Join Date: Jul 2011
Location: Bethel, Ohio USA
• Horses: 0
That "groove" is called the central sulci of the frog. The tissues of the frog are subject various anaerobic, opportunistic fungal and bacterial organisms. Those tissues are also subject mechanical stress created by load imbalance in the hoof.
Excess length in the caudal aspect of the walls (heels) contributes to a loss of passive stimulation of frog tissues and subsequent atrophy. Mechanical imbalance increases the depth and width of separation at the central sulci, sometimes shearing the heels proximal of the wall and into the area of the heel bulbs. The reduced health of those tissues increase the risk of microbial intrusion. The deeper infection at the central sulci is thrush. The reddening area closer to the heel bulbs is a general dermatitis called "scratches".
While I don't think it's applicable in this case (better photos would provide improved feedback), your farrier will want to assure no indications of canker. That is best done after the foot is cleaned up and better visuals are available.
Correct trimming and any potential orthotics are best applied by your farrier. Diagnostics and treatment of infection beyond routine thrush is better addressed by an attending veterinarian.