We've literally put three tonnes of donkey poop (ok, not horse, but very similar) a year on the garden for the past decade (and longer, but I've not been involv

We've literally put three tonnes of donkey poop (ok, not horse, but very similar) a year on the garden for the past decade (and longer, but I've not been involved in that prior) and had none of the issues you've raised. They might track back to your fields of feed origin. It's been fantastic for moisture retention, breaking down to increase soil complexity, weed suppression, making better figs, tomatoes, potatoes, oranges, lemons, grapefruit, grapes, etc. ( We've also used sheep manure scraped out from under shearing sheds, horse manure, sluiced out pig run waste, etc ) Call it a locale specific outcome perhaps.