I have grown some cauiflower from seed and transplanted them into the garden.

The grubs round the roots are almost certainly those of the cabbage root fly. This loves cauliflowers far more than cabbages, but does tend to leave kales alone.
The fly arrives by newly planted cauliflowers, lays eggs by the stem, and these hatch into grubs which then go down to the roots and devour them.
There is a fairly simple solution for future plantings: take a small square of cardboard (some use old carpet) and cut a slit into it so you can fit it tightly around the stem when you plant. I also find mulching heavily with straw around the plants helps.
It isn't likely the grubs came from the farmyard manure, but if it was 'earthy' then the grubs would pass through it. the collars are the best solution.