Opportunities for genetic improvement on specific traits require information on available diversity, together with knowledge on heritability estimates and possible trade-off relations among traits. Sixty-five sorghum inbred lines were evaluated for grain filling and other agronomic traits during 2008 and 29 re-evaluated in 2009. Time to anthesis, final grain weight (GW), grain growth rate, duration of grain filling, maximum water content, grain desiccation rate, moisture concentration at physiological maturity, plant height, panicle length, grain number per plant and final yield per plant were measured both years. Results highlighted the available variability for grain-filling patterns in sorghum, and genotypic differences (P < 0.05) for all traits were evident. Final GW variation (16–44 mg grain–1 in 2008, and 20–40 mg grain–1 in 2009) was achieved through different combinations of rate (3.27–9.78 mg degree-days grain–1 10-2) and duration of grain filling (413–853 degree-days). Calculated heritability for grain-filling traits ranged from 0.43 to 0.95, showing GW and maximum water content had the highest values. Grain number showed consistent negative associations with grain growth rate but not with GW due to grain-filling duration variability. This suggests selecting longer grain filling can increase GW (and yield) without negative trade-off relations with grain number.