Some years back, a big dairy operator put in a manure separator- gave the solids away free, even parked a loader tractor out by the pile for guys to use. Manure spreaders from neighboring small farms were lined up, hauling it away. Until they found out that the solids have virtually no nutrient value, and they didn't get any results at all. Turns out all the nutrients are in the liquid.
Horse manure, IMHO, isn't worth much as a fertilizer. I've been putting in on pasture(fresh) for years, and don't see much results. Cow is much better, chicken is best of all. And last year I put some goat manure on my garden, and I couldn't believe the results- I figured I put on too much because of the growth, so didn't put on any this year, and it's still crazy- tomatoes are 6 feet tall, zuchinni almost as high, its a jungle out there!