The main concern of southern africa's subsistence farmers is

The answer is pretty much contained within the question in this case.  "Subsistence" farming means you're doing enough farming to provide for your own family's needs, but not really any more than that.  Or on a village scale subsistence farming means to be able to provide enough for the subsistence of that village -- for that village to have enough to survive.

So, the main concern of southern Africa's subsistence farmers would be providing for their own family's needs.  They're not trying to produce excess crops for exporting.