Goodness, I have seen lots of horses barned and they are never kept in the conditions seen at Spindles farm.
If you are suggesting that horses normally turn a base into pure excrement in a matter of hours, then I expect there are far too many horses in a small space with inadequate bedding.
If you sew the plaits, you can also be clever about how they sit on the neck to flatter it for showing.
You can leave the start of the plait slightly less tight to allow the mane to bulk up a little to disguse a poor base to the neck.
Not that I'm saying that about your horse, btw, it's just a...