soil Use and Management, September 2009, 25, 224–233 doi: 10.1111/j.1475-2743.2009.00231.x 224 ª 2009 The Authors. Journal compilation ª 2009 British Society of Soil Science Soil Use and Management