August 2005: President George W. Bush signs the Energy Policy Act of 2005, requiring oil companies to add ethanol to their gasoline. Called the Renewable Fuels Standard, this mandate begins with a 4-billion-gallon requirement in 2006 and doubles by 2012. Corn is selling for $1.95 a bushel.

January 2007: In his State of the Union speech, President Bush calls on Congress to require production of 35 billion gallons of renewable and alternative fuels in 2017. It would effectively be a huge increase in the ethanol mandate. Corn is selling for $3.05 a bushel.

February 2007: Barack Obama, the junior senator from Illinois, the nation’s No. 2 corn-producing state, declares his candidacy for president. In his speech he hails “homegrown, alternative fuels like ethanol.” Obama is a strong supporter of passing a new, higher Renewable Fuels Standard.

December 2007: Congress passes the Energy Independence and Security Act of 2007. Bush signs it into law. It expands the renewable fuels standard to require 36 billion gallons of ethanol and other fuels to be blended into gasoline, diesel and jet fuel by 2022. Corn ethanol production would max out at 15 billion gallons in 2015. Corn is selling for $3.77.

January 2008: A study in the Proceedings of the National Academy of Sciences predicts that the ethanol mandate will increase nitrogen pollution in rivers, worsening the Gulf of Mexico’s dead zone, which cannot support sea life.

February 2008: A study in the journal Science warns that if U.S. biofuel policy encourages farmers to plow into untouched grassland or farmland that has been set aside for conservation, it will undermine efforts to reduce greenhouse gases. That’s because plowing into grassland releases carbon dioxide. The Department of Energy responds that the new fuel standard can be met without plowing into any conservation land.

2008: The amount of farmland set aside for conservation suddenly decreases. About 34 million acres are enrolled in the government’s voluntary Conservation Reserve Program, a drop of about 2 million from 2007.

May 2009: President Obama’s EPA takes the first steps toward implementing the new ethanol mandate. Government experts conclude that corn ethanol is, on average, 16 percent better than gasoline when it comes to greenhouse gas emissions. The law requires that new ethanol plants be 20 percent better.

2009: Enrollment in the Conservation Reserve Program falls again, this time by nearly 1 million acres.

March 2010: After lobbying from the agriculture industry, EPA publishes its final rule on the new ethanol mandate. The new analysis shows ethanol is 21 percent better than gasoline, slightly better than required by law. As part of the analysis, the government assumes corn prices will rise only slightly, to $3.59 a bushel, by 2022.

August 2010: Corn sells for $3.65, already eclipsing the government’s long-term price estimate.

2010: For the first time on record, ethanol is the No. 1 use for American corn, eclipsing livestock feed. Some 2.4 million more acres disappear from the Conservation Reserve Program.

February 2011: Corn sells for $5.65a bushel.

2011: Farmland acreage set aside for conservation continues to fall, this time by 173,000 acres. About 4.8 million acres have been lost since 2006.

January 2012: A 30-year-old federal subsidy for ethanol expires, along with a tariff on imported ethanol. Ethanol blenders were getting a tax credit of 45 cents per gallon. Corn sells for $6.07.

2012: Another 1.5 million acres of conservation land disappears, bringing the total to more than 6 million since 2006.

March 2013: A new study in the Proceedings of the National Academy of Sciences uses satellite data to show that rising corn prices have encouraged farmers to convert grassland to cropland, which releases carbon dioxide into the air. The Renewable Fuel Association responds that “the extremely high rate of error associated with the satellite imagery” makes the study’s results “highly questionable and irrelevant to the biofuels policy debate.” Corn sells for $7.13.

May 2013: Des Moines Water Works in Iowa reports historic levels of nitrates in the drinking-water supply, blames agricultural fertilizer.

July 2013: The National Oceanic and Atmospheric Administration announces the size of the Gulf of Mexico dead zone has increased. Larry McKinney, executive director of the Harte Research Institute for Gulf of Mexico Studies at Texas A&M University-Corpus Christi, blames ethanol production.

August 2013: EPA finalizes renewable fuel standard for 2013, requiring 16.55 billion gallons of biofuels, mostly ethanol, to be consumed in U.S. this year. Corn sells for $6.21.