If he’s getting 100mg copper total (in addition to whatever’s in the hay, which I assume you don’t know, then I might suggest 3x that. If he’s getting 400mg, I wouldn’t necessarily suggest more.
It also matters how much he weighs.
At some point more might be ok, but also at some point more isn’t always better.
1-2 IU per pound body weight, total. If synthetic especially, I’d aim for 2IU/lb. So, subtract the total in the feed/supplement, from that, and add the difference
My horse has been on (natural) Vitamin E supplement for years and still has chronic scratches.
I started with 2000i.u. and increased to 3000i.u. after the six month test. His feed gave him approximately 1000i.u. when I started supplementing. The weight tape said he was 1063lbs.
Thank you for the link and the suggestion - it is very reasonable for me to get to Davis, I will definitely talk to my vet about a referral and consult there. The current outbreak has gotten even a little bit worse in the last week. 
I agree with @CindyCRNA about the Shoo Fly boots. I have tried lots of things but nothing works as well as the Shoo Fly boots for my gelding. He had zero scratches episodes while wearing them last year.
Can you tell me more about the immunotherapy shots? Were they for the scratches or for other reasons, and what was given and how frequently? Thanks!
My mare is also very reactive to bug bites. I used a fly sheet last year, but she was absolutely miserable with it (hates clothing, very humid). I did the blood allergy test, and it came back highly reactive to basically all flies and mites—I know the blood allergy test is not as/very effective, but this was a pretty obvious result to me and my vet. However, since allergies have a compounding effect, it’s possible that addressing her bug allergies lessened the effect of other reactions. I don’t know enough about this science to really speak to it, so just noting that it’s possible it wasn’t just the zinc/copper for her.
My vet orders the immunotherapy mixture created for my horse (based on the results of testing) and ships it to me every 8-10 weeks. We are currently on the 1 shot/week schedule. It’s a 1.0mL subcutaneous shot, and she doesn’t mind it at all. There is a loading period where you give a shot every 4 days for for about 10 weeks I think. Ideally, you are supposed to be able to continually increase the interval between shots (every two weeks, every month, etc.) until you no longer need to administer the shots, but that’s dependent on the horse.
I deal with chronic scratches but can usually get it down to just a few tiny spots that I don’t even treat for weeks at a time.
This is probably me.
I have two horses that need just a LOT of copper and zinc. I don’t know why.
They get a full scoop each of Horsetech poly copper & poly zinc every day. I up that to a scoop and a half when they’re building coat in late summer.
The most effective treatment I’ve ever used is neem oil. I first discovered it in a commercial product which was just neem and a couple of calming herbs like chamomile and aloe vera (can’t remember now) mixed into a lanolin base. Since then I’ve just mixed up my own neem oil + lanolin or vaseline. Neem is an extremely effective anti bacterial and anti fungal and the lanolin provides a barrier.
After trying numerous other treatments and getting nowhere, I was shocked at how quickly this worked.
I’ve been dealing with some very resistant scratches on my mare’s white hind feet this winter. My usual go-to topical just wasn’t cutting it so I decided to try Fungasol ointment (which, as a bonus, is cheaper than my usual). It took maybe 2 applications and we’re back to mostly clean, pink skin. I’ve also been using the shampoo from the same “collection” to wash the affected areas before reapplying the ointment each day.
Well Sat night I tried something new - I washed with baby shampoo first to try to be gentle, then used the RX shampoo and let that sit a bit. Then I put on animax and let that soak in for a minute, then I put the homemade mixture (desitin + triple antibiotic + hydrocortisone) on top of that, plus the Shoo-fly boots. And last night when I washed with the baby shampoo again it looked way way better. I feel like it has done that to me in the past, though, given me false hope.
I also had a chat with my vet and discussed different options including ones mentioned in the thread and she’s going to email me with what we discussed with pricing for the things that require vet work (like the Eqstim injections) and then start trying some things.
I have clydesdales, and have always battled with scratches, or miscellaneous sores in the feathers, what I have found that works the best for me is mineral oil and powdered sulfur applied liberally once per week with a paint brush. messy but it keeps scratches at bay and is pretty cheap.
Have you tried Equiderma? It’s colloidal chlorhexidine. Same idea, no scrubbing. Just brush, keep dry and apply a couple times a week.
What JB said wrt alfalfa. It can cause photosensitization in some horses.
Sulphur paste.
Are you sure it’s scratches and not Equine cannon keratosis? They can look very similar, but keratosis is best treated by aggressive currying.
For true scratches, I spread a lot of furazone on the leg, wrap loosely in seran wrap, and then cover it all in a standing wrap. Leave on for about 10-12 hours, then wash the leg well the next day. It will gently remove all the scabs so your medication can get to more of the infected area.
I’d also try skipping the alfalfa to see if it’s sensitivity caused.
For zinc and copper, I use cal trace bulk zinc and copper. I have seen an improvement in my horses hooves with it. There’s a newer thought on copper/zinc supplementation that in some cases horses will need significantly more than the recommended amount. This may because the horse has a high iron intake, because iron can mess with the absorption of them. Sometimes your hay or your water will have high iron levels, and it can really vary even by region. For instance, the water from my well is considerably lower in iron than the water at the barn a few miles away.
Following up on this topic. Did you find a solution to your horse’s chronic scratches?
Thanks
It may also be worth testing for Cushings as it can cause immunosuppression. He’s a little young but it sounds like you’ve tried almost everything else.
If your vet isn’t already familiar, I always refer back the Tufts Equine Endocrinology Group website - I can never keep all the testing straight! It’s a great resource.
Agree with above posters that photosensitivity is another possibility and worth ruling out.
My farrier swears by powdered sulphur mixed into lard. I haven’t tried it yet but I might for some scratches in my herd now.
Unfortunately no. Still dealing with it and getting a bit worse rather than better.
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