More efficient nitrogen management in maize production systems is essential, because of the well-documented negative environmental impacts of N on water quality and greenhouse gas emissions. The web-based Adapt-N tool (http://adapt-n.cals.cornell.edu) was designed to improve maize N use efficiency by shrinking the uncertainty around the agronomic optimum N rate. It accounts for the dynamic, complex and locally-specific interactions among weather, soil and management that affect crop-available N in the soil and crop N uptake. Adapt-N uses near real-time high resolution climate data now available for the United State east of the 100^th meridian, and field-specific management, soils, and crop information supplied by users via a user-friendly web-interface, as inputs for a dynamic simulation model (Precision Nitrogen Management model). The model simulates daily soil C and N transformations, soil water storage and transport, and uptake of water and N by the maize crop. The Adapt-N web-interface provides a sidedress N recommendation and graphs visualizing seasonal dynamics over time of soil N, crop growth, temperature and precipitation for a user’s location. In collaboration with consultants, extension staff, and growers in the Northeast (mostly NY) and Midwest (mostly IA), we beta-tested the Adapt-N tool to compare Adapt-N recommended rates with current grower practice in replicated on-farm strip trials in grain and silage maize in 2011 and 2012, with further trials in progress during the 2013 growing season. On average, N application rates were reduced by 74 kg ha^-1 (66 lbs ac^-1) in NY and 36 kg ha^-1 (32 lbs ac^-1) in IA.  Yield losses with reduced N rates were negligible at 53 kg ha^-1 (1 bu ac^-1) on average across all trials, except in a small number of cases where the tool was improperly used, model adjustments were needed, or unpredictable late season factors influenced outcomes. Higher Adapt-N recommendations were justified by higher yields when drought was not the greater limiting factor. On average, Adapt-N use increased grower profits by about $77 ha^-1 ($31 ac^-1) in NY and $49 ha^-1 ($20 ac^-1) in IA, with Adapt-N increasing profits in 80% and 75% of trials, respectively. Adapt-N whole farm implementation on a NY grain farm led to savings of over $30,000 and over 30,000 kg of N fertilizer on 373 hectares of maize. Adapt-N also significantly reduced estimated N leaching and denitrification losses. Two years of on-farm testing thus show that the use of Adapt-N for sidedress N recommendations provides economic advantages to growers, improves N use efficiency, and reduces environmental impacts of N use in maize production.