We bale and wrap hundreds of bales of haylage, silage and hay.
As they are being stacked sometimes they get a rip in the wrap (we use 2 layers) and if air gets in during the fermentation period you'll get mold.
I go round with a big roll of black sticky tape, bought specifically for the job and tape up any holes.
If theres a not a lot of mold I feed to the bullocks, they pick amongst it, sheep are a bit more picky and will eat around it....and woe betide me if I sell a bale to the horse people, they throw fits !!

I find birds are a damned nuisance, particularly magpies, for pecking holes in the wrap, usually when its newly wrapped, and the wrap is shiney. If you miss these bird holes you'll get mold, for sure.