If it is 'mostly powdery' it may become hot with the addition of water. It may also be mostly saw dust or other bedding material and the clumps you are finding are the manure. As long as you have finished harvesting, you can use it as mulch on your asparagus bed. I mulch my asparagus beds every fall, after I cut them back, with composted horse bedding. Of course, I know that my neighbors, who supply me with the bedding, don't use vast amounts of drugs on their horses. I wouldn't want some of the residual drugs in my garden compost.

if you mound grass clippings much more that 3-4 inches high they will generate enough heat to cook what they touch. Same thing with manure. Where there is considerable distance between plants I mulch in place to save time and labor. I've gotten more weeds from horse manure than grass (the hay from the bedding sprouts) but it's easier than a compost pile. This has always worked for me. I don't have the time or space for a compost pile. I use grass clippings a lot around my tomatoes but as a mulch between the plants (about 6 inches from the plant with a smattering closer to it. )

When I did, in my ignorance, add animal manures to the garden beds without first composting those manures, I did see an increase in "weeds" and it made no difference if it was manure from dairy cows, horses, pigs, goats, sheep, llamas or alpacas, unless it was dehydrated manure. It seems the heat used to dry that manure was enough to kill any "weed" seeds. Since the easiest method to keep these "weed" seeds out of the garden is to compost the manure first it simply makes sense to do that, along with all the other very good reasons to compost manure before applying it to your garden soil.

Hmmmm... I think the lesson is never to assume that something is "compost there long enough." Give something unknown a year to "cook." I used horse poop, right out of the pasture, in my lasagna raised beds. I think the key to my success and weed avoidence was that it was layered under leaf-gro and mulch. I have the richest, crumbly beds in the world and am thrilled with them. Now if I could just stop the maples from rooting through the beds, changing them into mats of roots! Nothing's perfect, is it? Christine