If you have very little time and need to get horses on it quickly, then fertilize with Suregrow. You can put horses straight back on so long as it has had a few hours rain on it (won't be a problem at the moment). My paddocks look fabulous ATM after applying it in March.
I can highly recommend Suregrow fertiliser aswell, my grass has never looked so good. Its specifically designed for horse paddocks and safe to turn horses out straight after its put on the field, ideal for me as i just have the one field.