Thank you for the information. There are three things that come to mind with the history you detail.
1) Foot rot. Foot rot starts in the interdigital space and then can work its way north on the leg! Many times we see clinical cases with swelling of the hoofline and fetlock. They can erupt into infection, etc. Check in between the claws in the interdigital space to see if it may be underlying footrot. Stick your finger in there and feel the tissues (are they cracked, red, and raw). Is there a foul smell? Pain? That being said, if you have foot rot baths, make sure they have been mixed properly! I have seen a few instances where it was mixed wrong and led to a too strong mixture, causing irritation, infection, and swelling. Here is an article concerning foot rot as well as foot bath mixtures and treatment, etc:
2) Digital dermatitis (Hairy heel warts). This only affects the skin and typically is the hoofline and up. usually does not affect the interdigital space. This is usually controlled with an oxytetracycline spray. Here is picture of hairy heal warts: http://images.google.com/imgres?imgurl=http://www.prairielandag.com/wp-content/uploads/2009/07/cattle-hairy-heel-wart.gif&im grefurl=http://www.prairielandag.com/cattle-hoof-care&usg=__oLiwFf710xIoLe7vx4UHzyZmidE=&h=150&w=200&sz=29&hl=en&start=1&um=1&itbs=1&tbnid=UX82MbYU9lUmDM:&tbnh=78&tbnw=104&prev=/images%3Fq%3Dhairy%2Bheel%2Bwarts%2Bin%2Bcows%26um%3D1%26hl%3Den%26sa%3DN%26tbs%3Disch:1
3) Endophyte infected fescue pasture leading to fescue toxicity. This causes a decrease in blood circulation, especially in the extremities. It results in swelling that can erupt into infection and then eventually heals itself. I would take a walk around the pasture and look for any fescue in the pasture that they could be getting into.
Typically is a cow gets acidotic, they can have laminitis in their hooves. I have never seen a case get to the point where there is swelling and the swelling erupts into infection. Acidosis would be seen with dropped milk production, decreased feed intake, diarrhea, displaced abomasums, lameness. Make sure there is enough fiber in the diet and that the roughage is of good quality and has enough length on it (adequate stem). Be careful of the amount of carbs and concentrate available. I don't think this is a nutrition issue, but rather one of the above.
If you still have more cattle coming up with this problem, then I recommend having your vet out to check out a couple of the cows.