There are some countries where you are only permitted to land or take of from an "approved" airfield, and with the owner's or operator's permission (or combinations of the above). I understand that France is one, and therefore that there are very few "farm strips" in France.
In the UK, I can use my back garden as a runway and land and take off from there, as long as it's safe (captain's responsibility to confirm that). I'm sure the expert CFIs will know precise details, but my understanding is that for a student pilot, it's up to the instructor to decide if it's safe or not.
Permission from the landowner or his "agent" is still needed, or it's "trespass".