Good quality minerals before moving onto lush grass. Salt lick always available.
Ideally, graze lush grass with cattle first and then sheep, stops it being too lush for the sheep.
Do you need the grass to be this lush? Can you deplete it somewhat? (Eg., by taking a hay crop each year.)
And never fertilise it, if it's too lush for your sheep and you can't put cattle in first.
If you've just one lush field, can you take a hay crop first and then graze later?
Or overwinter them on there to knock it back, so it takes all spring and some of the summer to come back.