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.

Local adaptation and genotype by environment (G×E) interactions affect the expression of phenotypes in crop species. An investigation on the interplay between adaptation and G×E on sorghum heterosis phenotypes is lacking. To address this question, a set of 112 diverse grain sorghum hybrids and their 22 inbred parents of local and exotic origins, representing the primary female and male heterotic groups, were tested in five contrasting sorghum growing environments across two years in Queensland, Australia. Plant height, days to flowering, seed yield, grain weight, and grain number were measured and used in the estimation of heterosis. Mid parent heterosis for yield ranged from -25 to 217% and was highly influenced by grain number. In contrast to observations in maize the magnitude of heterosis for yield was not associated with site mean. Striking differences were observed in heterosis in hybrids from locally developed inbred parents compared with hybrids that were developed from exotic inbred parents developed in other countries. Heterosis in the latter combination was higher on average across all the test environments for the majority of traits. We hypothesise maladaptive phenotypic plasticity in the exotic parents contributed to the observed differences in heterosis estimates. These data confirm that heterosis estimates in sorghum must be obtained and interpreted in relevant genetic and environmental contexts. Breeders in developing countries with low sorghum hybrid seed uptake will find these insights useful when selecting hybrids for broader adaptation, improving efficiency of their breeding programs.