Yield increase in sorghum has been achieved primarily by increasing grain number. Scope exists to increase yield by increasing grain size, however this has been limited by the negative correlation between grain size and grain number. Extending the duration of the grain filling period has the potential to enable increased grain size without the trade-off with grain number. This study explored grain filling duration (GFD) in a diverse panel of 904 sorghum genotypes in three environments across two years. Significant variation in GFD observed, ranging from 400 to 680 degree-days, included entries with significantly longer GFD than current commercial hybrids. Longer GFD was shown to result in larger grain size.

Additionally,only low associations between GFD and grain number per panicle, flowering time or plant height were observed, indicating that GFD could be manipulated without adverse penalty to these traits. A simulation study to estimate the benefit of an increased GFD across Australian sorghum growing environments over 60 years revealed positive impacts on yield when GFD was increased by 10% or 20%. However, maintaining overall crop duration by shortening the time to flowering while extending GFD led to neutral or negative effects on yield. These results reveal opportunities to exploit GFD for improved genetic gains for yield in sorghum especially in environments or seasons where water does not become more limiting post anthesis.

Single plants of each genotype were tagged in each plot at the time of head exsertion prior to onset of flowering . All measurements for timing of flowering and maturity were recorded on the tagged plant. Flowering time was recorded as the date when the first anthers become visible at the tip of the panicle (Figure 1B). The tagged plant was monitored throughout the season and the date of physiological maturity was recorded as the date when a sampled grain from the tip of the panicle first showed the abscission layer (black layer) at the point of connection of the grain. Plant height was measured at HRF2 by selecting one plant at random from the plot and measuring the distance from the base of the plant to the tip of the panicle at physiological maturity. Single panicles were harvested at HRF2, threshed, and cleaned before grains per panicle and thousand kernel weight (TKW) were measured using an automatic seed counter and weighing machine (Ball Coleman Gen3 seed counter). Daily weather data was recorded using a portable weather station placed within the trial to record daily maximum and minimum air temperatures for the duration of the experiment. Overall, the trial at Gatton experienced lower temperatures during anthesis and post-anthesis in the grain filling period.