Iso,
hillrancher hit it pretty well.
Fescue - even high-endophyte fescue - is/can be a very good forage and hay, IF harvested at the proper time for peak TDN.
But, when folks here in KY wait until the 4th of July or Labor Day to cut it (we call it 'Holiday Hay'), TDN and protein levels will be in the toilet. I have seen heavy pregnants and fall-calving cows tough it through the winter on sorry hay of that sort(and nothing else) - with TDN<30 and CP in the 3-4% range - and just 'run out of gas', with green grass right around the corner.
Old-timers will say of that 'holiday hay', "It's better than a snowball!" - and it may be - but not much better. They can literally starve to death with a belly full of crappy, lignified 'residue'.
Performance on fescue hay has got nothing to do with 'being adjusted to it', and everything to do with forage quality at the time of harvest.