Corn, the Great American Crop
R.L. (Bob) Nielsen
Agronomy Dept., Purdue Univ.
West Lafayette, IN 47907-2054
Email address: rnielsen
orn is the only major cereal crop whose origin can be claimed by the Western Hemisphere. Centuries before Europeans "discovered" the New World, corn played a major role in the Mayan, Aztec, and Incan civilizations in Central and South America. In fact, the earliest archaeological evidence of corn was found in Mexico's Valley of Tehuacan and was dated at about 7,000 years old. Following corn's "discovery", the crop quickly spread to Europe, Africa, and Asia.
Corn's scientific name is Zea mays L. Native Americans called their crop "maize". After seeing maize for the first time, European settlers called it "Indian corn" in an attempt to compare the crop to the small grains grown in Europe that they called "corn". Today, most of the world still refers to the crop as maize, but the U.S. simply calls it corn.
Most of the field corn grown in the U.S. is "yellow dent ", but other types grown include popcorn, sweetcorn, and white corn. Popcorn is grown primarily in two states, Indiana and Nebraska. Sweetcorn is what many non-farm folks think of when "corn" is mentioned. White corn is grown mostly in the southern half of the U.S. and is used for producing foodstuffs such as corn flakes and corn meal. In recent years, other specialty corns have been developed for industrial uses. For example, waxy and high-amylose varieties contain high proportions of certain starch types useful for specific food and non-food uses.
Indiana traditionally ranks in the top 5 corn producing states in the country. In 2011, nearly 6 million acres of corn were harvested in Indiana, with an estimated average yield of 146 bushels per acre. Total statewide production was estimated at nearly 840 million bushels, valued at nearly 5.3 billion dollars. Total U.S. production harvested from 84 million acres was estimated to be over 12 billion bushels with a total worth of about 77 billion dollars!
The corn kernel is one of nature's most miraculous packages. From the time a 1/4 gram kernel is planted in late April, a 10-foot tall plant develops in about 56 days and 150 to 200 grams of seed are harvested in about 135 days. The return on investment for that one kernel is 600 to 800 percent!
Three important parts of the corn kernel are the pericarp (seed coat), endosperm, and embryo. The pericarp protects the enclosed endosperm and embryo from attack by fungi and bacteria. Damage to the pericarp during harvesting or processing can lead to decreased germination and seedling vigor. The endosperm occupies the bulk of the kernel and is the main energy reserve for the development of the young seedling. Starch is the primary component of the endosperm. The embryo contains a miniature plant with 4 to 5 preformed leaves, coleoptile, radicle root, and lateral seminal roots. The embryo also contains a cotyledon, which is an oil-rich storehouse of food important for the germination process.
When a kernel is placed in warm (50 degrees F or greater) and moist soil, germination will occur in 2 to 3 days. If watched closely, the first visible sign of germination is a slight swelling of the kernel. This is caused by water being absorbed through the pericarp. This moisture activates the enzymes that are necessary for germination. Under favorable conditions, the radicle root emerges near the tip of the kernel in several days and grows downward. Within a day or two, additional roots (called lateral seminal roots) emerge from near the base of the kernel and grow at about a 30 degree downward angle. The radicle and lateral seminal roots together serve to anchor the young seedling in the soil and take up a small amount of moisture and nutrients. Shortly after the radicle appears, the coleoptile emerges and grows toward the soil surface. The coleoptile (sometimes called the "spike") is a rigid piece of tissue that completely surrounds the 4 to 5 preformed leaves of the embryo. Just as the coleoptile nears the soil surface, exposure to sunlight causes the tip to split, allowing the rapidly expanding true leaves to emerge.
The growing point is an area of active cell division and elongation located at the top of the young stalk. The remainder of the leaves will be formed by the growing point - one leaf per stalk node, arranged alternatively, for a total of about 20 leaves per plant. After the last leaf is initiated, the tassel is formed by the growing point. Initially, little stalk tissue exists and the growing point is located about 1/2 inch below the soil surface, near the crown of the seedling. While the growing point is below ground, the seedling is relatively immune to above- ground damage. By 21 days after emergence, the stalk has begun to elongate, slowly elevating the growing point region. The growing point reaches the soil surface at about the time 5 to 6 fully exposed leaves are visible and rapidly moves upward after that.
