Where in Guns, Germs, and Steel does Diamond talk most about things like cultivation, crops, and agriculture?
Diamond talks about these topics throughout the book since he says that agriculture is the major driver of civilization.
In particular, Diamond talks about agriculture in the seven chapters that make up Part Two of the book. In Chapter 4, he talks about how farming allows societies to get the "guns, germs, and steel" that make them powerful. In Chapter 5, he talks about why some places got agriculture before others. In Chapters 6 and 10 he talks about how and why agriculture spreads more easily in some places than in others. In Chapters 7 and 8 he talks about how crops became domesticated and why this only happened in some places and not others. Finally, in Chapter 9, Diamond talks about why animal domestication did not happen in equal degrees across the world.
These are the chapters that are devoted most directly to agriculture.