while alfalfa does have its benefits, it’s certainly not a good forage to feed as the only forage. i’ve found best results with free choice quality grass hay - usually a timothy mix (that will sometimes have some alfalfa in it).
next, i shy away from beet pulp unless the horse needs the added moisture in their diet and/or it’s the best delivery for any supps a horse may need (such as if the horse really likes the beet pulp or something like that). on its own, beet pulp is just a filler and won’t put quality weight on a horse but WILL add water weight (i.e. "bloated weight") making a horse appear fatter without any additional quality nutrition. rather than beet pulp, i’ve had great success with a buckeye feeds product called ultimate finish shine & win 25% that’s a pelleted fat supplement. also in the way of feed, for a horse to put on weight, look at the fat content and always always look for a fixed formula (meaning the company won’t change the quality or the quantity of certain ingredients based on availability - purina for example is NOT a fixed formula for most of their feeds - correct me if i’m wrong here and they have recently changed this). for the most part for weight gain i’ve found something with a 10% fat content or higher will work better. the higher the fat content the sooner the horse will put on the weight - but be careful that it’s not a high carb feed (where the fat comes from sweets/sugar) as that kind of fat will be burned off and not help. i tend to prefer a pelleted feed over anything textured/sweet for just that reason. the fat content will be more readily digested and accepted into the body, thus helping the horse put on healthy weight faster that will stick to the horse longer.
finally as for BOSS, that alone is not a complete supplement. i’ve found it works for some horses, not for others. it is high fat, so certainly worth it, but make sure that you are feeding enough for your horse’s needs and that any other nutritional needs are being met through free choice hay (or as much quality forage as possible that isn’t JUST alfalfa), and whatever feed program you have.
good luck getting the weight on! if you’re looking at weight gain supps, i’ve also had great success with vapco’s fat cat that i order through equisupps.com for my hard keeper when he was in regular work. it added calories in quality not just quantity and didn’t make him hot or hyper.