At the time of writing no one had made any first cut silage but surely a lot will be clamped by the time this is read (the weather can’t get that bad, can it?). Removing a large bulk from the land depletes the potash in the soil and this must be replaced, either by slurry or bagged potash or a combination of both. Nitrogen is also needed to boost yield alongside the potash so a grade of 24-0-15 is ideal.
Soil with a potash index of 2- requires 90kg/ha (72units/acre). If slurry is applied, a 1000 gallon application of undiluted slurry will provide 13 kg (26 units). This can significantly reduce the amount of bagged K required.
Nitrogen has dropped in price since spring but cannot be thought “cheap”. However applying nitrogen during summer to boost yields is more cost effective than buying silage or moist feeds during winter. Having clamps full of silage or big piles of bales does make you feel more prepared for the winter ahead.
In multi-cut situations on light land sulphur is likely to be deficient. In this situation 24-0-14 + 7.5SO3 would be appropriate.

For further specific advice speak to your rep.