[Carry_Okie]

1. The expense of fencing, as mentioned above.
2. The need to have a source of water in each grazing area. It is harder to find water than to string fence.

IOW good ranching is work. It takes a people on horseback and maybe a good cattle dog or two to keep them moving properly.

[Iowa Granny]

You've made excellent points. Cattle tanks, on skids can be pulled from one pasture to another here in the midwest where ranches are smaller.

Portable fences are quite common here. They are used for both sheep and cattle.