First some definitions:
The food output is the output of a number of farms minus the research cost (200 for Domestication, 250 for Plough).
The wood input is the wood cost for those farms plus the research cost (50, 75 respectively).
The food/wood ratio for unupgraded farms is 3.33 (compare with FBs: > 5 if a single FB empties a single fish spot). So that's pretty low. Every upgrade adds 1 however.
Now when is upgrading farms worthwhile? The answer is: when their food/wood ratio exceeds that of unupgraded farms. Now you can find the critical number of farms by solving an equation (you'll hate that) or by drawing up a table with outputs.
Here's a table for normal farms, showing that the F/W ratio gets better at five farms, so if you build more than five, Domestication is worth researching, provided you can spare the resources for it.
Without Domestication With Domestication
Wood Food F/W Wood Food F/W
1 75 250 3.33 125 125 1.00
...
4 300 1000 3.33 350 1100 3.14
5 375 1250 3.33 425 1425 3.35
Of course this doesn't mean you should research it in the Tool Age - you probably can't miss that then. But in Iron there's a chance for it.
You can draw up a similar table for finding that the Plough is interesting if you build more than 7 farms (compared with farms with domestication) or more than 5 if you upgrade from scratch.
Without Plough With Plough
Wood Food F/W Wood Food F/W
1 75 325 4.33 150 150 1.00
...
7 525 2275 4.33 600 2550 4.25
8 600 2600 4.33 675 2950 4.37
9 675 2925 4.33 750 3350 4.47
Without Domestication With Plough
Wood Food F/W Wood Food F/W
1 75 250 3.33 200 -50 -0.25
...
5 375 1250 3.33 500 1550 3.10
6 450 1500 3.33 575 1950 3.39
Of course harvesting the food takes a while but these stats show that if you plan to do some serious farming in Iron, researching these techs may be worth considering, especially if you got a Palmy feeder who can provide you with the resources you need for researching them. Then very soon you can tribute them the surplus food back.
Flame away