The Farmers' Unions of Wales has criticised the Glastir scheme.
A new government scheme to pay farmers to manage the environment has been criticised by their representatives.
The Farmers' Union of Wales (FUW) said the Glastir scheme was a "missed opportunity" to make a real difference to climate change.
The scheme includes a tree planting programme to expand woodland to address greenhouse gas emissions.
Rural affairs Minister Elin Jones said Glastir would play a 'vital role' in responding to climate change.
The union's grand council at Aberystwyth called on Ms Jones to have a "complete rethink" of the scheme, which will replace five land management projects.
Glastir will have two elements, the first open to all farmers in Wales and the second targeted at areas delivering what the government regards as the most important environmental outcomes.
As part of the targeted element a tree planting programme aims to expand the area of Welsh woodland by 100,000 hectares over 20 years.
Farmers would receive a grant to establish permanent woodland on lower grade agricultural land, together with annual maintenance and income foregone payments for up to 15 years after planting.
An expansion of Welsh woodland was recommended by the independent group set up by the minister to examine how farming could address greenhouse gas emissions.
Ms Jones told AMs: "This is such an important issue that I have asked my officials to bring this option forward so that farmers can sign up to it from Autumn 2011, well in advance of the rest of the targeted element."
"Glastir represents a significant shift in approach for Wales and Welsh farming, but in a way that enables the industry to be at the forefront in delivering for Wales a land management agenda that fully justifies the investment of some £90m per year investment on the part of the taxpayer."
The government says the level of existing land management payments will be maintained under the new scheme.
But the FUW says there is a scarcity of information about Glastir and a more easily accessible, simpler scheme is needed.
The union's deputy policy director Rhian Nowell-Phillips said: "Glastir is missing the opportunity to make a real difference to climate change because it ignores the opportunities for sequestrating (isolating) carbon from managed grazing and concentrates on tree planting.
"Such a measure is unlikely to be taken up by farmers as they would be reluctant to see agricultural land taken out of production with little gain."
'More time needed'
Tywyn-based farmer Richard Vaughan, who chairs the FUW's land use and parliamentary committee, said: "Its high time that the assembly realised that farmers need time to adapt and to be given more details about the scheme so that they can make the right business decisions."
Ms Nowell-Phillips argued bringing forward just one element of the targeted scheme could create more confusion amongst farmers.
She said: "One of our concerns has been how difficult it will be for farmers to decide whether to go into the all-Wales element without knowing whether they will be accepted into the targeted element..."
She added that the early start of one element of the scheme leaves insufficient information available to farmers, who need to indicate their interest in the targeted element within the next few weeks.
She said: "The FUW continues to demand a full economic impact assessment of the new scheme...Unless Glastir is made accessible and simpler there could be severe consequences for Welsh communities and environments, especially in the uplands."
The assembly government denies the implementation of Glastir is being rushed, claiming officials have worked closely with farming unions.
It says ongoing training events have so far informed 2000 farmers about the scheme and information about it has been published on its website.