Nitrogen losses from grass ley after slurry application – surface broadcasting vs. injection

As the livestock numbers on Finnish dairy farms have increased and most fields on dairy farms are under grass, it has become common to spread cattle slurry over grasslands. To estimate environmental effects of recurrent slurry applications, a 5-year field study was performed to compare nitrogen (N) losses to water and ammonia losses to air by volatilization, when cattle slurry was either surface broadcast or injected into clay soil after grass cuttings. Slurry was spread on the grass in summer (1996-1997) or both in summer and autumn (1998-2000). Biomass N uptake before grass harvesting and amount of soil mineral N in spring and autumn were measured and field N balances were calculated. Despite cool weather, up to one third of the ammonium N of broadcast slurries was lost through ammonia volatilization after application in autumn, but injection effectively prevented losses. The mean surface runoff losses of total N were negligible (0.3-4.6 kg ha-1 yr-1) with the highest loss of 13 kg ha-1 yr-1 measured after slurry broadcasting to wet soil in autumn and followed with heavy rains. A substantial part (24-55%) of the applied mineral N was not recovered by the foregoing measurements