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Had my 13hh pony PTS by injection - approx £175 vet costs and £350 cremation costs (no ashes returned).
Had my 16hh horse shot, and "economic disposal", £200+VAT, by the same disposal company as above - they don't charge for shooting.
I chose the best method for each horse - the latter one hated vets and injections, so as distressing at it was for me, I chose to have her shot.
FWIW, I was hoping to give both of them a last summer - both were old, with failing teeth, and I felt it was the right time for them. It didn't work out like that though. The pony went downhill quickly, and with the weather getting hotter, she was PTS in July 2006, just before we had a massive heatwave which she wouldn't have coped with. I was glad the timing worked for the best. The horse didn't go downhill, but she lost her sparkle, and didn't seem her usual self. I was struggling to find feeds she could eat, and I was determined that she would die with a full belly and the sun on her back. I made the decision to let her go sooner, rather than risk her getting fussy about her feed, and ending up hungry. She was PTS in April this year, during a bout of lovely warm spring weather. I couldn't have asked for better for her.
I totally understand that you want your horse to have a last summer, but sometimes they make the decision for us. Definitely worth having a contingency plan in place, in case he does worsen quickly.