Supplements for an older horse

My horse Dunlap is right around 17. In the time that I've owned him he's been in good health... Nothing more than scrapes or cuts. My riding consists of mostly light ranch work and trail riding and my boyfriend does occasional (few times a year) rodeoing with him. I want to make sure Dunlap is healthy and feeling good so if anyone could recommend a good all around health supplement I would appreciate it. Thanks!
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