Something that seems to have been overlooked so far is the fact that such an acreage is going to be almost completely dark at night, and standard cameras are going to be useless after dark (forget about IR or even flood lights at this kind of range and area). A farm of this size is necessarily going to be far from any sort of "light pollution", so even the best low-light cameras are unlikely to give usable image, short of going to super-long shutter speeds.
If you want to be able to see what's happening at night, thermal is probably going to be required... potentially combined with PTZs that an operator can use to look in on areas of interest.
This is a clip of a site where we have three thermal cameras (one FLIR WideEye, two FLIR SR-19s) and a Pelco Esprit PTZ atop a 50' tower in the middle of a two-square-block yard; presets on the PTZ are configured to match the various FLIR camera views (PTZ in this shot is on Preset 5, matching Zone 5 FLIR).
Obviously this shot is during the day, but the FLIRs look almost the same day or night (they do lose some contrast in fog and heavy rain), and in this case it's a smaller area with SOME lighting, so the PTZ is still usable at night. People, animals and running or recently-running vehicles show bright-white in the scenes.
Here are some older day vs. night samples I have:
The other consideration, of course, is that you WILL need to get the cameras to some altitude for a good overview - as noted, the cameras in my examples are on a 50' high tower - so you'll need to factor that into your design as well.