Simulating seed number in grain sorghum from increases in plant dry weight

Simulation of seed number for crop models is important in identifying cultural practices, which enhance yield stability. Field and crop simulation studies examined the relationship between dry weight accumulation and seed number per plant to potentially improve the capability of the grain sorghum [Sorghum bicolor (L.) Moench] model, SORKAM, to simulate seed number. The best estimates of seed numbers were obtained from plant dry weight accumulated during the 360 growing degree day intervals encompassing panicle branch-spikelet formation (PBSI) and panicle elongation through anthesis (EA(I)). Comparison of observed vs. simulated seed numbers using SORKAM's original equations accounted for 57% of the variation in seed number, but it underestimated high seed numbers. Accumulated plant dry weight for the PBSI and EA(I) intervals accounted for 49 and 64% of the variation in seed number, respectively. Simulation of seed numbers improved when the more sensitive water stress coefficient (for leaf area, WATCO(Ie)) was applied to the interval (PBSI or EA(I)) experiencing the highest water stress while the less sensitive water stress coefficient (for dry weight, WATCO(dw)) was applied to the interval experiencing the lowest water stress. The slope from the regression of observed on simulated seed numbers was 0.80 (r(2) = 0.57) for SORKAM with the WATCO(Ie) switch compared with 0.59 (r(2) = 0.57) in the original SORKAM model. Hence, the timing and recovery of water stress during the panicle development period was important in estimating seed number of sorghum.