Apologies if something like this has already been asked, but I didn't find anything! This is going to be long winded, buckle up!

I just switched my two horses from Tribute feeds (one on Kalm Ultra, one on Essential K) to essentially a fortified forage product. I was so excited and have heard nothing but good things about this feed company. But now my horses have weird symptoms and I'm wondering what is going on.
The horse I had on Kalm Ultra previously has a history of various GI issues and I was hoping the switch would help. He started biting at his sides so I upped his GI supplements and added more probiotics. I don't know this horse very well, I haven't even had him a year, so while I'm trying to make him more comfortable and happy I don't know what works best for him. He also got what I and my farrier thought was an abcess forming from a stone bruise, figured that's just my luck. It never came through but lameness improved drastically. If he runs around and plays it will become aggravated so I have been keeping him from moving too much. It is probably worth noting that I power packed him around the same time that he reached what I think will be his full ration. I don't know his worming history, and we are trying to get to the bottom of his belly troubles. He has already been treated for gastric ulcers, and we take extra precautions for hindgut. 
My other horse had one bout of gas colic prior to starting the Tribute, but that is the extent of any belly issues. I've owned him for 6 years and know him very well. He bit at his sides for a bit which I've never seen him do, I added probiotics and Redmond Daily Gold and then his leg swelled up. 
I gave up and called the vet because all the symptoms were making me crazy. They came out the next morning. It was not my typical vet, but from the same practice and I really liked him. Vet said he really liked the feed tag of new feed and doesn't know why it would be a problem other than change in their microbiome, continue probiotics. He said he heard more gas in horse 1s gut sounds than he would like, gave injection of buscopan. Said foot problem is either a severe stone bruise or bruised coffin bone after exam. He took a fecal from horse 1 about 12 days out from PP (when he came out for everything else) and when he called me, said horse 1 worm load was still high side of moderate. Horse 2, advised to stay on probiotics gut sounds were good and his swollen leg was from an abcess that had come through overnight. He didn't act like he had an abcess at all, but he tends to get them every spring and fall so I wasn't overly surprised. Figured all of that was the best news that could have come out of all of this. 

My question is at what point do I give up on the new feed? I really like the ingredients and balance of it, but why the side biting?! Horse 1 has been pretty constant with having mark on his sides and horse 2 had marks maybe 3 days out of a month. Should I be worried I got a bad batch, is it truly microbiome change, and should I be worried about all the foot issues? Maybe I'm just being a bit much, but I don't ever want my horses guts to be hurting. There are so many things that can factor into all of this but I just keep coming back to the feed...

What is the ingredient list on the fortified feed you switched to?

I don't like to bad mouth any feed unless I have had a problem.  My issues were nothing like yours however on Tribute Kalm & Easy my horses would not shed out and looked poor, never happened before.  Mine are retired so I switched to straight oats and had have no problems and shiney coats.  Sometimes I think people make things way to complicated with supplements (guilty when showing) and such.  I love the KISS method saves me $$.

Good luck figuring out your issues.

I tried to look at the tag but I'm on my work phone and it's being difficult as always! Alfalfa, flax, rice bran, and some vitamins. In theory nothing that should be causing these issues. I'm thinking I'm just going to try something else, the symptoms and a PM about the same brand of feed has me all weirded out. The best part is the closest dealer is three hours away but I'd heard such good things that I made the drive and bought in bulk!! Not sure what to do with it, I don't want it causing someone else's horses issues! 

I'm definitely not wild about tribute, there's not very many good grain options near me! I like your style. I'm thinking my next step is just feed some alfalfa and a good fat supplement/vitamin and call it a day. 

ETA thank you! 

I have fed sweet feed, oats & a flake of alfalfa for 50+ years, to add to the horror I trained, showed, and kept them in stalls too.  I also had a mare on Bute for 2+ years and I never thought she had ulcers she is 23+ now and still looks good.  When I came on here in 2006 some people made me question everything, from sadlle fit to what I fed.  I have no problem with supplements I have fed many over the years. I wish I could find good alfalfa.  Now I feed oats grass hay and turn them out a lot.  Mainly because I'm lazy don't like to pick stalls, I live in the barn/house combo.  I am not saying you don't have ulcers at all trust your vet.  I'm just saying we never really worried about ulcers till we started throwing all the crap in their feed.  Also we now have the ability to test a lot like modern medicine.  I tend to stay away from the Dr afraid they will find sonething wrong.

Just pay attention to your horse and how they look.  On the biting issue does he do it when you cinch him up or just touch his side or standing around?  I also would have never thought of this but check him for ticks.  While trail riding in FL one of my friends horse went to stomping and she found a tick on his belly, we had just come down from a cold area. Never would have occured to me but if they can feel a fly?

I would feed it to him I don't think it's going to hurt him unless he is biting his sides really bad.  I guess i'm not understanding the side biting, I have one that will act like he's going to bite and you tighten the saddle to much you will wish you hadn't.  Owned him 8 years cinched 100's of times before I learned that lesson.  I did have him on supplements at the time.

I had some similar odd issues . Mine were not on grain but it was hay. I fed tribute in the past and liked it fine . If you want to message me I would be happy to share what I went through. It is probably not the same but something to consider.