Competitive ability of weedy rice against cultivated rice in the Philippines

Weedy rice is an important problem, particularly in direct-seeded rice where it is difficult to manage. Weedy rice is made up of the undesirable biotypes of rice with early and easy-shattering characteristics. These biotypes look very similar to cultivated rice but they actually cause reduction in its yield and grain quality. Greenhouse experiments conducted in 2008-2009 dry and wet seasons evaluated the effects on grain yield of rice of different densities and different period of removal/rouging of weedy rice. Results from all 2008 experiments showed that weedy rice densities of 5 and 20/m(2) reduced the number of panicles of cultivated rice (PSB Rc82) by 5 to 14% and 13 to 45%, respectively. Number of filled grains were reduced by 3 to 16% during dry season and 31 to 72% during wet season. These adverse effects caused up to 33% reduction in grain yield under a weedy rice density of 20/m2. For the 2009 wet season experiment, weedy rice that was allowed to grow after 45 days or later caused significant reductions in yield and yield components of cultivated rice. Weedy rice that grew from 30 to 75 days after sowing (DAS) with cultivated rice reduced the number of panicles by 13 to 26%; number of filled grains by 14 to 43% and percent of filled grains by 3-13%. Presence of weedy rice for more than 60 DAS in cultivated rice badly affected grain yield by 6-8%.