I wouldnt breed her as nivacular is passed on to yougstock hence why theese young horses get its not from jumping big tracks young and it only gets worse best option would be to sell her and start again. I have seen horses get slowly worse from nivacular and it is defantly not nice in the slitest the diese slowly eats away at the nivacular done until it is almost none exestint causing horses a lot of pain
exucse my spelling
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Spelling is excused, but not your assumptions!
Just being cheeky there, but would like to give my perspective on some of what you've said... Firstly, remember even for those that have studied this problem in depth, there are still a few unanswered questions about it, and there are also many incorrect or dubious assumptions abounding still. So IMO we have to be careful about being too definite & closed about what we 'know' about it.
Genetic factors; of course genetics plays a part in everything, and some breeds - such as heavies & arabs for eg do tend to innately have stronger caudal hooves than other breeds, according to research done(not sure how widespread that study...). There are also some unfortunate lines of horses, such as some QHs who have been bred to have extra small feet for eg, who tend to be innately problematic - should not be allowed to breed IMO.
BUT generally speaking, the evidence seems to have mounted far & away in favour of environment(eg. management, footing, hoofcare, work, etc) being by far the biggest 'cause' of strong or weak hooves, particularly in the caudal region. Dr Robert Bowker of University of Michigan is one person who has done a lot of research in this area and you can actually read the basics in a paper you'll find online titled "Contrasting Structural Morphologies of 'Good' and 'Bad' Footed Horses"
....So above explains 'hence' why young horses DO INDEED develop problems such as this from jumping or galloping young. One more interesting point that seems to be rather recently gleaned is that even healthy, well conditioned(such as mustangs for eg) horses don't BEGIN to develop caudal hoof strength(the fatty tissue of the digital cushion doesn't start to become fibrous cartilage) until the horse is around 4-5 years old!
There are many environmental factors which influence this development and unfortunately many horses never develop much strength & energy dissipation capacity in their heels. This leads to caudal hoof pain(NS), when the horse is forced to do more than his feet are capable of, and leads to problems of the actual bones(ND), tendons & joints(particularly navicular & P3 bones), which seems to be largely due to lack of circulation & lack of shock dissipation in those areas.
and it only gets worse best option would be to sell her and start again
Aside from me personally having a moral problem with just palming an animal like this off, especially selling her for money, 'navicular' is not at all necessarily a death sentence, or even a career finisher. Unfortunately conventional treatments(developed long before current understanding) are effective only palliatively - they (usually temporarily) treat the pain but not the cause, so the condition continues & can worsen. But with the right treatment there is evidence that it can be reversed. It is now clear that with good management(and time) horses can return to (pain free) usefulness, even assuming bone & tendon damage/changes couldn't be reversed. But recent research on bone remodelling(again you'll find some papers online; Dr Mark D Fisher's is one) suggests that even this may be able to be reversed!