Radiation-use efficiency among grain sorghum cultivars and plant densities

Radiation-use efficiency (RUE, g MJ-1) has often been used in many crop model equations to estimate total biomass and yield based on cumulative absorbed photosynthetically active radiation. Management practice effects on radiation-use efficiency have not been well documented for grain sorghum [Sorghum bicolor (L.) Moench]. A field study was conducted near Temple, TX to determine plant density and cultivar effects on RUE in grain sorghum. Early 'Cargill 4462' and late maturing 'Cargill 6670' cultivars were planted on a Houston Black clay (fine, montmorillonitic, thermic Udic Paleustert) on 16 Mar. and 1 Aug. 1989 at 5, 12, 19, and 26 plants per square meter. Incoming, reflected, and transmitted photosynthetically active radiation were measured weekly from 6 wk after emergence until full anthesis. Dry matter and leaf area index were determined from 0.5-m2 areas immediately after the radiation measurements. The calculated extinction coefficient, 0.51, for PAR transmittance was significantly lower than in the crop growth model, SORKAM, documentation. This new value or extinction coefficient reduces total dry matter in and increases RUE in SORKAM. The average RUE for all treatments was 3.5 g MJ-1 as compared with 3.2 g MJ-1 in SORKAM. The new values will be incorporated into SORKAM. No significant radiation use efficiency differences were detected among density and maturity class treatments