HI,
I posted this to your first posted, but it was buried so deep I thought you wouldn't see it. So here it is again:
Based on the limited information you have provided, I would NOTsuspect feed (i.e. excess carbohydrates/protein) or feed contamination (i.e. ergot). If that was the case I would expect the issue to show up in the front feet as well (laminitis). If it was one or two feet on one cow I might suspect it was due to an injury (an injury inspires a natural increase in blood flow for healing; this blood flow brings additional nutrients to the localized limb and thus you see an increase in hoof growth). Does the outside of the outside rear-foot toes want to grow underneath the foot? ( it is hard to spot without actually tying back the foot and looking underneath) It sounds like a classic case of corkscrew foot. It is becoming an all too common malady in the Angus breed, a genetic defect where the toes' bone (primarily the outside toes on the hind feet) actually start tilting inward. the whole structure actually starts to tilt, bone, coronary band, etc. and you get a toe growing down and inward. This causes discomfort and the limb responds as if experiencing an inury (increased blood flow) and thus long toes. The outside toe will wind up growing faster than it can wear down if you are pasturing on pasture with deep soil the hoof wall will wind up growing underneath the foot instead of wearing off. Once the toes' hoof wall grows underneath the foot, it harbors and conceals manure and dirt. Trapping detritus between the hoof wall and the sole of the hoof/toe. In short time this leads to a foot abcess. Cows with this condition will often start to show it around four years old and it will start to get more severe by the time they are six. The cows can continue to raise calves, but I would sell them for beef. If kept you will have to trim the hind feet at least twice a year depending on your conditions. You WILL have to tie the hind feet up and trim off the hoof wall that will want to grow out and over the bottom of the foot. Just trimming the ugly, long toe tips off will not suffice. Next time you see your vet ask him/her about it. If they are not primarily a large animal vet you might get a funny look. If they are you should get an, "Oh yeah, let me look into that."
Sounds now like your vet is aware of the possibility of a genetic "rotation", Corkscrew foot we call it.
I guess my first thought is, " limping is not normal." Don't mean to sound like a wiseacre, but you are right to be concerned. Keep following your instincts. I've been down this road with a group of genetically related cows we purchased (Angus x) . We still have some of them in our herd, but we are sure not to keep any of their calves for replacement heifers (I don't want to keep those genetics around). We have kept the original cows around because they raise incredible calves and they all go for beef so the issue is not an issue for these cows offspring. However, it is still a concern for our operation. Anything that adds extra labor hours is a concern. These 3 - 4 cows we still have around require trimming twice a year. We work the herd twice a year so we are sure to tie up each hind foot of each cow and trim off the excess horn (hoof wall is actually horn/protein) that tries to grow out and under the outside of the outside of each hind toe in this case. Sometimes one or two of them limp afterwards and sometimes not. We usually see the limping for three or four days. It is most likely caused by bruising of the sole. After trimming, the previously protected sole, will start taking more pressure and sometimes bruise. The soles eventually harden and the cow walks normally for the next six months (in our conditions).
If you are experiencing what I have described above (a conformation from your vet would be prudent) then I would advise getting used to restraining your cows in a chute, tying up the hind feet and learning to trim them properly. If that is not in the cards for you then I would really, really recommend that you find some new cows. Again, although what you descibe sounds all too familiar, before you go through and great stress, herd, or financial changes make sure that what you are dealing with is what I am describing by consulting a good, large animal vet.
Good luck!