OPTIMAL RATES OF NITROGEN-FERTILIZATION FOR 1ST-YEAR CORN AFTER ALFALFA

Alfalfa (Medicago sativa L.) contributes substantial N to grain crops that follow, but there is uncertainty concerning the amounts. The objective of this study was to determine optimal rates of N fertilization f or first-year corn (Zea mays L.) grown after alfalfa. Fertilizer N was applied at seven rates, ranging from 0 to 200 lb/acre, at 29 trials conducted over 4 yr in northeast Iowa. Concentrations of soil nitrate in late spring and concentration of nitrate in cornstalks at the end of the season were used to help determine optimal N rates. Fertilizer significantly increased yields at six of the 29 trials. The rate of N fertilization most profitable when applied across the 29 fields was mainly determined by cost of fertilization and.value of grain. At prices prevailing in the Corn Belt, applications of 0 and 25 lb N/acre were the most profitable. The soil test was most effective when used with a critical concentration of 14 ppm nitrate-N to distinguish between soils that should and should not be fertilized. Nitrogen fertilization was most profitable when 50 lb/acre was applied to the three low-testing soils, which gives a mean rate across all 29 trials of 5 lb N/acre. Both the soil test and the end-of-season cornstalk test should help producers recognize that little or no N is needed for first-year corn after alfalfa.