EFFECT OF COW SLURRY N ON HERBAGE PRODUCTIVITY, EFFICIENCY OF N UTILIZATION AND ON WHITE CLOVER CONTENT IN A NATURAL SWARD IN THE BASQUE COUNTRY, SPAIN

The effect of cow slurry N on productivity and white clover content wa s compared with that of mineral fertilization in a natural grassland s ward. Fertilizer was applied at two different N rates as split applica tions of ammonium nitrate, or as cow slurry (145 or 290 kg N ha(-1) in the first year and 120 or 240 kg N ha(-1) in the second year). In the first year, fertilizer N produced higher dry matter (DM) yields than the slurry treatments. Nitrogen efficiency and clover content reductio n were higher for the mineral treatments, the efficiency being lower f or the higher rate of N application. In the second year, N supplied by the slurry gave the same productivity, N efficiency and white clover content reduction as that from the mineral N fertilization; higher eff iciency in the second year being due to favourable weather conditions and low white clover contents. The high N efficiencies of the slurry c ompared with the fertilizer N treatments were attributed to the diluti on of the slurry and to split applications over the year. Yield differ ences between treatments were due mainly to the grass component of the sward. Competition between grasses and other species, and white clove r for mineral N resulted in a reduction in the white clover content in those treatments in which more N was supplied, whether it came from t he mineralization of slurry or from the fertilizer N applied.