The Impact of Climate Variability on Wheat Growth and Yield
Type of Degreethesis
Agronomy and Soils
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Understanding the environmental factors impacting wheat (Triticum aestivum L.) yield may lead to opportunities to increase yield potential. Variability in climatic conditions during the wheat growing season in the southeastern United States is strongly influenced by El Niño-Southern Oscillation (ENSO). Hence, ENSO forecast could potentially be used as a tool to adjust wheat management practices. Those adjustments focused on minimization of climate-related risks can be analyzed through the use of crop simulation models. To address this issue, this thesis studies the effect of planting date and variety selection on winter wheat production in Alabama. Additionally, evaluation of the Cropping System Model (CSM)-CERES-Wheat model was conducted for its ability to simulate growth, development, and grain yield of three different wheat varieties, as well as to determine yield response differences to planting date and variety selection combination based on ENSO phases. The field study was conducted during 2009-2010 and 2010-2011 growing seasons at three research stations across Alabama: Tennessee Valley (TVREC), Wiregrass (WGS), and E. V. Smith (EVS). Wheat was planted in a randomized complete block design with split-plots and five replications. Four planting dates at approximately 15 day intervals were assigned to the main plots, and three varieties with early (AGS 2060), medium (AGS 2035), and late maturity (Baldwin) were randomized within subplots. iii The simulation of wheat growth and yield was conducted using the Cropping System Model (CSM)-CERES-Wheat model, which was calibrated using data from three field studies. Data for the model evaluation was compiled from the 2008-2011 Alabama Performance Comparison of Small Grain Variety Trails. A seasonal analysis using 60 years of daily historic weather data was used to identify the impact of planting date and variety selection on yield as well as the wheat yield differences between ENSO phases. Results showed yield differences associated with location by planting date, maturity group and year interactions. Regardless of location and year, yield decreased as planting was delayed for the medium and late maturing cultivar. This research showed that seed yield could be increased if the wheat cultivars were planted 15 days earlier than the standard planting date used by farmers at each location. The medium and late maturities varieties had the highest yield at all locations for the early planting dates. At the central location, EVS, there was little yield impact due to changes in planting dates and all three varieties tended to performed in a similar fashion. Overall results across locations suggested that yield can be increased via a higher seed weight instead of increasing the number of seed per spike. This can be achieved more easily with early plantings. Results from simulation modeling showed that yield for all varieties decreased as planting was delayed at WGS and TVS. In contrast for EVS, the simulated average yield for the medium and late maturing varieties, AGS 2036 and Baldwin varieties, tended to be higher for later planting dates. During the La Niña years, the highest simulated wheat yield was observed compared to the other ENSO phases across all locations. The risk for yield losses associated with delayed planting was higher during El Niño phase than the iv other ENSO phases, especially for the early maturing variety. In contrast, during La Niña and Neutral phases, AGS 2060, the early maturing cultivar, exhibited the lowest yield reduction associated with late planting compared to the AGS 2035 and Baldwin varieties. At EVS, there was not a clear trend for higher yield associated with the specific variety to ENSO phase. At WGS, the early maturing variety, AGS2060, exhibited the highest yield reduction (16.9%), followed by AGS 2035 (16.25%) and Baldwin (12.8%) during the El Niño years when planting date 1 was compared to the latest planting date. During La Niña years, yield reductions when comparing the first planting date to the last planting date were smaller than for the El Niño years with differences between varieties of 10.45% for AGS 2060 followed by Baldwin with 11.89%, and AGS 2035 with 12.32%. Neutral years exhibited a broad range of yield reduction differences between locations and varieties. For TVS, AGS2060 had the lowest yield reduction (18.89%) followed by Baldwin (24.17%) and AGS 2035 (25.44%) for same planting dates comparisons. Further studies should focus on the evaluation and application of the CSM-CERES-Wheat model for other management practices and other agroclimatic regions where wheat is an important crop.