SAO PAULO, Dec 11 (Reuters) - A farmers' lobby in Brazil

is seeking to end a two-decade-long agreement that forbids grain

traders from buying soybeans from farms on deforested land in

the Amazon rainforest, claiming the deal has created an uneven

playing field.

Soybean farmer lobby Aprosoja-MT based in western Mato

Grosso state said on Wednesday that the agreement fostered "a

purchasing cartel" while harming farmers who strictly comply

with the South American nation's forest code.

The group said in a statement that it formally asked

antitrust watchdog CADE to end the deal.

Brazil is the world's largest soybean grower and exporter,

with Mato Grosso the country's top-producing state.

Global commodity giants including ADM and Bunge

voluntarily signed up for the "Amazon soy moratorium" in

the mid-2000s, pledging to stop buying soy from farms in the

rainforest that were deforested from 2008.

Under forestry rules, Amazon landowners can clear up to 20%

of their property. But an early 2000s deforestation surge

sparked calls for action by companies that feared a wider ban.

Scientists and conservationists have praised the moratorium

for slowing deforestation in the Amazon, the world's largest

rainforest and a bulwark against climate change since its trees

absorb vast amounts of climate-warming greenhouse gases.

Aprosoja-MT noted it filed a complaint to CADE after years

of failed negotiation attempts, adding that the moratorium

generated 20 billion reais ($3.3 billion) in losses for the

state.

CADE, which already has an open investigation over potential

market manipulation related to the moratorium, did not

immediately respond to a request for comment.

Trader lobby Abiove said the moratorium is "an instrument to

defend Brazilian agriculture" and does not bring harm to the

sector. Its lawyer Francisco Todorov told Reuters there is

plenty of land to farm soy in the Amazon in regions deforested

before 2008.

Earlier this year, Mato Grosso state lawmakers passed a law

stripping tax breaks from firms adhering to the moratorium.

"Our commitment is with the families who have been harmed

for almost 20 years," Aprosoja said in a separate statement.

Last week, environmentalists accused grain traders of

seeking to weaken the moratorium.

Abiove has said it was holding discussions on the

moratorium, but without going into further detail.

Asked for comment about the matter on Wednesday, Abiove said

it had no update.

($1 = 6.0195 reais)