Improving nitrogen management on grassland on commercial pilot dairy farms in the Netherlands

Nitrogen (N) use efficiency (NUE), the ratio of N output and N input, is rather low on dairy farms with high stocking densities and high N input on grassland resulting in high N losses to the environment. This study describes and analyses the development and variation in N management on grassland on 16 commercial pilot dairy farms in the project ‘Cows & Opportunities’ (C&O) over a 12-year period (1998–2009, with the aim that applying this knowledge to other farmers may provide insight in the (im)possibilities to improve management. Farm milk production ranged from 11 to 23 Mg ha-1 and grassland occupied ca. 80% of the total land area (between 63 and 97%). Mean N application rate (kg total N ha-1 year-1) on grassland (in manure, chemical fertilizer, excreta during grazing, biological N fixation and atmospheric deposition) on the pilot farms decreased from 540 in 1998 to 450 in 2001, while in the remainder of the period the inter-annual variation was low (between 400 and 450). Mean dry matter yields on grassland (11 Mg ha-1) varied among years and farms (between 7.7 and 16 Mg ha-1), without any significant temporal trend. We observed no trend of diminishing returns of dry matter yields at farm scale up to an N application rate on grassland of ca. 600 kg ha-1 because farms with a high production intensity (Mg milk ha-1) need more dry matter than farms with a lower intensity and were able to increase nitrogen management on grassland with high N input levels. Management options that result in improved NUE include reduced grazing time which results in increased dry matter yields and NUE as a consequence of better utilization of organic manure.