Laura,
I do not know what the "balance" of 20yr. old manure would be, but, it's probly some pretty darn good stuff.
It may over time, if left out in the open where rain especially can get at it, some of the "nutrients" must have leached out, but for the most part it'll be like good black dirt...
We had 3 horses when I was growing up so I can tell you our poop piles are still around a bit, and I'd say, they were about the same age to be honest with you.
I can remember the grass growing from them, oh, such beautiful grass, (the seeds never compost going through a horses' system.....)
But we still go there and grab some to incorporate with my home made compost and I get the rabbit poop too (same age)!!
I would think anything that likes a nice, reasonable rich in nutrients, would still find it a great growing medium. Mix with some topsoil and some compost...... and plant away....How fortunate for you to have that.
Do you have it or someone giving it to you??I am a zone 5 here and so pretty much anything you're already growing should do just fine. However, some plants don't like a rich soil and can cause lankyness or just lush growtha and no flowers....
Hope some of this info helps!!!!!
Weezie