Fiber and lignin is increasing in the plant which makes nutrients less available. Recommendation - As seed heads appear, speed up the grazing rotation to get ahead of the growth, or set aside some paddocks for hay or silage harvest. A key is to harvest the set aside paddocks soon. This will keep them near the same maturity as the other paddocks during the next rotation. Plants that are allowed to reach maturity will regrow much slower.
Tall plants are difficult for the animal to graze rapidly.
Recommendation - Try not to allow forages to mature to this stage. If they have, mechanically harvest them to allow new growth to begin. Attempt to have 2 to 4 inches of leaf remaining in the paddock after the animals are removed. This will increase the rate of regrowth of the forages and increase total production over the year. It's easy to see why grazing forages in the boot or prebud stage maintains quality and quantity of the plants.