A comparative analysis of the growth of sweet and forage sorghum crops. I. Dry matter production, phenology and morphology

Abstract

A sweet sorghum (cv. Wray) and a forage sorghum (cv. Silk) were grown in south-east Queensland without water deficit at two densities in 0.75 m rows on four occasions between late September and mid January. Tiller and branch numbers, and dry matter production, were recorded at appearance of the third leaf ligule, panicle initiation, anthesis, soft-dough stage, maturity and 3 weeks after maturity. Light interception was measured every 2 weeks. The time between all the studied phenological events up to anthesis was shortened with delay in sowing date except for that between appearance of the third ligule and initiation in cv. Silk. The time between anthesis and maturity lengthened with the delay in sowing date of both cultivars. Rate of development was a function of ambient temperature to the third ligule stage and again during maturation. Tiller and branch production were greatest in cv. Silk, decreased with delay in sowing date for both sorghums and was increased by the high density. The estimated assimilate flux required to sustain basal tillers was substantially lower in cv. Silk than in cv. Wray. This flux increased with temperature. In early sown crops, dry matter yields of cv. Wray were greater than those of cv. Silk. Close spacing increased dry matter yields up to anthesis, but by maturity the effect of spacing had been reduced. Cv. Wray produced a larger leaf canopy more rapidly than cv. Silk, although the partitioning of dry matter to leaf tissue only differed between cultivars after canopy closure. The efficiency with which crops used intercepted light energy to produce new above-ground dry matter changed during their ontogeny. It is suggested that early changes in efficiency were a consequence of altered partitioning to roots. The effect of sowing date and spacing on efficiency of light use was slight until the maturation phase, when efficiency decreased with delay in sowing date and at high density. Water use efficiency differed in a similar way to light use efficiency. The relationships found in this study indicate that sorghum growth models need to recognise differences in the morphological development of sorghum types.