RADIATION-USE EFFICIENCY AND GRAIN-YIELD OF MAIZE COMPETING WITH JOHNSONGRASS

Accurate simulation of the impact of weeds on crops requires adequate quantification of weed effects on crop biomass and on the partitioning of crop biomass into grain. The first objective of this study was to determine whether radiation-use efficiency (RUE) values of maize (Zea mays L.) and johnsongrass [Sorghum halepense (L.) Pers.] grown in monoculture could be applied to these species grows in mixed plantings. The second objective was to investigate how maize yield and harvest index (HI) respond to johnsongrass competition. Monoculture plots of maize and johnsongrass and plots with the two species competing were established on Houston black clay (fine, montmorillonitic, thermic Udic Pellustert) in the held at Temple, TX, in 1991 and 1992. Sequential measurements of light interception (IPAR) and of biomass were used to calculate values for RUE prior to anthesis. Yield and HI of maize were measured after physiological maturity. Johnsongrass reduced grain yield of maize by 5% in 1991 and by 33 to 49% in 1992. The greater competition in the second year also reduced maize HI from 0.55 in the first year to 0.43 in the second. Values of RUE of maize and johnsongrass growing together were similar to weighted means of monoculture RUE values. The weights were the relative fraction of each species in the mixture. The measured values of maize-johnsongrass mixtures differed from weighted means by 3 to 11% in both years.