[Lunatic666]

With perennial rice you lose the option to use the field for something else. Also timing the sowing with the rain is important.

(Source: my wife's family are rice farmers in the Northeast of Thailand)

[gus_massa]

Here in Argentina I think we plant in the same spot soy and corn, or soy and sunflower, or other combinations. Each crop is better for the temperatures and rain of each season. So I also guess it's more effective to use a annual crop (that is actually only a few months in the field, not the whole year).