HORT410 - Vegetable Crops
Corn - Notes
Corn, Zea mays L. (2n = 20, diploid) (maize), is a monocotyledon of the family Gramineae (Poaceae) [Poaceae Images], and represents one of the world's most important human and stock food crops (Jugenheimer, 1992). It is one of the largest of the cereals and can reach heights of 15 ft (Jugenheimer, 1992). The plant is monoecious, possessing separately located male and female flowers; the male (staminate) flowers are borne in the tassel at the top of the stalk, and the female (pistillate) infloresence is a cluster, called a cob, at a joint of the stalk (Jugenheimer, 1992). The corn silks hanging from the husk of each cob are the pollen receptors; each silk must receive a grain of pollen in order for its fruit, or kernel, to develop (Jugenheimer, 1992). A fertilized cob, or ear, contains eight or more rows of kernels (Jugenheimer, 1992). Typically the stalks have one to three cobs (Jugenheimer, 1992).
(see: National Corn Growers Association)
Corn is one of the few economic plants native to America (Jugenheimer, 1992). Archaeological and geological excavations indicate that a type of primitive corn was used as a food in Tehuacan, Mexico at least 7,000 years ago (Jugenheimer, 1992; Galinat, 1988). No wild forms of corn have been found, and the origin of domesticated corn remains speculative (Jugenheimer, 1992). It is probable that corn's progenitor may be a domesticated version of teosinte (a wild grass) still found in Mexico and Guatemala (Galinat, 1988). It is thought that the processes of mutation, natural selection, and mass selection by the American Indians gradually transformed certain varieties of wild corn into the cultivated plant called maize (mahiz) (Jugenheimer, 1992). Corn was unknown outside the New World before the arrival of Columbus, but the plant was extensively cultivated in all its present forms by the American Indians (Jugenheimer, 1992). Indian maize was brought to Europe and Africa by 16th-century explorers (Jugenheimer, 1992). Since the 1930's, the development of hybrid varieties of corn, use of nitrogen fertilizers, and increased mechanization have resulted in greatly increased corn yields (Jugenheimer, 1992). Modern varieties have improved quality, are larger in cob size, and have a greater number and weight of the kernels per ear in comparison to the corn grown by the American Indians (Jugenheimer, 1992).
Corn grows best in deep, well-aerated, warm, loam soils rich in organic matter, and with a high nitrogen, phosphorus, and potassium content (Jugenheimer, 1992). Optimum crop performance is achieved under moderately high summer temperatures with warm nights, and adequate
(but not excessive) rainfall that is evenly distributed during the growing season (Jugenheimer, 1992). The Corn Belt of the midwestern U.S. is ideally suited to corn production because it satisfies these climatic requirements (Jugenheimer, 1992). The flat topograhy of this region has facilitated mechanization of production. Specialized equipment for corn production includes wheel-track planters (which dig the furrows, plant the seed, and cover the seed in one operation), mechanical corn pickers (which harvest (and shell) only the
ears), and field choppers (which cut and chop the stalks into silage) (Jugenheimer, 1992). Use of high yielding, disease resistant hybrids, nitrogen fertilizers, heavy plant populations, efficient mechanization, effective pest control and improved water management, have
contributed to an annual increase of 2.5 bushels/acre of corn during each of the past 25 years in the United States (Jugenheimer, 1992). Before 1930, more than 30 minutes of labor were required to produce a bushel (25.4 kg) of corn in the U.S.; less than 3 minutes is now
required (Jugenheimer, 1992).
