We do not add glucosamine or chondroitin sulfate to our equine formulas. Prescription and over-the-counter (OTC) medications need to be administered by body weight of the animal. Feeds that contain OTC arthritic medication thus would have different levels of the ingredients relative to how much is being fed. For example a horse fed 2 lbs. of a commercial formula would receive – the glucosamine or chondroitin sulfate of a horse that is fed 4 lbs. In addition, studies strongly indicate that feeds containing glucosamine or chondroitin sulfate vary greatly in levels and in the vast majority of products are less than what is actually advertised on the product.
There is also the question of effectiveness. The injectable forms have demonstrated success but the oral forms are only supported by anecdotal stories. I wish I could provide more encouragement to the benefits. Also, there is zero evidence that administering oral glucosamine or chondroitin sulfate will prevent joint issues. Even if you do decide to use one of the OTC products you would be better serve to top dress the ingredients and feed relative to body weight.