What a seed bank actually is
Many invasive weeds and brush species produce seed that stays dormant in the soil for years — sometimes decades — waiting for the right combination of light, moisture, and disturbance to germinate. This dormant reserve is called a seed bank, and it exists whether or not the mature plant it came from is still standing. Clearing the visible plant, by any method, does nothing to the seed already banked in the soil.
This is a universal limit of vegetation management, not something unique to grazing — mowing, herbicide, and hand-clearing all face the exact same problem. A property cleared once and left alone will regrow from its seed bank regardless of which method did the clearing.
Why repeat, seasonally-timed grazing actually works
The way to shrink a seed bank over time is to prevent the plant from reaching seed-set, year after year, until the stored reserve is exhausted or reduced to a manageable level. That's why professional prescribed grazing on an invasive-control project is typically planned as a multi-year program with grazing timed before the target weed sets seed each season, rather than a single clearing event.
Understanding this changes what a realistic expectation looks like: a property with an established invasive-weed problem should expect visible improvement after the first pass, but real, durable control usually takes two to four grazing seasons, with each pass timed to hit the target plant at its most vulnerable stage.