Seeding practices and cultivar maturity effects on simulated dryland grain sorghum yield

Typical planting recommendations for dryland grain sorghum [Sorghum bicolor (L.) Moench] in the southern High Plains are to delay until soil water is adequate for crop establishment, but no population or cultivar maturity class are specified. Our objectives were to use the SORKAM simulation model, long-term (1958-1998) weather records at Bushland, TX, and known Pullman soil (fine, mixed, superactive, thermic Torrertic Paleustolls) properties to identify an optimum planting date, population, row spacing, and cultivar maturity combination to maximize dryland grain sorghum yield. We simulated sorghum grain yields for combinations of planting dates (15 May, 5 June, and 25 June), populations (3,6, and 12 plants m(-2)), row spacings (0.38 and 0.76 m), and cultivar maturity class (early, medium, and late). SORKAM consistently (r(2) = 0.69, RMSE = 792 kg ha(-1)) simulated grain yields that averaged about 5% more than measured values and correctly simulated row width and population effects on yield. Simulated grain yields increased with narrow row-spacing similar to 9%, independent of planting date or cultivar. Increasing plant population significantly decreased panicle seed number, seed mass, and plant tillers; however, the simulated grain yield was unchanged (39964106 kg ha(-1)) by plant populations. Mean simulated grain yields were greatest for the 5 June planting dates with early and medium maturity cultivars that avoided late summer heat or water deficit stresses and matured before freezing weather. Our results show early or medium maturity cultivars, planted 5 June, in 0.38-m row widths, using 3 or 6 plants m(-2), achieve the greatest dryland grain yield on a southern High Plains clay loam soil.