THe real problem with horse manure isn't anything to do with the manure itself, it can be applied fresh if you want, it's nothing like as "strong" as cow or pig in that regard, it's lying gits of farmers selling hay contaminated with aminopyralid. THe hay agents (who might well be in on it) then sell the hay onto unknowing equestrianists, and suddenly what should be one of the best soil improvers out there is damaging potato, tomato and bean crops.... I've had massive problems the last two years with it, I've coped by planting extra spuds and moving the beans to my home plot (which isn't big enough for all the beans I grow. The stuff sticks in the soil for a couple of years in bad cases too... Doesn't seem to affect brassicas or cucurbits much (I've got 80-100lb pumpkins on a pile I know to be probably contaminated. THe best thing to do is to pile it up and take a sample and do a bean test.... hatch a dozen broad bean seeds and plant half out in pots with just plain soil and half with a 50/50 mix of soil and manure... the weird feathery growth if there's contamination is very obvous... trouble is you have to test each batch you take as it only takes one bad bale of hay to contaminate a few tons of manure....
Last year I lost 80%+ of my beans, all my outdoor tomatoes and about 50% of my potato crop (estimated against what I thought I should get) , this year I've lost about 30% of the spuds, and I can only grow about a third of the beans at home that I can on the alloment.... I try to keep on top of bean tests, but sometimes you jsut run out of time.... that said if I can get through another year, the massive improvemetn in soil structure will be in place and I can be a bit more fussy, my soil needs about a foot of manure on it and the only sane way of doing it is to put it on the spud bed, rtor it in and then backfill and earth up with more of it during the season..... end result is great soil but reduced spud harvests.....