If you fertilize a field, is it necessary to keep horses off that field until it rains to prevent horses from getting sick?

Ideally horses should be removed from newly fertilized pastures and be reintroduced to the pasture after a light rain. However, that may not be feasible due to housing space for the horse(s). Under normal conditions when ammonium nitrate (33%N) is used and a good even distribution of fertilizer is obtained, there should not be a threat to horses in that pasture. Horses have a greater tolerance to nitrogen than most people think (Meadows et al., 1978). However, if a fertilizer truck dropped a big pile of urea (82%N) on the pasture and the horse consumed a large quantity of the fertilizer, a problem could persist. With proper spreading of fertilizer and under normal conditions, a horse grazing recently fertilized pasture will not even have loose manure as a result.