From this point on towards pollination, the plant grows rapidly and its demands for water and nutrients increase dramatically. This rapid growth phase occurs throughout the month of June, when daylengths are near their maximum. Optimum moisture and temperatures during this time will help ensure the development of a large corn "factory" by the time flowering begins.
Corn flowers about 60 days after planting in the central Cornbelt of the U.S. The corn plant has separate male and female flowers. The tassel, found at the top of the plant, contains the male flowers. These flowers form tiny double-barrelled structures called anthers that contain pollen grains. Under the proper moisture conditions, tiny pores open at the anther tips, allowing the pollen to "shed". The female flowers are contained in the ear shoots located about 6 to 8 leaves below the tassel. Each female flower consists of an ovule (potential kernel) on the cob and an attached silk (stigma) that elongates towards the tip of the ear shoot.
Pollination is the act of transferring the pollen from the tassels to the exposed silks on the ears. Corn is called a cross-pollinated species because pollen freely moves from plant to plant throughout a field. Within minutes of landing on a silk, a pollen grain "germinates" and develops a pollen tube that contains the male genetic material. Within 24 hours, the pollen tube grows down the inside of the silk and the ovule is fertilized. Kernel development continues over the next 55 days. Visible stages of kernel development are blister, milk (roasting ear stage), dough, dent, and physiological maturity. Physiological maturity occurs when a thin black layer develops near the cob end of the kernels. This "black layer" is simply a layer of cells that die, collapse, and turn black at the end of kernel development.
Until the early part of the 20th Century, corn varieties were developed and maintained by farmers themselves. During harvest, the farmer would set aside the best-looking ears from the field for planting the following year. In this way, the farmer was also a plant breeder, developing varieties by a method called " mass selection". Because the varieties were allowed to cross-pollinate normally and openly, they were called " open- pollinated" varieties.
Beginning in the late 1920's and early 1930's, the concept of hybrid seed corn was developed by several researchers. As practiced today, hybrid varieties are developed by seed companies through carefully controlled cross-pollination between two "parent" lines, called inbreds. These inbreds are developed from several years of self-pollination and selection for desirable characteristics. In hybrid seed production fields, rows of the "male" parent are planted every 4 to 6 rows and provide the pollen to fertilize the ear shoots of the female plants. Rows of the "female" parent must usually be detasseled to prevent them from pollinating themselves. Because hybrids are produced by controlled pollination, farmers must purchase new seed each year rather than saving their own. However, the agronomic performance of hybrids is vastly superior to that of the older open-pollinated varieties.
Development and implementation of genetic engineering techniques in corn breeding resulted in the first transgenic traits available to farmers in the mid-1990's. These traits are labeled "transgenic" because the genes responsible for the new traits were transferred from a different biological organism. Transgenic traits available to farmers today include those for biological resistance to certain insects, resistance to certain herbicides, and improved efficiency in ethanol production from the grain. To date, none of the transgenic traits literally improve yield. Rather, they simplify the control of insects and weeds, potentially reduce the need for pesticides, or improve the efficiency of the ethanol production process.
Until about the 1950's, corn was planted in hills, with several kernels per hill, and the hills spaced 40 inches apart. This was called a "check row" planting pattern and allowed the crop to be cross-cultivated at right angles throughout the field. The rows were spaced wide enough for the horses used to pull the farm machinery. With the introduction of tractors and chemical herbicides, row spacings have since gradually narrowed. Today, corn is typically planted 1 to 2 inches deep, 7 to 10 inches apart within rows spaced 30 inches apart.
Corn requires 20 to 25 inches of water during the growing season, the majority of it coming from rainfall in May, June, July, and August in Indiana. Corn removes about 120 lb. of nitrogen, 70 lb. of phosphorus, and 46 lb. of potassium from the soil in the harvested grain. Much of this nutrient requirement is supplied by soil reserves, but some must be annually supplied by organic or inorganic fertilizers. For example, nitrogen may be supplied by legume crops (soybean, alfalfa) grown previous to corn, inorganic nitrogen fertilizers, animal manures, and municipal waste sludges. The optimum soil pH range is 6.0 to 7.0. Lime can be applied to acidic soils (less than pH 6.0) to increase the soil pH to optimum levels.