In article .com,
wrote:
Thanks for the information which I can follow up.

I stayed at an arable farm's B&B for 2 1/2 years, and am interested in
the result of the kind of calculations done to maximise net profit per
acre, since the high cost of fuel may change the kind of crops grown.
(The money yield is more important that the tonnage.)

Read up on "permaculture", a system of farming that depends mainly on
perennial plants.

You asked earlier about "no-tillage" systems. In these the crop
residue is knocked down and perhaps disked in, and herbicides are used
heavily to control weeds and volunteers. There are a lot of trade-offs
in expenses here, not the least that weeds rapidly develop resistance
to each new herbicide, and new herbicides that are still under patent
are sold for whatever the market will bear. Some herbicides are
persistent, and limit your choice of crop in subsequent years. E.g. if
you use a broadleaf herbicide like atrazine, you can't follow your
maize crop with soybeans or alfalfa. If you plant Roundup-resistant
canola, you can't use Roundup to control the innumerable canola
volunteers that appear the next spring.

IIRC, there are efforts in progress to develop perennial forms of maize
and some small grains, but none have been successful yet. Another
approach would be to domesticate a perennial grass into a grain crop.
It could take millennia by standard methods, but it might be possible
to speed it up by gene transfer methods. Overall, it's remarkable how
very few food plants have been domesticated out of the enormous number
of edible species.

One problem with perennial crops is the length of time it takes to get
a reasonable profit per acre. This is one of the motivations for
developing dwarfing rootstocks for fruit trees -- they make the trees
bear sooner. Note that starchy nuts like acorns and chestnuts are
fairly similar to grains in nutritional value, but the trees are slow
to bear, and the farmer still has to pay the mortgage while waiting for
the trees to mature.

(People used to be able to plan far into the future, but it's not
usually economically possible now. Pepys recorded in the 1660's that
the French were planting large acreages of oak trees, so that 300 years
later, in the 1960's, France would have a domestic supply of oak masts
for sailing vessels, to better compete with the British navy.)