It yields earlier and seeds more freely, so when they run the harvesting process, all of the weedy rice hits the ground, which reduces the total yield per acre by whatever % weedy rice is in that field. So if weedy rice eats up 5% of the field then at minimum that’s 5% of the field lost, before the mixed-harvest problems.

It has a harder composition, so that the force used to turn it into white rice is damaging to most other kinds; and if processed as brown rice, will have a longer cooking time than the planted crop it’s mixed with, leading to uneven cooked results. Like mixing two pastas with two different cooking times into the same pot.

“There are a number of ways that weedy rice can impact rice production:”: https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=2293...