Which biofuel plants help global warming the most?

http://www.sciencedaily.com/releases/2008/12/081202133228.htm

It's not the most common big three. It's estimated that half the carbon once sequestered in the Mid-western soil has been depleted. Louisiana and S America think the answer is sugar cane. WRONG. It has now been determined that perennial grasses can sequester carbon most rapidly AND make good yields of biofuel.

This article also clearly explains the CO2 dilemma exacerbated by tilling the soil for our crops.