Absolutely.
There are mandatory, what they call compulsory, articles of UPOV, that in order to accede or ratify a certain convention you must have that in your law. Failure to even have one of those compulsory elements means you don't meet the requirements of UPOV 91. Then there are also what we call optional articles to UPOV 91. The farmer's privilege is one of them. You can have the choice of whether to implement a farmer's privilege or not in your law. In those areas, countries do have latitude in terms of how they implement.