Yield response of long-term mixed grassland swards and nutrient cycling under different nutrient sources and management regimes

The response of a long-term, mixed-species hayfield in Maine, USA, to commercial fertilizers and liquid dairy manure was evaluated over a 6-year period, including the effects on yield, nutrient concentration and cycling, forage species composition and soil nutrient levels. Nutrient treatments included an unamended control, N fertilizer, NPK fertilizer and liquid dairy manure (LDM). The application rates of plant-available N, P, and K were constant across treatments. Application of nutrients in any form increased forage yield, generally by 2-4 t dry matter (DM) ha(-1) year(-1). Yield from NPK fertilizer was 0.05-0.25 higher than from LDM, due to differences in N availability. Yield responses to P and K were minimal and there appeared to be no difference between P and K in fertilizer and manure. The forage sward became increasingly dominated by grass species as the experiment progressed; application of P and K in fertilizer or LDM allowed Agropyron repens to increase at the expense of Poa pratensis. Forage nutrient removal accounted for all applied N and K, and nearly all applied P, throughout the study period, demonstrating the important role these forages can play in whole-farm nutrient management.