10 cows - 370 per minute (37 per cow/minute)
7900 8270
9000 9368

20 cows - 680 per minute (34 per cow/minute)
10100 10780

30 cows - 850 per minute (28.33 per cow/minute)
3500 4350

40 cows - 1063 per minute (26.575 per cow/minute)
5000 - 6063
7100 - 8163

50 cows - 1150 per minute (23 per cow/minute)
9000 - 10150

60 cows - 1237 per minute (20 per cow/minute)
12000 - 13237

65 cows - 1157 per minute (17.8 per cow/minute)
16500 - 17657
19000 - 20136
23400 - 24595 (1195 per minute)

70 cows - 1130 per minute (16.14 per cow/minute)
28000 - 29130

As you can see, sometime between 60 and 70, the total income breaks even and starts to decline. Now, this can be for any number of reasons, like latency due to the amount of cow models, and if you ever want to add any data to this guide, just PM me, I'll be more than happy to add it.

I would say it's almost never worth it to go for more than 40, because by then the difference in food per minute is lower than the cost of a single cow, which means you need at least 20 minutes to make up the difference in those 10 extra cows you need to get to 50.

20 minutes might be feasible if your opponent is really bunkering in and won't budge, but you should always aim to make more troops instead if that is the case, as you will need them to keep up the pressure on the opponent. If your troops don't die very fast, sure, go for economy, but other than that I'd say, stick to 40 or less.