Sorry I'm going to (possibly) disagree with the others, although I find it hard to tell from your description. You said it was healing with no heat or swelling but now is open and yellow. If the scab has come off and it doesn't scab over again quickly, or the scab keeps coming off it could well be that it is breaking down due to infection. When you say yellow, is it clear 'runny' yellow in which case yes it could be serum from having the scab knocked off but if it is thick or cottage cheesy then this sounds more like infection.
I think I missed whereabouts the cut is? As the position can make a difference to how well it heals.
Yes skin is skin, but horses are out there getting dirty and flied up so they are more prone to getting infection. And the flies are evil at the moment, at least around our way.
It is up to you and as I say it's hard for me at least to tell from your description, but if it were me (and I have done this today in fact) and it's been going on for a while I'd be asking the vet for antibiotics, in my humble experience I've battled with cuts before that look like they're healing and then keep falling apart, tried all sorts of creams, cut n heal, wound gel...and eventually resorted to a few days antibiotics which has sorted them out (and wished that I'd done that sooner). Now they only get one chance to heal themselves then I reach for the phone. Of course, there is a cost attached and so if it's not been that long (a couple of days) that it's not healed then it may just need a bit more time, try some of the creams on here to keep the flies out, just make sure the wound is clean underneath otherwise you'll be trapping in any germs.
Sorry forgot to say, the advice these days is not to dry out cuts and that they are better in a moist environment so I'd go with the dermagel that you have rather than anything drying. Flamazine is also good for healing but again I think you will have to ask your vet for it although I don't think it is a prescription on ly drug so you might not need a call out to get some.