Before I irrigate grassland/bonus grassland, the food produced is two per turn. But for some reason, when I irrigate the grasslands, they still only produce two food per turn! Why? I don't understand. I want it to produce three food per turn.

Grasslands do not produce three food when irrigated under despotism or anarchy. You likely are still in one of those forms of government--if not, then there might be something wrong.

Oh, thanks! That is the problem:)

In the MesoAmerica, Fall of Rome and Age of Exploration Conquests in C3C, Blood Cult and Tribal Council governments also have the -1 rule. I'm not sure about the Mesopotamia Conquest, haven't tried it yet.