Twelve hybrids, twelve parents of six hybrids and four detillered hybrids of grain sorghum were grown at a density of 75,000 plants per hectare, at the Cooper Research Station, Lawes, Southeast Queensland.
Sampling was conducted on four occasions; at flag leaf emergence, anthesis, 2 ½ weeks after anthesis and maturity. Plant dry weight, (main stem and tillers separately) leaf area and number were measured at each occasion. Crop growth rates were estimated from changes in total dry matter with time. Light transmission by the canopy was measured approximately every two weeks, from two weeks before flag leaf emergence to maturity.
There was considerable variation both among hybrids and between hybrids and parents in the time taken to reach flag leaf stage and anthesis, but all genotypes, except one, matured at between 101 and 107 days after sowing. Generally the hybrids reached
anthesis earlier than their parents with the majority of hybrids (9 out of 12) having a relatively constant grain filling period of 38-39 days. The removal of tillers did not affect developmental times.
The male parent on average produced and maintained a greater number of tillers than the hybrid while the female parent in all cases produced the same or lower numbers of tillers than its hybrid. Tiller number declined by 50-•70% in all cases with most hybrids having 2-4 tillers m_2 at maturity. Tiller contribution to total dry matter (TDM) was over 50% in the male line, while tiller contributions ranged from 40-50% in the hybrids and were generally under 40% in the female parents.
The removal of tillers significantly reduced TDM production at maturity but the reduction (24-26%) in dry matter was less than the contribution made be tillers (40-50%) in the control. Tiller removal significantly reduced leaf area index by 1-3 LAI
units and light interception by 10-20% at anthesis and maturity.
All hybrids, except one, grew similarly and showed on approximately linear growth between flag leaf stage and maturity. The hybrid and the female parent had a similar pattern of dry matter production up to mid grain filling, when the growth rate of the female declined.
While the male parents had a lower final TDM than the female and the hybrid, they had a consistently higher LAI than the female but the same or a slightly higher LAI than the hybrid. However, both the female parents and the hybrids on average intercepted more light at all stages of growth than the male lines.