If they're in there now let them bare it down tight, put out slurry or fert then. The tighter its grazed the better the silage will be. If cutting in June it will prob have headed out anyway. Given the way ration prices are likely heading it may be worth cutting it earlier to get as good as quality as you can
Depends on whether you have enough grass without this field until aftergrass comes back, if yes remove now, no slurry and cut mid june then spread slurry. you'll have to eat it bare if you want to spread slurry now