Worms love horse manure. It is 95% of my worms feedstock. I use it fresh with no worries. If you purchased worms for your bin, they probably came from a bed of horse manure.
If your bin is on the wet side, then add manure dry. Otherwise, wet it well before adding.
The only caution is to beware of adding too much and creating heat. To avoid overheating a small enclosed bin, just add small amounts at a time. Also, mixing manure into existing bed can cause heat. Maybe try a little sprinkled on top, or a couple handfuls in a corner. No bedding should be mixed with manure, as it will cause heat. Use it straight.

One other warning. Horse manure is like worm candy. They will get addicted, breed like crazy and want more!

Good luck and happy wormin' Pete

I agree with Pete. If you like to fuss, you can screen/seive it and make it fluffy.
It still needs to be damp. I had a live worm in some very dry horse manure yesterday. I put it in the garden to celebrate its stamina. I would recommend a new bin with just HM -- no paper products -- in a more-is-better frame of thinking. Depending on what is in your present bin and how dense the worm population is, adding HM slowly, one corner at a time, seems a good idea.

I have 3 HM bins: 2 plastic 1 Worm Inn. Two of the three are getting "sticky" which suggests to me it is time to move some worms to fresh bedding.
I'm gonna post this as a question and see what others think.

After about 11 years of having access to as much horse manure as I wanted, my source of it was lost when the producers of **it were sold.

I feel about as bad as a person can feel about losing horse manure worm magic.I'm kinda lost and really feel absolutely like I'm just starting over.

The wormies are on the verge of revolt because, as Mendopete said, they get addicted to it.

As for your good fortune.....enjoy.

chuckiebtoo

chuckiebtoo: I agree it would be difficult to recovers from such devastation. I hope we can give away some tips such as keep a second bin just in case.

A couple weeks ago I went to a horse farm, (see my post Maure worms) That was a type, it was supposed to be Manure worms in the heading.

VERY little luck finding worms, and I tried a number of piles, some 6 months old, 1 year and 3 years old. The winter was very cold, and the piles froze, which is the first time the farmer saw this happen.

I did see a number of fat dew worms. I brought a couple home to see if they would survive in my bins.

I'm going out again in a few weeks to see if there are any this time.

I put what worms I had and some "apples" and fluffy poop in my bins. It might have been my imagination, but I swear I could smell the poop in my home office, where the bins are located, for a few days afterwards.