That is GOING to get infected unless you dress it somehow. Too late to stitch it now, even if stitches were going to hold it...
Honestly I would wrap the whole foot, and pastern, and fetlock. Tape it at the top with an adhesive tape or bandage that will stretch a little but not a lot. And change that dressing at least every 2 days, preferably daily.
AND I would not keep her in the pasture as too much movement will inhibit healing and may prevent it entirely.
I will share a story of a broken bone - my mother's - to put into perspective how much movement can damage the healing process.
She came off a horse, snapped her humerus. Had a plastic removable "cast" put on it, which was moulded to her arm shape so that it sat the break nicely for it to knit. Was given physio exercises to make sure she didn't lose muscle while it healed.
Did the physio exercises. Every single day, as directed.
The bone would knit, and then when she did the exercises it would snap again from the movement. Eventually it gave up trying to heal. 6 months later, the specialist decided she needed surgery. 5 weeks after that, she got in for her op. It's since been ANOTHER 8 AND A HALF MONTHS and it's STILL not 100%.
Movement inhibits healing. And may stop it entirely. I know bones are different to skin but if it tries to heal and then is ripped open every time it begins to knit it will eventually give up trying. SOME movement is good, but too much is detrimental. Pasture will not be good for this mare, a stall and hand-walking daily will.
A CLEAN SLATE FOR THE FUTURE