Federated Farmers says the Labour Party's decision to bring agriculture into the emissions trading scheme in 2013 would lead to more emissions.
Labour has reaffirmed its promise to tax animal emissions from 2013, if it becomes the next government.
Its environmental spokesperson Charles Chauvel says nearly half of New Zealand's greenhouse gas emssions come from grazing animals and it is environmentally irresponsible to leave agriculture out of the emissions trading scheme.
Federated Farmers vice president Dr William Rolleston says the tax would cost farmers $20,000 to $30,000 a year.
He says they can't afford that after years of losses.
Dr Rolleston says the extra costs would drive production to countries with less-efficient production systems.