Neither overgrazing or undergrazing is desirable. The rate of plant recovery depends on the growing conditions. When plants are severely grazed their recovery is slow. When grazing is less severe, the recovery is relatively rapid. The forage consumed and the quality of the diet changes during an animal's stay in the pasture.
Cows are gourmets.
They graze selectively, eating the best plants and plant parts first, avoiding coarser, less palatable, less nutritious feed. As the days pass, the forage gets older and less digestible, and stock spoil more and more grass through trampling and dung and urine contamination, so they eat less. In heavily stocked continuously grazed pastures, regrowth will be grazed as soon as it's available. The phase 1 regrowth is highly nutritious, but there is generally not enough of it to support high levels of animal production. If animals are forced to eat phase III material, which passes through their gut very slowly, their daily intake will drop because they simply can't fit any more feed in their rumen.