The Plant Protection Products (Sustainable Use) Regulations 2012 aim to encourage best practice and reduce possible environmental or other risks of using pesticides. But the NFU said occasional sprayers could lose out.
Deputy president Meurig Raymond said: "For operators adhering to best-practice guidelines, this will make little difference. But we are concerned over the impact on occasional sprayers, who may have to do a range of measures such as testing.
"Defra must be clear on what will be required and the time frame," he added. "Spray operators may not be sure if their continuous professional development under schemes such as the national register will be sufficient."
Meurig added: "We are pleased that the emphasis in the legislation is on reducing risks by responsible and considered use of pesticides, rather than reducing use without taking best practice into consideration."
Amenity Forum secretary Alan Spedding said: "We are working with the Chemicals Regulation Directorate, which is producing guidance. We will help it to produce an amenity sector action plan and are meeting later this month to discuss how this might work to ensure the safe and effective use of pesticides in our sector."