Are you guys (pardon my french) beating the crap out of them first before you wash them?
I nearly throw my neck out every saturday banging the towels along the fence in the back yard to get the poop off.
By the time I'm done there is little to no solid dropping left on them, just the poop color "stain" if you will.
So with one wash on the hot & heavy cycle (lol, that sounds funny

) they come out completely clean without needing a second wash. There is never even any residue in the washer afterwards.
I wouldn't say I have an expensive/fancy washing machine, either, it's somewhat dated.. at least 6 years old if not more.
I guess it's more the elbow grease before hand of getting the towels as clean/rid of poo before they go in the washing machine, but I'm so confident with how clean they are I've thrown in the door mats/rugs with them too and everything still comes clean with nothing left over..
Also, since I move along the fence as I do it, and I do it once a week, the poop biodegrades fast, especially if it rains, so it's not like I have bird crap outlining my back yard ;)