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An Extended Model of Forage Grass Response to Applied Nitrogen

Agricultural Engineering Dep., Univ. of Florida, Gainesville, FL 32611
USDA-ARS Southern Piedmont Conserv. Res. Center, Watkinsville, GA 30677
Agricultural Engineering Dep., Univ. of Florida, Gainesville, FL 32611

^* Corresponding author.

A simple model is needed that relates forage grass production (yield and N removal) to management factors (applied N, harvest interval, and water availability). The objective of this analysis was to extend a previous model to include quantitative coupling between yield and N removal in response to applied N for perennial grasses. The extended model was developed from three postulates: (1) annual dry matter yield follows logistic response to applied N, (2) annual plant N removal follows logistic response to applied N, and (3) the N response coefficients are 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 concentration and plant N removal follow a linear relationship. Data from a field study in Louisiana with dallisgrass [Paspalum dilatation Poir.] grown on Olivier silt loam (fine-silty, mixed, thermic Aquic Fragiudalf) were used to demonstrate applicability of the model and to illustrate procedures. Analysis of variance supported Postulate 3 for these data, with an overall correlation coefficient of 0.9990. Plant N concentration (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 mechanisms that control forage production.

Received for publication March 29, 1993.

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