6 quarts of cubes isn’t likely more than a couple pounds, but it all depends on their size - big or small. The smaller they are, the more 1qt will weigh. Not knowing what you were wanting to accomplish with the alfalfa, I can’t say how much to give her. More calories? 3-5lb. Mixing in supplements? 1lb or so.
I suggest you watch this before you do anything. https://youtu.be/hrZgtrqbMVI Posted in this section and very eye opening in spotting how horses tell us they are in pain and showing us that many experienced/ professional horse people have no idea when the saddle fits and when it doesn’t.
4 Nexium a day in her dry feed ration for 30 days won’t cost much and won’t hurt her a bit if you want to try and see if it helps. If it helps go a month and then taper off.
ETA: your trainer needs a saddle that fits her and the horse if she is doing all the riding? Stuffing herself into your saddle can’t be doing the horse ( or her) any favors…
Interesting article by an equine nutritionist you might find helpful. I didn’t see if it gave an amount.
But it appears the downside of pellets is the reduced chewing, as opposed to hay, which promotes protective saliva.
There’s also the idea of the stem, either hay or cubes, vs pellets creating a “mat” in the gut that stays there longer and offers a physical barrier to stomach acid.
I knew a horse like this. He was a lesson horse at a barn where I boarded and worked many years ago. TC was as saintly a creature as you could ever ask for. He packed tiny kids around (despite being a big boy himself) and was always so careful. A true babysitter who was worth his weight in gold.
He belonged to a lovely woman who let the barn basically free lease him and use him in the lesson program. She knew he’d be well cared for and that it would be good for him to stay active (as he was getting on in years and had some creakiness as one often does). Her one rule was NO ALFALFA. She promised that he would become an unrecognizable fire breathing dragon if he got so much as a half of a flake of alfalfa.
I don’t remember how or why he wound up getting some, but boy! Sweet ol’ TC became bug-eyed, fidgety, and scared of his shadow overnight.
Took him off the alfalfa and he was right back to his bombproof, reliable self.
I know my horses over the years have had a little more oomph and reactivity when substantial amounts of alfalfa are added. I am careful with it now with my current gelding because he tends to present as magnesium deficient (he’s supplemented with MagRestore, which does wonders for him). I worry about the calcium content of alfalfa with him. Not sure if it would matter, but I prefer to play it safe.
I wondered about magnesium with the OP’s horse. It sure makes a difference in my gelding.
Forgot to add before that Previcox makes mine ulcery. He can tolerate 14 days and then he’s a mess. If she started having these symptoms after beginning the Previcox, I’d also stop it and see what you get.