CONTRIBUTIONS OF ALFALFA TO A SUBSEQUENT CORN CROP: AGRONOMIC AND ECONOMIC EVALUATION (CROP ROTATION, NITROGEN, LEGUME)

The availability of new alfalfa germplasm with increased storage of root N has accentuated the importance of determining the value of short-term alfalfa-corn rotations. The objectives were to compare effects of two harvest systems on forage yield and quality, and N incorporated from an experimental ('MN ROOT N') and a standard ('Saranac AR') alfalfa, to evaluate effects of previous cropping treatments on grain yield of a subsequent corn crop, to develop a method to discern between N contributions (NC) and rotation effects (RC) in legume-nonlegume sequences, and to determine the economically optimum rotation/N-fertilization combination of corn following alfalfa at four Minnesota locations.; A field study was conducted in 1982 and 1983 in which first-phase rotation components were alfalfa, corn, and soybean, and the second-phase component was corn. Alfalfas were subjected to two harvest systems; either one or three cuts removed. Nitrogen fertilizer was applied to corn across all first-phase treatments.; The alfalfas did not differ in total forage production, but Saranac AR had higher forage quality. MN ROOT N had greater root dry matter and N concentration. Differences in second-phase corn yields were not associated with differences in alfalfa N incorporation at three of four locations. With no N applied, corn yields were greater after all alfalfa treatments than after corn.; Alfalfas did not differ in NC or RC within either harvest system, and all alfalfa treatments provided greater NC than soybean. At least 50% of the corn yield enhancement from the legumes in rotation was attributable to RC.; A linear programming model was formulated to select the optimum two-year rotation/N-fertilization combination at each location, based on gross margin. An alfalfa-corn sequence with alfalfa subjected to a three-cut system was the economically optimum rotation at all locations. In comparing gross margins of the two alfalfas, the optimum rotation was determined more by the quantity of alfalfa produced than by differences in second-phase corn grain yield.