We call it deferred grazing here (ryegrass and clover) and basically what happens is the grass grows tall, seeds, dies and falls over and an undergrowth of clover, weeds and less mature grass comes through the dead stuff... depends what your growth rates are and how long you leave it ungrazed.
Defering from late spring through to early autumn if you have a surplus, can be a useful way of allowing a pasture to renew itself via reseeding while storing up some low quality tucker for low production animals to eat at a time when the summer dry has drastically reduced pasture availability.
