Beet pulp is a regular item in all my horses' diets. I like feeding it for a variety of reasons. As with any other new food, it should be introduced to your horses gradually and slowly. Some horses may like it more than others. It can be fed alone with vitamins (to really fat horses) or in combination with your feed.
Here are some reasons why I feed it:
1) hydration - especially in the winter when they tend to not drink as much. When soaking beet pulp shreds or pellets, make sure to cover the dry product completely with water then wait 15-60 minutes so it soaks up as much water as possible. Some horses will enjoy eating their beet pulp very wet, almost like soup, while some will prefer some of the water drained off. Either way is okay.
2) Calories. Beet pulp contain a good amount of calories. For horses who have a hard time keeping weight on, like older horses or nursing mothers, beet pulp can provide the extra calories without the risks of adding grains which aren't digested as easily. Beet pulp can be fed in amounts that equal up to 40% of the total diet by dry weight -- and that's a lot of beet pulp. Minis should probably be kept to under the 1 lb level (but 1 lb of dry weight beet pulp is a LOT of soaked beet pulp!). Fat horses can benefit from beet pulp, too, when it is fed IN PLACE of grains. Just add vitamins/minerals and some grass hay or grazing, and your horse has a satisfying meal without the addition of grains. Many of my personal maintenance and pet minis are on an all-forage diet like this one.
3) Carbohydrates. Beet pulp contains carbohydrates just like grains, but in a different form (pectins rather than fructans). The carbs from beet pulp provide slow release energy whereas the carbs from grains like oats provide quick bursts of energy. That's one of the reasons race horses eat a lot of oats.
4) Fiber. Horses thrive on high fiber diets. However, in our minis, a lot of hay and grass can sometimes lead to big bellies (via gut fill). Horses need a lot of fiber in their diets to make their guts work properly, so every day they should have at least 1% of their ideal body weight in long stem fiber (hay/grass). Beet pulp can susbtitute for part of that, however, so that's why a lot of show horses are fed beet pulp -- so they can still get the fiber their bodies need, but they don't get the big bellies that a pasture horse might get. Beet pulp is also highly digestible so doesn't sit around in the gut as long as hay. When hay sits around in the gut getting digested, it forms gas and fluid which swells the belly. Beet pulp doesn't do that.
5) Carrier for vitamins. Lots of vitamins and other supplements come in powdery forms which many horses turn their noses up at. Once the beet pulp is wet, the powdery supplements like vitamins, minerals, even medications, will stick to the beet pulp and make it more difficult for the horse to sort out.
As far as when to feed it, beet pulp can be fed year round. Many people will feed more in the winter and less in the summer, especially if they have pasture. Beet pulp contains a good amount of calories so in the winter when the grass is gone, it can keep weight on your horse and in the summer, can be cut back to just enough to carry supplements.
I'm sure others will add their experiences with beet pulp. Here is a good link for some excellent information on why and how to feed it:
Myths and Reality of Beet Pulp
Robin C