Maybe Vitamins and minerals are not what is needed here, could be from boredom or needing more fiber in the diet. Try feeding more hay or better hay or put toys in with the horses. But if you think it is vitamin/mineral deficient diet, there are plenty of vitamin supplement products available but not really needed unless you are feeding poor quality hay or your horse is perfoming heavy exercises. A mineral block will supply most minerals except for salt, calcium, and phosphorous. Ca and P can be added to the diet to balance out any deficiencies. Lime stone is a good additive for a calcium and disodium phosphate for phosphorous or dicalcium phosphate for both.  

A product that is popular around here are the Key-Lix tubs of minerals.

Most of the local feed stores carry this in a 200 lb tub for about $60. I just drop the tub out in the paddock and give the horses free use of it.   It last my 4 horses about a month.

http://key-lix.com/Equine.html

How do these hold up for you?  When it rains, does it become a big slurry?  These (or something similar) are sold at the local feed store, and I've always been curious, but worry about it turning to soup.  Thanks.

A good quality feed should always be considered in a horse's balanced diet also.

I'm sorry,but no matter what the quality of hay or grass it cannot meet the full mineral requirements of domestic and well bred horses of today.For one,our soil has been depleted,so that even we as humans do not reap the full nutritional benefits of our food that we once did.I have been taking glyconutrients myself since learning this while studying nutrition during chemotherapy.We are missing the simple sugars that once were in our foodstuffs and they are vital for our immunity and cell support.

And PS: my horses won't touch a mineral block in the wintertime.If you'll read the label on a "red" mineral block or even the other types it won't equal the minerals in a quality supplement.A red mineral block is largely made up of salt.The other minerals will be of a trace content.

If you have questions such as this I recommend that you consult your veternarian and your county extension agent as to the particular qualities of your hay and feeding program concerns.

I have a friend that uses a very similar product up in Oklahoma.  From what I've seen, the horses just seem to come over and slurp the water out of it.  Didn't seem to last long enough to get really soupy.  But they do get a little melty in the summer so they needed to be put in a shady area (like under a tree).  Horses still loved them.  I never saw any waste to them.  BUT they will make you a little crazy when the horses get down to the last little bits that are in the corners of the tub.  They would be so impatient with getting it out they would paw at the tub (these were made out of the 1/3 bottom of a steel drum and were refundable when you would go to get your next batch).  CLANG, CLANG, CLANG..... but her horses looked great.

They come in a Poly tub. No return.  I use the empties as muck buckets, extra water buckets, feed buckets when I'm traveling so the horses don't have to eat off the ground in strange locations.

The product is very hard, especially in the winter. Even in the summer it's pretty hard.  Horse can only lick it. They occassionally try to bite, but just scratch the surface. If it sits out in the rain or sprinklers, the water disolves some of the product. The horses just slurp it up as mentioned. But it doesn't disolve very deep into the product. I've not had any wasted.

One downside, In the summer when the horses are licking it. Then turn and lick or bite at flies on their side or legs, they leave some of the sticky stuff on themselves.

I've shown horses on both Stride and Accel, one at a time not both together and had excellent results!

I don't offer mineral blocks in the stall only in the pastures, I do however offer yellow or sulphur salt ALL the time.

Another WONDERFUL product that I've had clients use with great results is Bluebonnet feeds Boost-em.  They also make feeds with Stride in the blend so measuring isn't a problem for kiddos.

Provided your horses are receiving a balanced diet....there is a product on the market called Quitt. It is meant specifically for horses that chew on wood (trees included) It often will stop them from chewing by the time you have finished the tub and costs around $15. I had a horse that was de-barking some of our young trees and it stopped him from doing it.

Good luck!

It is more likely these horses are chewing due to boredom then a dietary reason.

While I do agree that horses chew out of boredom I will say that the period of time that I had them on Source they did not chew - nothing else in their diet or turnout had changed in that time. 

Sarah