COPENHAGEN - An increasing shift away from fossil fuels such as gasoline to alternatives derived from plants and waste need not produce an increase in food prices, U.S. Agriculture Secretary Tom Vilsack told Reuters.

Speaking on the sidelines of a U.N. climate conference in Copenhagen, Vilsack added he expected the U.S. Department of Agriculture to run a farm carbon offsetting scheme under a prospective U.S. climate law.

Farm crops such as corn are used to produce alternative transport fuels such as ethanol, while wood is likely to becoming more in demand for both solid biomass and liquid biofuels as new technologies emerge.

Vilsack said any increasing competition for cropland may be compensated by increases in farm productivity, thereby restraining food prices.

Every time people start talking about food versus fuel it's always based on the fact (assumption) that where we are remains static relative to production, he said.

Higher food prices in 2008 led to food riots in some developing countries and were partly blamed on biofuels such as ethanol consuming part of the U.S. corn crop, for example. Vilsack cautioned against blaming biofuels for higher food prices.

But advances in crop productivity could keep pace with increasing demand for land, said Vilsack

Crop production experts are absolutely convinced that within 10 years with just seed technology they can produce a 100 bushel increase in yields on average.

It actually has happened, we've seen substantial increases in production even though the amount of cropland in the United States is slightly less than it was 10, 20, 30 years ago.

Vilsack added that new technologies would see biofuels made of livestock and household waste, which would not compete for land.

There are so many new opportunities for second and third generation biofuels. One is the use of agricultural waste.

Vilsack announced an agreement on Tuesday with U.S. dairy producers to target a 25 percent cut in emissions of methane -- a powerful greenhouse gas -- by 2020, by using manure to generate electricity.

Such emissions cuts may qualify for carbon offsets under a prospective climate bill, with legislation currently stalled in the U.S. Senate.

I think there'll be a recognition that ... what qualifies for an offset in the agriculture area USDA will be heavily involved in that. There are other areas of offsets. We don't know anything about landfills ... that should be another agency.