ARTHUR MAX June 1: The 2,700 pigs on the farm that John Horrevorts manages yield more than ham and bacon. A biogas plant makes enough electricity from their waste to run the farm and feeds extra wattage into the Dutch national grid. He even gets bonus payments for reducing greenhouse gas emissions. As the world struggles to reduce pollution causing climate change, attention has focused on the burning of fossil fuels in factories, power stations, and vehicles. But UN scientists says farming and forestry accou-nt for more than 30 per cent of the greenhouse gases that are gradually heating the earth. Much of that pollution comes from cattle, sheep and pigs that belch or excrete methane, a heat-trapping gas more than 20 times as potent as carbon dioxide, the most common global warming gas. Negotiators from 190 countries have been working to reach a new climate change agreement in Dec-ember on ways to reduce emissions and help countries adapt to changes in climate. They will reconvene June 1 in Bonn, Germany, for another two-week session. Yet it is uncertain whether cutting agricultural emissions will be part of the agreement expected to emerge at the final meetings in Copenhagen, Denmark. The subject is complex, emissions are difficult to measure, and the whole question is politically sensitive, touching on the distrust between the world