Optimal Rates of Nitrogen Fertilization for First‐Year Corn after Alfalfa

Abstract

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 for 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. Research Question Alfalfa 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 for corn after alfalfa in northeast Iowa. Concentrations of soil nitrate in late spring and concentrations of nitrate in cornstalks at the end of the season were used to help identify optimal N rates. Literature Summary Numerous studies have shown that little yield response to applied N should be expected on first‐year corn after alfalfa. Other studies indicate that farmers frequently apply substantially more fertilizer N to corn after alfalfa than is required to attain maximum yields. Overapplication of fertilizer N reduces profitability of corn production and can cause nitrate contamination of water supplies. Study Description Nitrogen response trials were established on 29 fields from 1987 through 1990. Fertilizer N was applied at seven rates, ranging from 0 to 200 lb/acre. Except for fertilizer application and harvest, each trial was managed by the farmers. Alfalfa management usually consisted of three cuttings per year in all but the establishment year, and no field received a manure application for at least 3 yr before corn planting. Soil samples were collected when corn was 6 to 12 in. tall from the surface 1 ft layer of soil. Cornstalk samples were collected 1 to 3 wk after physiological maturity (black layer formation) by removing 8‐in. segments of stalk starting 6 in. above the ground. Applied Questions What was the corn grain yield response to N fertilization? Applications of fertilizer N significantly increased yields at six of the 29 trials. At each of these responsive trials there was a yield increase only for the first 25 lb N/acre applied. What was the most profitable rate of N fertilization for corn after alfalfa? The rate of N fertilization that was most profitable when applied across 29 trials was mainly determined by price received per bushel of grain, price paid per pound of N fertilizer, and application costs (Table ). At prices prevailing in the Corn Belt, application of 0 to 25 lb N/acre were the most profitable. Did concentrations of soil nitrate in late spring and concentrations of nitrate in cornstalks at the end of the season help identify optimal rates of fertilization? The soil test was most effective when a critical concentration of 14 ppm nitrate‐N was used to distinguish between soils that should and should not be fertilized. The cornstalk test correctly indicated that excess N had been applied to most treatments. 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. Net returns from N fertilization across the 29 sites at various price scenarios. Net returns from fertilization Corn = $2.00/bu Corn = $2.50/bu N rate N = $0.10/lb N = $0.20/lb N = $0.10/lb N = $0.20/lb lb/acre ‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐$/acre‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐ 25 −1.15 −0.65 0.56 1.06 50 −5.43 −7.43 −4.16 −6.16 75 −4.26 −8.76 −2.08 −6.58 100 −6.82 −13.82 −4.66 −11.66 150 −5.71 −17.71 −2.02 −14.02 200 −15.30 −32.30 −12.75 −29.75 Application cost of $5.50/acre for $0.1071b N (common for anhydrous ammonia) and $2.50/acre for $0.20/lb N (common for urea‐ammonium‐nitrate solutions).

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