Optimum Seeding Dates for Fall Cover Crops Based on Site Specific Growing Degree Days.

Planting cover crops after corn silage harvest could have a critical role in the recovery of residual nitrogen (N) and N from fall applied manure, which would otherwise be lost to the environment. 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. 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 for their ability to accumulate nutrients. Our research (2004 to 2013) indicated the seeding date for maximum N accumulation was two weeks earlier than the date for adequate soil erosion control. Based on these results and our research many cover crops planted by farmers are not timely and thus are not effective in taking up end-of-season N and losing N to leaching over winter. Optimal planting decisions depend on accumulated fall Growing Degree Days (GDD) which varies with geographic location. Our research has shown more than 100 kg N/ha can be accumulated in the fall after corn harvest on fields where manure is fall-applied and that approximately 1100 GDDs are needed for adequate growth of winter rye for optimum N accumulation. The GDD analysis is based on daily time series information from meteorological stations within and around the specific region. The GDD time series for each station are spline interpolated and accumulated to develop geographically continuous information and used in the development of a Zonal model. Local variation in GDD resulting from changes in elevation is assessed using a Local model. A Hybrid model uses both regional and local predictions in developing planting recommendations.