Manure Management in an Irrigated Silage Corn Field: Experiment and Modeling

On agricultural lands, animal waste disposal as fertilizer has been practiced since the beginning of agriculture. However, the practice has been an environmental concern in recent years due to over disposal of animal waste in some instances. This study evaluated soil NO₃ response to beef-manure application on a corn (Zea mays L.) field and tested the Root Zone Water Quality Model (RZWQM) for manure management. The experiment site was located in Northeastern Colorado on a silage-corn field with a history of fertilization with beef manure every fall after corn harvest. To study the residual effect of long-term manure application, 582 kg ha⁻¹ of manure-N was applied to the east side of the field in the Fall of 1993, 1994, and 1995, while the west side received manure in 1993 only. Average silage-corn yields from the west site were 25.4, 31.9, and 22.5 Mg ha⁻¹ for 1994, 1995, and 1996, respectively, which were not significantly different from that harvested from the east site (25.1, 30.9, and 24.3 Mg ha⁻¹, respectively). Average soil NO₃ concentrations decreased significantly from 14.9 to 8.5 mg N kg⁻¹ in the top 30 cm of soil, and from 5.4 to 3.7 mg N kg⁻¹ in the 30- to 60-cm soil profile after stopping manure application. No significant difference in soil NO₃ concentrations between the manured and not-manured sites was found below 60 cm. Average plant N uptake ranged from 140 to 362 kg N ha⁻¹ and was not significantly different between the two sites. The RZWQM was calibrated on the basis of the measured silage-corn yield and plant N uptake, and was then used to predict soil NO₃ concentration and total water storage in the soil profile. Generally, the calibrated model provided adequate predictions for both NO₃ and soil water content with r² > 0.83. The model was further used to evaluate alternative scenarios of manure and water management.