Effect of Shade on Growth and Yield of Weedy Rice (Oryza sativa L.) Biotypes and a Rice (Oryza sativa L.) Cultivar from Asia

Four different experiments were conducted in a screen house to determine the effects of shade on growth and grain yield of three weedy rice biotypes (WRM from Malaysia, WRS from Sri Lanka, and WRV from Vietnam) and one cultivated rice variety (Rc 222 from the Philippines). The shade regimes did not influence plant height of any weedy rice biotypes and cultivated rice; however, the height was influenced by the biotypes. At full sunlight or continuous shade of 50%, shoot and root biomass of the WRV accession was greater than other three biotypes. However, the continuous shade of 75% eliminated these differences among the biotypes. Compared with the full sunlight, shade treatments significantly reduced shoot and root biomass of cultivated rice; however, such differences for weedy rice biotypes were observed at only 75% of shade or when plants were transferred from full sunlight to 75% of shade. Compared with the full sunlight, shoot biomass of cultivated rice declined by 57% and 72% when grown in 50% and 75% of shade, respectively. For weedy rice, however, these values were much lower. Compared with full sunlight, continuous shade of 50% reduced shoot biomass of the Malaysia weedy rice accession by only 19%. Compared with full sunlight, continuous shade of 75% significantly reduced weedy rice biomass and grain yield. The results of the study show that shade through weed-competitive cultivars and use of high seeding rate of cultivated rice can reduce weedy rice growth.