Dear Khrossed -- The answer to your query is "no". Although diet does have a significant impact on growth and development, I know of no dietary deficiency (or toxic substance) that specifically causes a horse's back to drop or sway.
The usual cause for this is weak deep vertebral ligamentous sheathing, which is inherited from the parents, who may or may not show low backs themselves.
If you have a 2 year old horse that already shows a low back, I would step back and take a look at his whole conformation before deciding what to do with him. Particularly, you will want to look at the neck, the tail, and the chest:
(1) Tail: After familiarizing the horse with having his tail handled (so that he does not feel inclined to protest or kick), gently lift the tailbone, grip it, and lean back and pull on it with a gradual pull. What you are testing for here is to see if the tailbone feels stretchy or rubbery -- if it does, this is a very accurate indicator of weak deep vertebral ligaments. Also, note the thickness at the top of the dock -- stronger backs have thicker docks. Compare to a variety of other horses to give yourself a scale of comparison.
(2) Neck: How long is the horse's neck? Longer necks sometimes presage weaker backs, particularly if the base of the neck is set low (i.e. functional ewe-neck).
(3) Chest: Narrow body also tends to signify weaker back. Are both front legs "coming out of the same hole"? HOw much width is there between the inner surfaces of the horse's upper arms, at the height of the chest?
You might also like, in your reply to this, to tell me why you ask the question. Does this pertain to a horse you own? Are you wondering what he might be good for, or if it's OK to begin riding? Let me know. -- Dr. Deb
This reply may apply to my inquiry in another topic (Whip-like hoof movement) regarding my 5-yr-old Connemara who lacks back. I've always been fond of the 'double-back' with the crease down the spine, which is common in this breed. Thinking it was a sign of lack of muscling, Dr.Deb corrected my erroneous thinking in her reply saying it's the exact opposite. If you recall, I said he had a low back when I looked at him as a long 2-yr-old.
I did the tail pull on him, and it seems that it does have a rubbery-ness to it at the level of the dock. His neck has been amont his best features, and I will re-read the chapter in Conformation Analysis, but I believe his neck is okay. His chest is quite adequate with space between his legs when viewed from the front.
If his low back (looks like an inverted "V" from a 45-degree angle looking from his tail) is indeed caused by weak deep vertebral ligaments, is lunging over cavaletti as outlined in your book the answer? To include more detail, I notice that when I take off the saddle, his back seems even lower.
Perhaps ground work is in order, but I hate the thought of not being able to ride him. I will do whatever I can to remedy this situation...I love this horse. Kathy