That's one of the difficulties with mods that alter the way the game is played. For the AI to make full use of the new options, the requisite components in the SDK need to be tweaked. Unfortunately, that's no mean feat. Even with C++ knowledge, there's still a mountain of code that needs to be read just to locate the right sections. Unfortunately, Firaxis didn't give us a comprehensive manual or "ready reckoner" to help us find the sections of the DLL we need for various modding scenarios. It's like having to read all 400,000 pages of tax legislation just to find out your marginal % tax rate. After the first 200 pages of irrelevancies, all the writing starts to look the same , and you're as likely to skim over it without noticing. Unless someone has already figured out where the changes would be needed (possibly while trying to do something else vaguely related), we're probably on a hiding to nothing getting it perfected before Civ V is released.
But for game balance reasons, I would prefer to do it another way: have a separate unit to plant/herd units...
Planter/Herder unit: an expensive unit that coststo build, like Workers/Settlers. That unit will destroy itself to create one crop or herd that you already have access to on a suitable square. Then issues of game balance can be addressed simply by tweaking the amount of
the unit costs.
So you can't create a herd of cows if you have no access to cows; but if you set up a trade for cows, then build this unit and plant it, you get your Cow square!
It's probably best that we lobby for reform of the food management system in the Civ V fora. A system where crops or animals can only be farmed in the plots where they have grown/roamed wild since the dawn of time is ludicrous. The whole point of farming is that the crops/animals are not wild and, hence, can graze/be grown in any convenient arable land. Since the development of pickling, this has occurred away from cities as the produce was transportable. Besides, farming is not labour intensive (on a people per sq. km basis), so a city full of farms would have a low population. Post-industrial cities have high populations where there are dense, labour-intensive workplaces, like factories, mines, quarries, foundries, and (later) offices. The food is/was transported via wagon, barge or (now) lorry. Civ IV, in its current form, forces your population into your breadbaskets, whilst your mines (and engineer specialists from forges) remain disused. Quite unrealistic. However, without all the necessary SDK changes, our knowledge of the workings of any mods we make will give us an exploitable advantage over the AI. The advantage could be decisive enough to require moving the difficulty level up to compensate. I encourage anyone who can fill the knowledge gap to post. My programming skills are intermediate (ie: I'll likely comprehend programming suggestions, but I can't read through all the code quickly, so to read through the SDK code to find what we need would take me a very long time indeed). But, I also anyone passionate about good food resource management to lobby for it's Civ V inclusion. This will see the AI utilising its advantages as we certainly will. This is a post I have already made on the Civ V announcement forum, which touches on these issues as part of a broader discussion on resource/labour management and transportation.
Please add to the discussion.
There is absolutely no code in this mod to spread irrigation. None. All the agriculture mod does is allow you to plant crops (corn, wheat or rice) on land plots, with 
