When carrying-out a farmer strat should I worry about negative income? Should I offset this with Bus/Res Tech?

yes after you have maxed military and agritech (ahgri 1st then military and rest).

how long has this agriculture/military tech strategy existed?

military tech has always (at least for a long time) reduced expenses and in earthempires it also increases priv market prices (helpful for demo farmers and non-demo de-stockers with near maxed mil tech if bushels price is under 38 on public market).

Bus yes, res not especially

No, don´t worry about negative income.

You will make enough money from selling your bushells to cover running costs. Spend your surplus cash on turrets and on agriculture tech.

is cashing at the end of game good for farmer?

yes if you have positive income.

Thanks for the info

Cashing is the right thing to do once you have reached your land goal. Whether you have positive or negative income does not really affect this. Of course, if things have gone well you have made so much money that you have maxed out Agriculture and have been able to afford lots of Bus/Res too (buy that in roughly equal quantities subject to prices). If things have gone badly you did not manage that and you have a negative income. The point is that you have to use turns so that your farms turn out the bushels. If income is positive cashing increases the cash gain, when you are in deficit cashing reduces the deficit.

The judgement aspect lies in when to use turns to explore and build and when to stop doing that (and use turns to cash).

This is a judgement which needs to be made at the start of the set. The reason for that is csites. If you adopt a high land goal you need more csites (which you build early) whereas if your land goal is more modest you need less csites.

Only experience will help set a sensible land goal. In general a farmer must aim for lots of land. On the other hand a (relatively) inexperienced player should sensibly set their sights low. I suggest 10k acres in tourney or express; 30k in Primary; in Alliance your clan mates can advise; and who the hell knows in the madhouse (FFA). You can look up the number of csites apropriate to those various numbers.

Thanks for the info