In Copenhagen, Ag Secretary Tom Vilsack promised to cut greenhouse gas emissions on farms. He said the government will be giving farmers more money for methane digesters. They're machines that capture methane from manure.

Katie Feeney is with the environmental group Clean Air Council.

"If you can make it easy for them and cost effective for them to be sustainable, to reduce their emissions, then I foresee a lot more people participating in programs such as that."

But some environmentalists say voluntary programs are not enough. They say big dairy farms should be regulated more.

Starting in the New Year, all kinds of businesses will have to report their greenhouse gas emissions. But there's a big exception: large concentrated animal farms don't have to.