that's why i disband the army, in friendly territory if possible, as soon as they get too battered up from the fights and then either focus on defense abit while they recover strength or call fresh lords available that didn't join the first army and are not currently busy in another AI army.Not at the expense of the quality...
If you're sending fodder in to just be killed from your AI parties, then your AI parties end up leaving the battle with like 10~ troops leftover, while you still have 200. They then encounter one other AI party (probably merc party), and they're instantly captured, and no more AI party, which means you have to go roaming around to go find them, in the middle of a war.
It just isn't practical to make an army.
Most times i don't really take more than like 50 guys myself leaving the rest in garrisons to not waste them, they are all high level so will resist more (specially using RBM mod) and i don't have to worry about too much expenses with soldiers if i can easily summon 1000+ men armies just using a bit of influence that i soon recover in a few succesful sieges.