So what are rules for farming and establishing a base of operations? My players are wanting to start and its kind of a part of our ongoing discussion right now. Feedback is appreciated thank  you for your time. We may switch to only war right now we are in Dark Heresy.

Farming?
You mean like planting crops, raising cattle and thelikes?

Only War is about being part of a large organization. You don't built a base of operation, you are assigned to one at best, maybe you construct one as part of an assignment but unless you are a high ranking officer you won't have any say in where to dig the trenches and where to deploy turrets.

Nor would the Guard be caught dead farming on a large scale. That is reserved for, well, farmers.

An Imperial Navy transport full of Imperial Guard goes off course in the Warp and is terribly damaged. It makes planetfall on an uncharted life-sustaining world. Too bad the planet already has orks/eldar exodites/whatever…

Now you can have Only War + farming!

You could invent some trade skills or give additional functions to the one you use for riding beasts. I can see how on a desert world a stronghold with good water and fuel would be crucial, but the game mechanics don''t really favour anything so dull…. try inventing a series of flashpoints (raiders stealing crops, stampede of local wildlife to be caught, orks poisoning the water from upstream) and run it like that.

It is likely that, unless under a prolonged and massive siege, any large guard regiment would depend entirely upon off-world support.

Unless severely isolated in some way, you'd probably be hard pressed to turn any part of the guard into farmers.

I like the idea of fighting on a young Knight/Exodites planet, I've loved that lore since Epic Space Marine and fighting a campaign there would be cool. The sustainability of agriculture and livestock on these worlds is key to their importance. Not sure that a Guard Regiment would be wasted on digging anything that wasn't a trench, but I could see them fighting on these types of worlds. Helping establish and protect colonies and secure large areas would be the key goal, after that I suspect there would be a lot of scouting and skirmishing. I've always envisaged these worlds to be sparsely populated so the Regiment would probably have quite a bit of ground to cover. Once the major threats had been dealt with and some decent PDFs had been raised I think the Regiment would be lifted out of their quite quickly, assuming the Departmento Munitorum didn't forget about them.

As for worrying about agricultural output, that's not really a Guardsman's remit until his Regiment gets disbanded and left to found its own home.

DW