AN EXTENDED MODEL OF FORAGE GRASS RESPONSE TO APPLIED NITROGEN

A simple model is needed that relates forage grass production (yield a nd N removal) to management factors (applied N, harvest interval, and water availability). The objective of this analysis was to extend a pr evious model to include quantitative coupling between yield and N remo val in response to applied N for perennial grasses. The extended model was developed from three postulates: (1) annual dry matter yield foll ows logistic response to applied N, (2) annual plant N removal follows logistic response to applied N, and (3) the N response coefficients a re the same for both. Three additional consequences derive from these postulates: (i) plant N concentration response to applied N follows a ratio of logistic functions, (ii) annual dry matter yield and plant N removal are related by a hyperbolic equation, and (iii) plant N concen tration and plant N removal follow a linear relationship. Data from a field study in Louisiana with dallisgrass [Paspalum dilatatum Poir.] g rown on Olivier silt loam (fine-silty, mixed, thermic Aquic Fragiudalf ) were used to demonstrate applicability of the model and to illustrat e procedures. Analysis of variance supported Postulate 3 for these dat a, with an overall correlation coefficient of 0.9990. Plant N concentr ation (N(c)) for this study was bounded by 11.8 < N(c) < 27.5 g kg-1. Maximum yield for the study reached 57% of potential yield at plant N removal of 427 kg ha-1. Potential yield of 27.5 Mg ha-1 appears to be in the range of values reported in the literature. Further use of this model should provide added insight into the coupling among various me chanisms that control forage production.