It is the period of most rapid growth. During phase II, leaves contain sufficient protein and energy to meet the nutritional needs of most livestock. Remember phase II - high quality and high quantity. As plants continue to grow, leaves become more and more shaded. Lower leaves die and decompose.
Leaves use more energy for respiration than they can produce through photosynthesis.
Phase III material is stemmy and fibrous. Nutrient content, palatability, and digestibility of leaves in phase III material is poor. Remember phase III - low quality but high quantity. In phase III, shading and senescence begin to detract from efficiency of photosynthesis. But the effect on the paddocks would be much different.