The next request you get, simply reply and say something along the lines of:
' That would be a FABULOUS time to come, OH desperately needs a few extra hands to (crutch the sheep/drench the lambs/fix the fences/mow the lawn/tip the cows) and I've got a STACK of (composting/manuring/seeding/weeding/painting) to do.
Bad news is that we're running really short of wine/beer, meat and money, so if you don't want a dry, vegetarian diet then please make sure to bring along a decent selection to share '
If they're true friends they won't mind helping and bringing stuff along