Sean I think I'd be more inclined to buy some beets at a farmer's market, or even the local grocery store. I don't know about feeding horses, but I believe that there are different varieties of beets used to produce sugar, that don't look (or taste) much like those we're used to. Beets are pretty darned easy to grow, however -- we've grown them off and on for 20 years or more. They only need about 60 days or so, I think - a lot shorter season than most vegetables. And they are not especially susceptible to pests, as far as I've found. About the only annoying aspect is the need to thin them after germination, because of the compound seeds -- you plant one, and four or five little beet plants germinate. But after that, they're really pretty foolproof. Sunlight and occasional watering - they are not fussy about soil conditions, either. But if they don't work for you, buy some. I have to think that beets intended for human consumption would tend to have better color and flavor than beets grown for livestock fodder.
Doug