RADIATION-USE EFFICIENCY AMONG GRAIN-SORGHUM CULTIVARS AND PLANT DENSITIES

Radiation-use efficiency (RUE, g MJ-1) has often been used in many cro p model equations to estimate total biomass and yield based on cumulat ive 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 conduct ed 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, montmorill onitic, thermic Udic Paleustert) on 16 Mar. and 1 Aug. 1989 at 5, 12, 19, and 26 plants per square meter. Incoming, reflected, and transmitt ed photosynthetically active radiation were measured weekly from 6 wk after emergence until full anthesis. Dry matter and leaf area index we re determined from 0.5-m2 areas immediately after the radiation measur ements. The calculated extinction coefficient, 0.51, for PAR transmitt ance was significantly lower than in the crop growth model, SORKAM, do cumentation. This new value or extinction coefficient reduces total dr y matter in and increases RUE in SORKAM. The average RUE for all treat ments was 3.5 g MJ-1 as compared with 3.2 g MJ-1 in SORKAM. The new va lues will be incorporated into SORKAM. No significant radiation use ef ficiency differences were detected among density and maturity class tr eatments.