Those two jobs are not mutually exclusive. Programming is mentally demanding, farm work physically (with some sleep deprivation). I think it is manageable with good planning.
Farm work is exclusively physically demanding only if you want to be the village idiot for ever. Machines (or for the matter, mules/oxen) are better suited for this than humans.
While there is a lot of physical work in farming, you need to keep a fair amount of tacit knowledge in your head and recall it almost instantly on an unpredictable basis. This is specially true if you are working with animals (which the OP is). It is not overly taxing to remember it all once you already know it, but when you are a newbie you have to put the effort and learn it in the first place.
More over, if you are learning it the traditional way (aka, following around an old hand and do as told) you are expected to figure out the patterns and learn on your own. This guys are no experts in pedagogy and since they are just mimicking their own mentors, an explanation of what you've just seen/done may or may not follow.
