Forages going dormant is significant. USDA-NRCS defines stockpiling as 'allowing standing forage to accumulate for grazing at a later period, often for fall and winter grazing after dormancy.' The key word here is dormancy.

Once dormant, they can be grazed with less harm to energy reserves. Forage plants are sensitive to grazing in the fall. That is why I usually promote the use of annuals or crop residue, or ideally the combination of the two in the early fall. It allows the pastures to recuperate and build reserves.