Clover isn't good for them to eat too much but as far as I know it doesn't cause any external problems unless he is allergic to it. Are there many buttercups?. My fields are covered in clover, I am spraying to get rid of as much as I can because I have horses with metabolic problems.
Do you feed alfalfa? My gelding had terrible photosensitivity on his white leg markings one summer and after spraying for buttercups (it's good that you don't have this) and taking him off alfalfa, his skin has been much, much better. The spray for the buttercups has reduced the amount of clover in the field too.
It might be worth looking up Alsike clover (Trifolium hybridum) and see if the pics online look to be what you have in the field. Alsike clover may cause photosensitivity which can certainly manifest itself as pastern dermatitis. Worth a quick look perhaps?
I feed him 2 feeds of approx 500g of saracen releve and lucerne pellets are one of the ingredients. It's the only feed he will eat mixed with all his supplements. I have fed him Alfalfa from dengie before with no problems. I'm wondering if it's the soil.
If you've got lots of clover then I would Google Mycotoxins and start feeding a toxin binder. My 3 horses were poorly last year, vets couldn't figure out what it was, only happened in one particular paddock. It was only after we left that mentioned it to the vet about the paddock/clover and he came back with mycotoxins. They can cause all sorts of internal and external issues.
I started mine on Mycosorb and they are doing amazingly well, including the one who had hind gut problems that the vet thinks were made worse by them. They are at a different yard now which also has clover and we've got no probems at all, it's quite a difference from this time last year!
Clover is high in sugar so not good for fatties or laminitics! I have a lot if it and my girl has ignored most of it, the only real problem I have is that the flowers make her sneeze a lot which is cured now as they have died off.
I loathe clover-as above it's awful for fatties (think sugar-coated lard!), horrid for flies and I've had it cause slobbers and photosensitivity (white clover, just way too much of it). The only thing that helped to bring it under control was constant topping and fertilising the ground.
It was the only time my last horse ever got pastern dermatitis in 7 years-worse than buttercup IMO and I had that as well. One of my ponies is a pig for the stuff as well.
Clovers easily sprayed out with a specialist herbicide I do mine every few years when it begins to look an issue .
Horses don't like clover if you watch them they chose not to graze it unless they need to .
I don't have any skin problems, mine all have slobbers (slaframine poisoning) due to our warm and wet weather recently, basically, it looks as if I have a herd of rabid horses
Many people just leave the horses on the pastures, I can't stand to do that, so I have them on dry lots eating double the hay they usually do :/