David Hare-Scott wrote:
I presume a 'below ground' crop is a root ot tuber and that 'well cooked'
means well composted and degraded..

Yes.

Why should the manure be well cooked in that particular case?

There is some risk that the horse manure includes fecal matter from other
animals, domestic and wild, and possibly even human feces (in some barns this
is likely, because there is no toilet anywhere on the premises). You'll
want to avoid bacteria from non-horse feces being in direct contact with any
food you may consume raw. Also, plants will make better use of composted
manure. Composting involves heavy bacterial growth, which briefly consumes
available nitrogen. The nitrogen is released when the bacteria die off at
the end of the composting cycle.

Solar radiation over time takes care of most above-ground contaminations.

Many manure piles are aged yet poorly composted, due to too much moisture
and too little air. They are much improved by composting. Whereas fresh
manure is "green" matter, high in nitrogen, aged manure is "brown" matter.
I like to compost all manure before I apply it, if only to break down the
balls.

Una