In regard to weed control— at what maturity or stage of development are seeds viable?

I try to control the foxtail and other stickers population on my property by mowing, burning, and weeding before they start to make seeds. However, I wonder if the seeds are even viable at the stages I often decide to weed them by hand instead of mowing them and spreading the seeds.

What level of maturity do the seeds become viable? Is it once the plant is fully developed and starts to die? Or is it much sooner? Thank you