Optimizing Agronomic Inputs in Corn–Wheat–Soybean Cropping Systems in Alabama: Implications for Productivity and Nitrogen Use

Abstract

Wheat (Triticum aestivum L.), corn (Zea mays L.), and soybean (Glycine max L.) are economically important row crops in Alabama, yet region-specific information on highmanagement wheat systems and soybean nitrogen (N) credits remain limited. This thesis addresses these gaps through two independent field studies conducted across contrasting Alabama environments. The first study evaluated the agronomic and economic responses of two wheat cultivars (Jamestown and DG 9332) to a stepwise gradient of management intensification during the 2023– 2024 and 2024–2025 growing seasons at three locations (North, Central, and South Alabama). Treatments included a baseline practice (BP) and stepwise modifications in seeding rate, fertility, fungicide, and micronutrient supplementation, plus a comprehensive practice (CP) combining all inputs. Site-year environment was the dominant source of variation, with location yield rankings reversing between seasons. DG 9332 consistently produced greater yield than Jamestown, while Jamestown produced greater grain protein, confirming a yield–protein tradeoff supported by a significant negative correlation between plot-level yield and protein (r = −0.50, P ≤ 0.001). Management practice responses were expressed as main effects with no significant interactions. Enhanced fertility and CP consistently reduced net return relative to BP across environments and years, while lower-cost adjustments (+Pop and +Fung) produced positive gains in roughly half of environments. Under the price scenarios evaluated, maximizing yield through high-input management did not maximize profit, indicating that producers are more likely to benefit from targeted, lower-cost adjustments than from comprehensive high-input practices. The second study quantified the soybean N credit to corn using a two-year rotational experiment at two Alabama locations (North and South). In 2024, previous crop treatments (corn, 3 soybean, and fallow) were established, and in 2025, corn was grown across all plots at six N rates (0–336 kg N ha⁻¹). Quadratic-plateau response functions were used to estimate agronomic and economic optimum N rates (AONR and EONR) under three N-to-corn price ratios. Location, previous crop, N rate, and all interactions significantly affected corn yield. At the North location, soybean increased plateau yield by 1.78 Mg ha⁻¹ relative to continuous corn, but EONR did not decline, indicating that the rotation effect was expressed through increased yield potential rather than reduced N requirement. At the South location, soybean consistently reduced EONR relative to continuous corn by 27 to 40 kg N ha⁻¹ depending on fertilizer price conditions, representing a positive and economically meaningful soybean N credit. These contrasting outcomes reflect differences in soil buffering capacity between sites and demonstrate that soybean N credit in Alabama is site dependent. Together, these studies emphasize that wheat management and corn N management recommendations in Alabama should be tailored to site-specific environmental and soil conditions rather than applied uniformly across the state.