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that looks pretty bad, put some anti-septic on it and stitch it up yourself or taker to the vet
Your dilemma is why everyone should have a skin stapler,I got one from my vet and instruction and hands on training on nailing it shut,it's very easy to do and the few times I've had cause to put one back together it's come out fine.You line the cut sections up and start stapling at the top of the wound,staple down and leave the bottom 1/4 inch open so it can drain and not create an abcess.I hit em with cephalexin antibiotics afterwards and about a week later ascess the healing and if it's looking good I take the staples out. The wound your dog has needs staples or stitches IMO. 5-10 of them would be my guess.
IMO - stitches but you might have waited too long now and the skin flap will go necrotic and need to be cut off. Stitching is easy to do with the right stuff and the stapler is easy too. Build a canine first aid kit and the skills to use it. It will more than pay for itself for a field dog.