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.