Nitrogen Fertilizer Response Potential of Corn and Sorghum in Continuous and Rotated Crop Sequences

Crop management systems need to be designed to maintain economic profitability and minimize negative environmental impact. The objective of this study was to determine the effects of previous crop, yield potential, and residual soil nitrate (RSN) on grain yield response to N fertilizer of sorghum [Sorghum bicolor (L.) Moench.] and corn (Zea mays L.). Trials were conducted on 38 farms in 14 counties in eastern Nebraska during 1988 to 1990 and separated by previous crop into three groups, including (i) cereal [either sorghum, corn, oat (Avena sativa L.), wheat (Triticum aestivum L.), or rye (Secale cereale L.)], (ii) soybean [Glycine max (L.) Merr.], and (iii) forage legume [either alfalfa (Medicago sativa L.), sweet clover (Melilotus officinalis Lam.), or red clover (Trifolium pratense L.)]. The potential for a response to N fertilizer was described by relating initially available N to yield level (N/Y). Initially available N included RSN to a depth of 40 in., preplant and starter fertillizer N, and NO₃-N in irrigation water. Yield level was assumed to be equal to the maximum predicted yield from regression analysis in individual trials. The critical level of N/Y at which 95% of maximum predicted yield was attained without N fertilizer application, was 0.80 lb initial N/bu grain for sorghum following cereal, 1.44 lb initial N/bu grain for corn following cereal, 0.65 lb initial N/bu grain for corn following soybean, and zero lb initial N/bu for corn following forage legume. Knowledge of initially available N relative to expected yield for individual fields will help farmers make sound economic and environmental decisions n the need for N fertilizer in continuous and rotated crop sequences. Research QuestionCrop management systems need to be designed to maintain economic profitability and minimize negative environmental impact. Crop rotations are an integral part of sustainable agricultural systems, but satisfactory N fertilizer recommendations for cereals in rotation with legumes are not available. This study was conducted to determine the combined effects of previous crop, initially available N, and yield level on grain yield response to N fertilizer of sorghum and corn grown in eastern Nebraska. Literature SummaryCurrent N fertilizer recommendations for both continuous and rotated crop sequences are based upon yield response observed in monoculture with subtraction of an average N credit for a particular previous crop when grown in rotation. Highly variable N credit values have been observed for a particular previous crop within the same study. One possible explanation for this large variation in observed N credit may have been differences among years and crop sequences in residual soil nitrate (RSN), although data supporting this hypothesis are absent. In addition, grain yield in rotation is often greater than in monoculture due to a rotation effect, which leads to greater N requirement, despite observation of less N fertilizer required in rotation compared with monoculture. Sufficient data relating N credit values to initially available N and yield level are lacking. Study DescriptionThirty-eight farmers in 14 counties of eastern Nebraska participated in 86 N fertilizer trials as part of a University of Nebraska Cooperative Extension program during 1988 through 1990. The ratio of initially available N to yield level (N/Y) was calculated for each individual trial. Initially available N consisted of RSN to a depth of 40 in., preplant and starter fertilizer N (if applicable), and NO₃-N in 9 in. of irrigation water (if applicable). Yield level of each farm was assumed to be the maximum predicted yield within the range of N fertilizer rates tested for each trial based upon a regression equation. Sorghum and corn grain yield response to N fertilizer was evaluated with respect to previous crop (i.e., cereal, soybean, or forage legume) and N/Y. Applied QuestionCan the combined use of residual soil nitrate and yield level improve N fertilizer recommendations for sorghum and corn grown in monoculture and in rotation with legumes? Average sorghum and corn grain yield responded positively to addition of N fertilizer with cereal as previous crop. The magnitude of the linear response, however, decreased with increasing N/Y. With soybean and forage legume as previous crop, N fertilizer and N/Y had no effect on sorghum grain yield. Corn grain yield was also unaffected by the addition of N fertilizer with soybean or forage legume as previous crop. With soybean as previous crop, however, the interaction of N/Y with N fertilizer rate indicated that grain yield responded to N fertilizer only under conditions of very low N/Y. Relative sorghum and corn grain yield without N fertilizer increased with increasing level of N/Y with cereal as previous crop. This relationship was used to identify the critical level of N/Y at which yield response to N fertilizer was minimal. Considering 95% of maximum yield level as optimum to meet economic and environmental criteria, the optimum yield of sorghum and corn with cereal as previous crop was attained at a level of N/Y of 0.80 and 1.44 lb N/bu, respectively. At least 95% of maximum yield level was achieved at a level of N/Y of 0.65 lb N/bu for corn following soybean and 0 lb N/bu for corn following forage legume. The results of this study suggest that initially available N relative to yield level is an important parameter for predicting N fertilizer response potential of sorghum and corn grown in eastern Nebraska. Major difference in the critical level of N/Y observed with respect to previous crop coincides with previous observations that cereals preceded by leguminous crops generally exhibit a reduced requirement for N fertilizer. The concept of using a different critical level of N/Y with respect to previous crop, allows for a fluctuating N credit that is dependent upon initially available N and yield level. This fluctuating N credit with legume as previous crop is consistent with highly variable N credit values found in previous research. Although the exact reason for the reduced requirement of N fertilizer of sorghum and corn grown with legume as previous crop compared with cereal as previous crop was not mechanistically defined, the rotation effect can still be exploited with an improvement in determining appropriate N fertilizer application in rotated crop sequences to achieve economically and environmentally optimum yield (Table 1). The combined effects of measuring RSN, selecting a realistic yield goal, and accounting for previous cropping history are management strategies tat can increase the profitability of farming operations and decrease the potential negative environmental impact of N fertilizer carryover. Table 1Nitrogen fertilizer recommendations for corn based upon previous crop, expected yield, and initially available N (assuming a N requirement coefficient at 95% maximum yield).† Previous cropCornSoybeanForage legumeExpected yieldInitially available NN required‡Fert. N§N requiredFert. NN requiredFert. Nbu/acre-------------------------------------------- lb N/acre ------------------------------------6050863639000601008603900060150860390009050130805990090100130305900090150130059000120501731237828 0012010017373780001201501732378000150502161669848 001501002161169800015015021666980002005028823813080 0020010028818813030 00200150288138130000†Initially available N = NO₃-N/acre-40 in. + preplant N + starter N + NO₃-Nlacre-9 in. irrigation water. ‡N required = expected yield × N requirement coefficient (i.e., 1.44 following corn, 0.65 following soybean, and 0.00 following forage legume). §Fertilizer N (i.e., recommended amount of fertilzier) = N required — initially available N.