| 1. | Primary branches of inflorescence not divided, bearing only terminal racemes | 2 |
| Primary branches of inflorescence divided, bearing terminal and lateral racemes | 3 | |
| 2. | Spikelets awnless | S. nitidum f. nitidum (QLD) |
| Spikelets awned | S. nitidum f. aristatum (QLD) S. serratum |
|
| 3. | Inflorescence axis much thicker than branches; grains large, exposed at maturity | *S. bicolor (NSW VIC NT SA WA QLD) |
| Inflorescence axis slightly thicker than branches; grains small, enclosed by glumes at maturity | 4 | |
| 4. | Rhizome present | 5 |
| Rhizome absent | 6 | |
| 5. | Sessile spikelet 5-6.5 x 2-5mm | *S. x almum (NSW NT WA QLD) |
| Sessile spikelet 4.5-6 x 1.8-2mm | *S. halepense (NSW VIC SA WA QLD) S. halepense forma muticum, S. miliaceum |
|
| 6. | Sessile spikelet breaking clean from apex of rachis internode | *S. arundinaceum (QLD) S. bicolor subsp. arundinaceum, S. verticilliflorum |
| Sessile spikelet breaking together with upper portion of rachis internode | *S. x drummondii (NSW WA QLD) S. bicolor subsp. drummondii, S. sudanense |
Key to Sorghum derived from Garber 1950; Simon 1979; Lazarides et
al. 1991; Spangler
Taxonomic name: