Abstract:
Weedy rice (Oryza sativa f. spontaneae) is a recently emerged major threat in many rice-growing areas in
tropical Asia including Sri Lanka since 1990s. Though there are scientific investigations on origin, diversity,
distribution and controlling technique of weedy rice in regional and global scales, studies on the interactions
between weedy rice and cultivated rice varieties under local situations are limited. A green-house experiment
was conducted at the Open University, Nawala, Sri Lanka (30oC, 40% humidity) in 2015 using an improvedcultivated rice variety (Bg379-2) with varying percentages of cultivated: a black-hulled weedy rice bio-type
(0, 20, 40, 60 80 100 /pot). Influence of weedy rice on growth and yield of Bg379-2 was assessed by measuring
plant height and number of tillers through 2, 4, 6 and 8 weekly. Dry shoot-and root- biomass of uprooted plants
at 8 weeks after sowing (8WAS)were determined through oven-dried to a constant weight at70 º C for 72 hrs
and weighed to the nearest 0.01 g. Yield components of Bg379-2 including mean weight of 100 grains/plant,
number of panicles/plant, number of grains/panicle and yield weight/plant were calculated. Data were
statistically analyzed using SAS Ver. 9.0. Significant interactions between weedy rice bio-type and Bg379-2
were apparent after 6 weeks after sowing (WAS). A yield loss of 30% was observed even at a percentage of
20 of weedy rice at 20 /pot. The competition of weedy rice resulted in 90% loss of yield of variety Bg379-2at
80% plants/pot. For the purpose of generalization, further research is recommended to determine the effect of
weedy rice density on growth and yield components of different new improved rice varieties.
