I guess you are wanting to get some N to your established brassica.
The easiest way is to broadcast urea right before a substantial rain. But urea degrades in heat and high humidity if left on the surface, that's why we wait until we expect a half inch or so of rain.
If you spread urea and it doesn't rain as much as expected, it still isn't a waste, but you will lose N.
They also sell coated urea which has a longer "surface life", but I've never tried it so I can't comment on it's effectiveness.
Following a legume with brassica is also a good idea. Clovers, beans, sunn hemp put N in your soil so overseeing brassica into legumes is one approach.
I've never spread cow manure, but it is a good source of N so it probably won't hurt.
Be aware, high N content encourages grass weeds. You need to use up unused N.
Rye is great for that, too. Brassica uses N. Some N is good. Too much leads to other issues.