What kind of slurry? Human from a bio digester type thing,; cattle from a liquid manure pit? Our cattle will not graze when we spread liquid manure from a dairy for weeks.... and yes, rain will help. But it makes the grass taste bad... best to do that at the end of grazing season or put it on hayfields at the beginning of the growth. They will balk at the hay sometimes too. Most slurry is better for crop ground.