You need to keep in contact with your vet about how to treat your horse's wound.
That is a very nasty cut to go putting sugar in it, please talk to your vet, s/he knows better what to do, than anyone on the internet.
I think your vet needs to stay very pro-active with this wound.
Did s/he give you instructions on changing bandages? How often? What topical meds?
Did the vet give you or tell you what bandaging material you need, telfa-pads?
I personally would have that wound wrapped from fetlock to hip.
That wound can go south in a hurry.