I've built a couple of raised beds in my back garden and my intentions are to grow onions, garlic, beans & peas in them. So this weekend I'm thinking off emptying my old grow bags into the beds then laying a whole load of horse manure on top to hopefully put some nutrient back into the spent compost. Will this work ?.
I think I would be adding some decent topsoil and mixing it well Zab. You may find that using just using old grow bag compost does not give encourage deep and strong root growth for future crops. In fact I would want 50/50 mix of topsoil and whatever compost.
My father-in-law, oone of the old-school who had an immaculate garden (you could see people on the bus turn their heads to look as they passed) surprised me by being absolutely against horse manure. Horses eat seeds, which are designed to pass through digestive systems unscathed as their method of dispersal, and the result is a weedy garden. Only chemical fertilisers on his garden and it makes us very careful what we put on our compost heap.
Your idea is fine as long as the horse manure is very well rotted - if you put fresh manure on you won't be able to grow anything for a couple of years.