Nitrogen content curves for small grain forage crops

The export of N as a pollutant from dairies or other animal facilities can be reduced by land application programs that synchronize plant-available soil N with crop N demand. A simple, but reasonable, representation of N content over time by small grain forages is needed to assist in management of manures in dairy forage systems. Our objective in this study was to discover whether simple parameterized models averaged across a range of conditions and species could adequately predict cumulative N uptake by small grain forages over time. In order to describe N accumulation within the small grain forages commonly used for landspreading in California's Central Valley, 22 replicated field plots, representing a variety of small grain cultivars, were established and sampled between 1997 and 2002. Crop N development over thermal time was fitted to a highly flexible logistic equation that can be used by planners as a convenient design tool. Historic harvest data can be used to parameterize the resulting function for specific field conditions. Data from the different experiments were combined in four ways so that parameterizations can be selected by users to reflect available information. When all data from all experiments were lumped together, r2 = 0.91. Use of cultivar-specific shape factors and field-specific times of maximum N uptake increased the overall r2 to 0.95. While many uncertainties due to environmental, management, and weather factors limit the precision of prediction for crop N accumulation, the use of the simple logistic relationship may be a reasonable planning alternative. © 2009 American Society of Agricultural and Biological Engineers.