On the ground, usually until it rains and washes it in, in the grass - until that grass is cut. Fertilised (especially agricultural fertilisers) and horses don't mix well. I only fertilise paddocks I'm making hay from - not ones I want to graze.
Thanks, I have just been offered a field for the summer, the farmer fertilised it last year then cut it for hay he has had sheep on over winter and I wanted to check that the fertilizer wouldn't still be in the grass this year.