Lowland Ecotype Cyperus rotundus L. Affects Growth and Yield of Rice under Flooded Conditions in the Philippines

Lowland ecotype Cyperus rotundus L. has been reported to infest irrigated lowland ricefields in the Philippines. Its effects on the growth and yield of rice under flooded conditions are unknown. Two experimental runs were conducted in a screenhouse to determine the growth of lowland ecotype C. rotundus with transplanted and wet direct-seeded rice at sowing densities of 0, 22, 44, 66, and 88 initial germinating tubers m–2, as well as its effects on the growth and yield of rice. Except for height, growth variables of lowland ecotype C. rotundus were increased with the increase in its germinating tuber densities. Compared with 22 germinating tubers m–2, its number of off-shoots and tubers plus shoot and root biomass at 44–88 germinating tubers m–2 increased by up to three times. Growth variables of rice were reduced by the weed. Grain yield of transplanted rice was reduced by 14–38%, whereas that of wet direct-seeded rice was reduced by 11–28%. The estimated yield losses of transplanted rice per weed plant were 0.8 and 0.1% as germinating tuber density and number of off-shoots approached zero (i) and 75.7 and 66.1% as the variables approached infinity (a). Estimated yield losses of wet direct-seeded rice per weed plant were 0.6 and 0.1% as the two variables approached i and 63.8 and 100% as variables approached a. Results suggest that lowland ecotype C. rotundus can grow well and reduce the growth and yield of rice under flooded conditions. Although the findings were obtained under screenhouse conditions, the results imply the need of developing a holistic weed control strategy against the weed.