Most farmers and extension and UDSA-NRCS staff may know the appropriate date for seeding cover crops, but this is based on achieving 30% cover prior to winter for control of soil erosion. In the Deerfield area of Massachusetts in the Connecticut River Valley the seeding date has been taken to be September 15, although some earlier extension bulletins have a much later date. These seeding dates for cover crops have been determined for controlling water and wind erosion and were mainly based on plant canopy cover, not on root development ¬or their ability to accumulate nutrients. Our research indicated the seeding date for maximum N accumulation was two to three weeks earlier than the date for adequate soil erosion control. Based on these results many cover crops planted by farmers are not effective in taking up end-of-season N. Studies showed more than 100 kg N/ha can be accumulated in the fall after corn harvest on fields where manure is fall-applied. If nitrate-N is not taken up by a cover crop in the fall it will be lost to leaching over winter. This is especially problematic on dairy farms with fall-applied manure being both an environmental issue, and a real economic cost. Approximately 1100 GDDs are needed for adequate growth of winter rye for optimum N accumulation.