Just to review- rice weed control is complicated. I recently killed a small patch of rice because I forgot about a nit-picky restriction on one of the herbicides we were testing. So, pay close attention to label warnings. It will keep weeds from germinating, but really won't kill weeds that are already growing. Levee building problems, pumping problems and late flood establishments are one of the biggest weed control problems in rice.
Timing is critical in rice.
In general, grass cannot be allowed to get bigger than the 3 leaf stage. You need to think in terms of a fuzzy green carpet instead of thinking about grass plants. There are inexpensive weed control programs, expensive programs and really expensive programs. Inexpensive programs can work, but they quickly become the most expensive ones when when they fail. Expensive, properly executed programs are cheaper than clean up programs.