Author ORCID Identifier

https://orcid.org/0009-0004-9555-6235

Date Available

12-12-2025

Year of Publication

2025

Document Type

Doctoral Dissertation

Degree Name

Doctor of Philosophy (PhD)

College

Agriculture, Food and Environment

Department/School/Program

Plant and Soil Sciences

Faculty

Montse Salmeron

Faculty

Arthur Hunt

Abstract

Soybean (Glycine max [L.] Merr.), a major crop in the upper U.S. Mid-South, is predominantly cultivated under rainfed conditions, where variable precipitation frequently induces water stress and limits yield potential. Consequently, irrigation adoption has been increasing across this region. Determining the optimal cultivar maturity group (MG) and planting date (PD) for both rainfed and irrigated systems is essential for maximizing yield and improving water use metrics. Evaluating crop water use efficiency (WUE; yield per unit of seasonal evapotranspiration) and total water productivity (WP; yield per unit of total water input from precipitation and irrigation) is critical to developing such recommendations. Field experiments were conducted from 2017 to 2020 in Lexington, Kentucky, under irrigated and rainfed conditions, using two PDs (May and June) and 16 cultivars representing MG 2 through 5. Experimental data from 2017 and 2019 were used to calibrate, and 2018 and 2020 to validate, the CROPGRO-Soybean model within the Decision Support System for Agrotechnology Transfer (DSSAT) v4.8.5. Across four years, irrigation increased soybean yield by 12–117%, depending on PD, MG, and seasonal rainfall distribution. Irrigation enhanced both WUE and WP during the driest year but decreased WP for some MG x PD combinations in wetter years. Results indicated that irrigation effectively mitigates yield losses under water stress and enhances yield stability across variable climates. However, limited information exists on how irrigation alters the PD and MG recommendations, or how it influences WUE and WP dynamics. Using Lexington, KY, as a representative irrigation-transition region, the calibrated CROPGRO model was used to simulate 30 years (1991–2020) of yield and water balance responses under 13 PDs, four MGs (2–5), and two water regimes (irrigated, rainfed). The 30-year sensitivity analysis revealed that (i) under irrigation, the optimum planting window for maximum yield occurred approximately 20 days earlier and favored shorter MGs compared with rainfed conditions, particularly in dry years; (ii) early planting in April increased yield potential but reduced WUE and WP, especially under rainfed management, suggesting that delayed planting may conserve water in dry years; (iii) similar PD and MG combinations could be identified that simultaneously optimized yield, WUE, and WP under both water regimes; and (iv) the relative benefit of irrigation depended on the chosen water-use metric, with WUE and WP responding differently. To identify efficient irrigation strategies, several irrigation scheduling approaches were evaluated against an optimized irrigation scheduler. These include soil moisture threshold-based irrigation (100% replenishment at 55% soil water threshold), evapotranspiration (ET) deficit-based irrigation (100% and 60% ET replacement), and delayed irrigation strategies initiated at reproductive stages R3 and R5. A 7-day forecast–based optimization tool was also tested to simulate practical irrigation decisions under short-term weather predictions. Results indicated that delaying irrigation until the R3 stage reduced total irrigation volume without yield penalties. Mild water stress during early vegetative stages did not significantly affect photosynthesis, and had slight effect on leaf expansion suggesting opportunities to conserve irrigation water without yield loss. Overall, optimized irrigation scheduling improved irrigation water productivity through better timing and allocation of water resources, offering a framework for sustainable irrigation expansion in soybean systems transitioning from rainfed to irrigated production. Among the other irrigation scheduling methods, soil threshold-based scheduling, in combination with delaying it to reproductive stages maximized yield with minimized water use.

Digital Object Identifier (DOI)

https://doi.org/10.13023/etd.2025.605

Funding Information

Kentucky Soybean Promotion Board Grant

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