"Off-panel fields and farms" is not said in error. Those are not "just-off-panel". Those are in the distance off-panel. From that location, however, the livestock are "just-off-panel", though this does get confusing, because if the farms and fields themselves are off-panel, one would, logically, never actually be near enough to them for the cows to be "just-off-panel".
It is worth noting, however, that Rhoda refers only to the vast majority of cows, and cows may well wander, or transported for various reasons. There could have been cows in the very town Susan was spending time in earlier, and those would, in fact, be "just-off-panel".
I believe you'll find everything about this adds up logically and should never be questioned.