AIs are building too many farms on their colonies

More food means more population and more population means more production. Don't see what's wrong.

The amount of food determines the maximum population of a colony (up to a maximum of 30). It's not building farms to feed the existing population. It's building them to allow for growth.

the layout of that planet is a bit inefficient, but the farms aren't that bad actually. would have been smarter to put the farms next to the hospital to actually get some more growth. but overall that layout is already much better than what they did 2 patches ago

I understand and that is the point. You don't need 3 farms to support 8 population, it is waste of space on a planet. In this case 1 farm is enough: the planet would have 10 food and 9.5 pop, when the pop grows to 10/10 you build another farm or research an upgrade.

they also built 2 approval buildings, so their strategy was probably to grow a big population on that planet. the rules regarding prodction and population were recently changed (again) and high population is now more efficient than before. it's a valid strategy. maybe not the most efficient, but i've seen them do worse.

I've noticed that the AI seems to be stuck in the mode of building a structure up to its maximum available form before moving on to the next hex, as if every planet is stuck on 'auto-upgrade'.  The net result is often a horribly retarded empire because it's usually far more efficient to fill your planet with tier 1 versions of everything before you embark upon an upgrade program.

... it's still going to have 3 farms in the end to hit 20 bil. Why not get them built early and avoid wasting turns of valuable growth?

Because the space the farms are being used for can be better utilised with research or anything else but just waiting for the population to increase. Growth modifiers suck, good luck getting to .3 per turn without every growth mod in the book thrown at it.

Because building them before you come close to the pre-farm population cap delays building much more immediately valuable structures. Furthermore, even if your timing is somewhat off, a planet with a decent manufacturing level can fairly easily make up for the lost turns of growth; it's only 0.1 population growth per turn as a base value, modified by approval, colonial hospitals, other growth bonuses, and birthing subsidies, which, except on a high-manufacturing world, probably won't add up to more than 0.2-0.3 population growth per turn.

Checking against the in-game numbers, it appears that population is not capped at 30, nor is the production bonus from population capped at 30. I don't know what PlanetFullPopulation in GalCiv3GlobalDefs.XML is supposed to do, but it's either not supposed to cap the population or population-based production, or it's not working.

I can get to +0.6 growth per turn on my breadbasket world, I put a lot of manufacturing on it, build a transport take 3 pop off the planet, then build population growth for a few turns to recover the pop. Alternately I can fine tune the manufacturing to pop out one transport every 5 turns.

I too am seeing way too many farms on planets. I recently took a planet that had 6 farms on it. Its population was still only at like 11 before I took it over, so I guess the AI should have been building more defenses instead.

It is exactly what i get, insane amount of farms when population is really low.