Good day all. I believe horses find beet pulp very digestible and thus it works well with the hard keeper or one that is dentally deficient. I also believe it's imperative to soak it prior to feeding (either in a one to one or two to one ratio of water to pulp). It is very absorbant and expands greatly in size when soaked. Thus if eaten dry in large quantity there is risk of expansion in the horse's stomach when water is taken in after the fact leading to a mighty ache. Golden Horse, in your case, it sounds the quantity your horse ingested was not in itself alarming and I'm wondering if what did happen was that your boy was perhaps a little anxious or nervous about a new learning situation and was doing a grab and gulp rather than chewing properly -- all of which will change once you progressed with his trailer loading skills.
The Myths and Reality of Beet Pulp - Susan Evans Garlinghouse
"Many horse owners are also concerned that, due to the amount of water that beet pulp soaks up, and the volume that it expands to, a large meal of dry beet pulp will somehow cause the stomach to swell up and rupture. A simple explanation of the equine stomach will allay this particular concern. The capacity of the equine stomach is 2-4 gallons, equivalent to approximately 4 ½ to 9 ½ pounds of dry beet pulp. Movement of food from the stomach to the small intestine can vary depending on a number of factors, but as the stomach begins to reach maximum capacity, stretch receptors in the walls of the stomach will trigger the release of motilin, a hormone which in turn stimulates the emptying of the stomach and passage of food into the small intestine, cecum and colon. As the capacity of the gastrointestinal system-approximately 38 to 48 gallons-is more than sufficient to adequately contain even a very large meal of beet pulp (or any other feed), the only horse in danger of a gastric rupture is one suffering from impaction or other severe lack of normal peristaltic movement. " (not by me, copied from The Myths and Reality of Beet Pulp - Susan Evans Garlinghouse)
Feeding Myths Debunked