NFU CYMRU is calling for concerted action to curb illegal, damaging and dangerous use of common land by drivers of off-road vehicles and motorbikes.
Because cross-compliance regulations apply to commons, the union fears that the single farm payments of farmers with grazing rights will be compromised.
In Wales, 180,000ha, (445,000 acres) or 10.7% of the land area, is registered common, and a significant part of it is close to the heavily populated M4 corridor in south Wales.
Graziers have recently reported a sharp rise in the number of stock killed by motorists speeding on roads crossing common land, and about growing use by motorbikes and 4x4s.
William Jenkins of Monmouthshire NFU, described off-road driving as an escalating menace.
“I envisage a scenario where farmers will no longer wish to graze livestock on the common due to the reckless behaviour of a small number of people, and this could have a significant impact on our incomes and the environment of the common,” he said.