By letter I mean that your farmer can write a letter that looks something like this:
(Place, dd.mm.yyyy)
To whom it may concern
I, (name of farmer), certify that (your name), date of birth (dd.mm.yyyy) has worked on my pig farm in (location, country) since (date1) until present. (your name) has experience with (task1), (task2) and (task3). He is hard working and reliable (or whatever else he wants to say about you).
(signature of farmer)
mailing address of farm
phone number
If the farmer does not or can not write the letter in English, you can get him to write it in his language and get it translated by a certified translator. The farmer if he is a nice guy may also not care if you write the letter yourself and he will sign. That is what my employer told me when I wanted a reference letter. Just write what you want, I will sign.