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. (C) 2012 Elsevier B.V. All rights reserved.