Impact of hydrology and effluent quality on the management of woodchip pads for overwintering cattle. II. Effluent analysis and nutrient balance

Woodchip pads can be a sustainable alternative to the overwintering of stock on grassland or in conventional housing and can offer benefits in improved animal performance, improved health and environmentally sustainable options for the management of the effluent resulting from the animal excreta (dung, urine and rainfall over the pad). Detailed observations were made on effluent flow and quality from woodchip pads on two commercial farms in the UK, one in Powys (Wales) and the other in Leicestershire (England), over a period of 8 months in 2009/10. Flow data and hydrological characteristics, reported in the companion to the current paper (Jackson & Smith 2012), were combined with the results of effluent sample and soiled woodchip analyses, together with records of animal numbers and activity on the pads, to calculate nutrient fluxes and nutrient balances across the pad for defined periods. Nutrient balances showed that, of the estimated nutrient inputs in animal excreta deposited on the pad, only 0·05–0·10 of the N and P were contained in the effluent draining from the pad, with the rest (>0·90 of N and P inputs) retained in the solids accumulating in the surface layers of soiled woodchip, ‘spent timber residues’ (STR). The STR was similar in analysis to straw-based farmyard manure (FYM), high in organic N, and land spreading of this material should be managed in a similar way to FYM. It also appears suitable for application to grassland, except when based on coarse woodchips. These results confirm the hypothesis that the effluent draining from the pads should be considered as consistently similar to dirty water rather than slurry, as in the current rules associated with Nitrate Vulnerable Zones (NVZs) in England and Wales.