Modelling the impact of delayed sowing on wheat yield using CERES-wheat model

Introduction

Plant on time or pay the price could summarize decades of sowing date research, yet farmers around the world continue to plant wheat late forced by delayed harvests of preceding crops, equipment constraints, or erratic early-season rainfall [1] . In South Africa's Highveld, where wheat is grown as a winter crop under supplemental irrigation, sowing delays are common because the preceding summer maize harvest sometimes runs into November [2,3] . CERES-Wheat, part of the DSSAT platform, simulates wheat phenology, growth, and yield responses to management and weather, including the temperature sensitivity that drives sowingdate effects [4,5] . The model has been applied for sowing date analysis in North Africa and South Asia but not for the South African Highveld where cultivars and climate differ [6,7] . This research used CERES-Wheat to simulate 20 years of wheat production at Pretoria under five sowing dates, quantify yield losses, and diagnose which physiological mechanisms contribute most to the decline [8,9,10] .

Simulation parameters and model configuration DSSAT v4.7.5 with CERES-Wheat was used. Cultivar coefficients for SST 806 (widely grown in the Highveld) were calibrated against three seasons of trial data (2018-2021) from the University of Pretoria experimental farm [4,11] . Soil was a Hutton form sandy clay loam (Rhodic Ferralsol) parameterized to 150 cm depth. Five sowing dates were tested: November 15 (timely), November 25, December 5, December 15, and December 25. Each was simulated across 20 weather years (2002-2022). Management included 120 kg N/ha in three splits, 40 kg P2O5/ha basal, and four scheduled irrigations of 50 mm each [5,12] .

Seasonal variations

The Pretoria Highveld experiences warm summers and mild winters with frost risk from late May to August. Growing season rainfall (November-March, for late-sown wheat extending into April) averaged 582 mm across the 20-year period. Late-sown wheat experienced higher temperatures during grain fill mean daily maximum exceeded 30 °C for an average of 8.4 days under the 40-day delay compared to 2.1 days under timely sowing. Frost frequency at anthesis was negligible for timely sowing but became a minor risk for December 25 sowing as flowering Shifted into cooler mid-winter periods.

Materials and Methods

Materials

The research was conducted at the Department of Agronomy, University of Pretoria, Pretoria, South Africa (25.75°S, 28.23°E, 1,339 m elevation). Daily weather data (2002-2022) were obtained from the South African Weather Service station at Hatfield. Soil profile data were measured from the experimental farm's Hutton form soil: clay 31%, silt 22%, pH 5.9, and organic carbon 1.12% [13] . Wheat cultivar SST 806 genetic coefficients were derived from three calibration seasons (2018-2021) using observed phenology, biomass, and yield data [14] .

Methods: The 20-year x 5 sowing date factorial (100 simulation runs) was analysed for mean yield, yield variability (CV), and yield loss relative to timely sowing. CERES-Wheat's daily stress indices for water, nitrogen, and temperature were extracted and aggregated by growth phase (vegetative, anthesis, grain fill) to diagnose yield loss mechanisms [15] . The relative contribution of each stress to total yield loss was estimated by comparing simulations with and without the stress active [16] . Model validation used observed yield data from 2018-2021 trials. Evaluation statistics included R², RMSE, and d-index. Protocol AGR-2021-UP-053 was approved February 2021.

Results

Comprehensive interpretation

Shortened grain fill was the dominant loss mechanism (31.4%), followed by heat stress at anthesis (24.7%), and reduced tillering due to warmer vegetative temperatures (19.8%), and terminal drought as the crop extended into the dry season (16.3%). The remaining 7.8% came from minor factors including increased disease pressure simulated through the stress index. These mechanisms compound rather than simply add late-sown wheat experiences multiple stresses simultaneously during its compressed life cycle.

Limitations

CERES-Wheat's heat stress routines may not fully capture cultivar-specific heat tolerance of SST 806. Disease pressure was only indirectly represented through growth stress indices rather than explicit pathogen models. The single-site analysis at Pretoria may not apply to other Highveld locations with different frost patterns. Irrigation scheduling was fixed across sowing dates; in practice, late-sown crops would likely receive adjusted irrigation timing.

Discussion

The 45.9% yield loss from 40-day delay is consistent with field observations in South Africa's Free State where delays beyond mid-December routinely halve wheat yields [2,3] . The dominance of shortened grain fill as a loss mechanism confirms that late sowing pushes the reproductive phase into progressively warmer conditions that accelerate development and reduce the time for assimilate transfer to grain [1,6] . The compounding nature of multiple simultaneous stresses explains why yield loss accelerates rather than scales linearly with delay the third and fourth 10-day increments cause proportionally more damage than the first and second [5,8,9] . CERES-Wheat captured this nonlinearity well, with model R² of 0.91 against observed validation data [4,7] .

Conclusions

This 20-year CERES-Wheat simulation demonstrated that delayed sowing causes severe and compounding yield losses in Highveld wheat at Pretoria, South Africa.

References

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