If it's field tailings then it can just be dug in, if it's stable manure then it'll need rotting... the difference is cause dby all the nitrogen from the horses urine on th estraw apparently.... I grow fairly good spuds and epic pumpkins on fresh field tailings... sweetcorn likes it too... you will get weeds in with field tailings and you don't want to try and grow carrots or parsnips on a bed that's been treated cos they'll fork....
Also the big caveat with horse manure at the moment.... unscrupulous farmers are selling hay to hay agents that shouldn't be leaving their farms as it's been sprayed with Aminopyralid -based weedkillers.... the stuff goes straight through a horse unaffected and will seriously damage beans, tomatoes and gives spuds a hard time... brassicas, onions and cucurbits seem much less affected, sweetcorn doesn't care about it cos it's only there to kill broadleaves, it doesn't affect grasses (of which maize is an example) ... I'd suggest stacking it and sowing some broad beans in pots, once they're up about four incehs plant them in the manure, aminopyralid will show up quickly as growth deformation it goes all twisted and feathery.... the weedkiller will last in the soil for a couple of years, so it's useable, but rotate carefully....
