Rural affairs secretary Huw Irranca-Davies called it "a landmark moment for Wales" and said an early test would be how many farmers sign up.
New Year 2026 is "really significant" for Welsh agriculture, said Abi Reader, NFU Cymru's deputy president, who owns a dairy farm in Wenvoe, Vale of Glamorgan.
"This is a once in a generation change to farm support payments and there's a lot of expectation riding on this," she said.
For decades farms had received more than £300m a year in EU subsidies, shared out largely based on how much land they had.
Getting a replacement plan in place after Brexit - known as the Sustainable Farming Scheme (SFS) - had been "an absolutely enormous rollercoaster", Ms Reader said.
Unions had branded earlier versions of the government's plans as "unworkable" and they contributed to a wave of protests in 2024, including the largest ever seen outside the Senedd.
After further negotiations, final details were unveiled this summer.
Farms joining the SFS will have to commit to 12 so-called universal actions in exchange for their entry-level payment.
These range from maintaining hedgerows and reducing pesticide use to attending online courses on sustainable farming practices.
Additional funding would be available for more ambitious environmental work and farms working together.
"I think we're largely pleased with the framework now, but there are still a number of tweaks [to be made] and we still need a budget that will reflect inflation and really deliver for farms and for the countryside," Ms Reader said.
Farmers now have to prepare to apply for the new scheme by 15 May.
Alternatively they could stick to their old subsidies as they are phased out, but see a 40% cut to their payments this year.