Corn (Zea mays) production in the U.S. includes corn produced for silage (corn which is harvested wet and fed to livestock much like hay and forage crops) and corn produced for the grain (92% of total harvested acres). On average, around 71 million acres of corn are harvested for grain annually although acres vary by year. Average national grain yields have ranged from 129 to 160 bushels/acre between 2000 and 2006, mostly due to weather (table 1). Production is concentrated in the Midwest (about 80% in the states of Illinois, Indiana, Iowa, Kansas, Minnesota, Missouri, Nebraska, South Dakota, Ohio, and Wisconsin), but occurs in nearly every state except New England (USDA-NASS) (figure 1). Currently, about 80% of corn is produced in rotation with another crop, typically soybeans (75%), although other crops (e.g., wheat, rye, barley, and oats) are also used (Kim, 2003; USDA-ERS, 2006). Corn tillage practices vary substantially by region—at a national level, 62% of corn acres used conventional till operations, 18% used reduced till operations, and 20% employed no-till methods in 2004 (Conservation Technology Information Center, 2004).
In the Midwest, corn is typically planted between mid-April and early May in 30 to 38 inch wide rows at a rate that yields 28,000 to 32,000 plants/acre. Most corn acres are fertilized (96% nitrogen, 81% phosphorous, and 65% potassium in 2005) at national average applications rates of 138 lbs N/acre (range of 67 to 171 lbs/ac; 138 to 147 lbs/ac in five largest producing states), 58 lb P2O5/acre (range of 35 to 77 lb/ac), and 84 lb K2O/acre (range of 19 to 124 lb/acre) (USDA ERS). Pounds of active ingredients of pesticides (i.e., insecticides, herbicides) applied to corn acres decreased from 227.3 million lbs in 1997 to 174.6 million lbs in 2004 due to changes in management, use of newer herbicides that are applied at lower rates, and the adoption of genetically modified corn varieties for weed and insect control. In 2005, about 25% of U.S. corn acres were planted with herbicide tolerant corn varieties and 35% of corn acres were planted to bt corn varieties (corn varieties that contain bacillus thuringensis to control root cutworms) (Fernandez-Cornejo, 2006).
Corn production costs vary widely averaging $1.08/bushel and $2.98/bushel for the 25% of producers with the lowest and highest costs respectively (year 2001). Low cost producers generally have larger farms and more corn acres, have higher yields/acre, are less likely to irrigate, and more frequently utilize reduced tillage and crop rotations than do high cost producers (Foreman, 2006).
Most corn produced in the U.S. is used as livestock feed. About 15% is exported and around 7% is used to produce sweeteners (e.g., high fructose corn syrup used in soft drinks). A significant and increasing portion of corn production is used to produce bioenergy and bioproducts. In 2005, about 2.15 billion bushels of corn (of the 11.11 billion bushels produced) were used to produce 3.9 billion gallons of fuel ethanol (USDA-ERS; RFA, 2006). Corn is also used to produce bioproducts, such as polylactic acid (biodegradable plastic) and 1,3-propanediol (used to produce Sorona®, a synthetic fabric). Additionally, corn stover (the above ground, non-grain portion of the corn plant) could be collected and used as a bioenergy and bioproduct feedstock. About 1 dry ton of corn stover is produced for every 1 ton of corn grain. After leaving sufficient quantities of stover on the field to maintain the health and productivity of the soil, several million dry tons of corn stover could potentially be available.
Corn production has steadily increased over time (from 4.17 to 11.11 billion bushels since 1996). About 20% of the increase is due to increased acres with the remainder due to increased yields resulting from genetic improvements and changes in management (figure 2). Yield increases have averaged 1.8 bushels/ac/yr between 1965 and 2005 (CAST, 2006).
Corn yields are expected to increase in the coming years. A workshop of crop experts (English, 1997) projected average national yields of 215 and 260 bu/ac by 2030 and 2050 respectively (compared to 1990-1992 yields of 120 bu/ac) under the most likely scenario as a result of genetic, management, and equipment improvements. The USDA projects average national yields of 163.9 bushels/acres and 77.2 million harvested acres by 2015 (USDA OCE). FAPRI projects average national corn grain yields of 164 bu/ac and 78.7 million harvested acres by 2015 (FAPRI, 2006). Industry sources indicate the potential of substantially higher grain yields resulting from improved moisture and nutrient use, cold tolerance, and resistance to pests, diseases, and weeds. Seed producers are incorporating multiple characteristics (stacking) into the new varieties under development (Pioneer, 2007; Fraley, 2006; Syngenta, 2007).
New varieties with traits more conducive to the use of corn for bioenergy and bioproducts are also being developed. Approaches include modifying the corn to have new or improved functions, more uniform functions, or decreased processing costs. Examples include improving the starch content and/or the fermentation efficiency (i.e., the amount of total starch broken down into glucose) such as waxy corn (which contains higher proportions of amylopectin--starch made up of branched chains of glucose molecules); high amylose corn (amylose is starch composed of linear chains of glucose molecules) (Johnson, 1999; Pioneer, 2007); highly extractable starch corn that yields about 2% more extractable starch than commodity corn varieties in wet milling applications (University of Illinois, 2003; Pioneer, 2007); and high total fermentable hybrids that provide a increase in ethanol yields per bushel of corn (about 2-4%) in dry grind applications (Fraley, 2006; Pioneer, 2007). Modifications to the starch are also being explored such as introducing functional groups (such as aldehyde groups) or lengthening the starch chains (Johnson, 1999). Other approaches include development of corn varieties containing amylase enzymes (enzymes that break down the corn starch into sugars). Use of these varieties could reduce the amount of amylase enzymes that would need to be added in the conversion process (BBI, 2006b; Syngenta, 2007).