In hybrid corn seed production common planting patterns include 4:1 (4 rows of female parent to one row of male parent), 4:2, 4:1:4:2, 6:2 and solid female with interplanted male (Wych, 1988). Split-date planting of parents is often used to ensure that the two parents "nick", or reach flowering stage concurrently (knowledge of days to flowering, and/or heat units to flowering is critical) (Wych, 1988). Pollen control is essential (Wych, 1988). Hybrid seed production fields are isolated from other corn fields which could provide pollen which would contaminate the hybrid seed production field (Wych, 1988). Often commercial hybrid seed production fields are planted with extra male parent border rows for the purpose of providing pollen saturation adequate to ensure genetic purity of the hybrid seed (Wych, 1988). Soybean is often used to provide the required isolation distance (125 to 201 m) from the nearest "contaminant" corn (Wych, 1988). Many vegetable growers are also contract growers for hybrid seed companies because they can provide well isolated fields and are used to dealing with labor intensive crop production (Wych, 1988). They must be willing to cooperate with seed companies to alter their cultural practices and/or timing, rate and kind of herbicides, insecticides, or fungicides applied (Wych, 1988). Equipment modifications are often necessary (Wych, 1988). The most labor-intensive time is detasselling of the female parent (Wych, 1988). Detasselling of the female parent before they shed pollen is required to ensure that only the male parent provides pollen to the emerging silks of the female (Wych, 1988). Major seed corn companies hire and train seed field inspectors to observe pollen control operations, report irregularities, and assist in interpeting rules for the detasseling/rogueing supervisors (Wych, 1988). When detasseling starts, the inspectors check fields to be sure that female parent tassels and off-types in male rows are properly removed (Wych, 1988). Genetic purity of the hybrid seed is essential (Wych, 1988). Teenage youth are often employed by seed companies to perform hand detasseling operations (Wych, 1988). Mechanical detasselers are available, and typically are self-propelled, high clearance machines (capable of operating even in extremely muddy fields) which either cut or pull the tassels off (Wych, 1988).
Corn is among the world's four most important crops (the others are wheat, rice, and potatoes) (Jugenheimer, 1992). The United States produces over 40 percent of the world output (Jugenheimer, 1992). The U.S. corn crop averages 8 billion bushels, worth $20 billion per year (exceeding that of wheat, oats, rice, rye, barley and sorghum combined) (Jugenheimer, 1992). Countries with large areas devoted to corn include the United States, China, Brazil, the USSR, Mexico, Argentina, Romania, Yugoslavia, France, India, and South Africa (Jugenheimer, 1992).
Corn is used primarily as a food in many areas of the world, although in the United States well over half of the crop is fed to livestock (Watson, 1988; Ruskin, 1988). Major industrial users of corn are feed manufacturers, millers, and the distilling and fermentation industries (Watson, 1988). Corn oil is obtained from the germ of the kernel by wet or dry milling (Watson, 1988). Corn sugars and syrups, are derived from the starch contained in the endosperm (Watson, 1988). In many areas of the developing world, maize is a vital staple, particularly for the rural poor, because it has many valuable properties (Ruskin, 1988):
- Gives one of the highest yields per hour of labor spent.
- Provides nutrients in compact form.
- Is easily transportable.
- Is protected against birds and rain by its husks.
- Is easy to harvest and can be shelled by hand.
- Stores well if properly dried.
- Is relatively free of major disease epidemics.
- Competes with weeds better than other cereals.
- Does not shatter and thus can be left standing in the field at maturity.
- Has cultivars with different maturing periods.
About half of the world's chronically undernourished people live in countries where maize is a staple (Ruskin, 1988). In Central America a typical diet often consists of 20 or more tortillas (an unleavened pancake-like corn bread) per day, supplemented with hot peppers, some native greens, and little or nothing else (Ruskin, 1988). Because corn is deficient in protein, both in quantity and in quality, nutritional problems often arise when it makes up a large proportion of the diet (Ruskin, 1988). Maize kernels contain only about 9.5% protein (low in comparison to beans (23%), peanuts (26%) or soybeans (38%)), but an even more serious concern is that maize protein is low in two nutritionally vital amino acids; lysine and tryptophan (Alexander, 1988; Ruskin, 1988). Maize is also deficient in the B-vitamin, niacin (Ruskin, 1988). Bodies deficient in niacin develop pellagra, characterized by dermatitis, diarrhea, and dementia (Ruskin, 1988). In 1963 researchers at Purdue University (E.T. Mertz, O.E. Nelson & L.S. Bates) discovered a corn mutant (opaque-2) which caused increased levels of lysine and tryptophan (Ruskin, 1988). The nutritionally improved lines developed from the opaque-2 mutant are now widely used in Central America and India, where these lines have been shown to significantly increase body weight gain in children (Ruskin, 1988). These lines are increasingly being used by U.S. farmers for livestock feed (Ruskin, 1988).
Corn may be divided into several groups, each differing primarily in the properties of the starch accumulated by the seeds (Alexander, 1988; Jugenheimer, 1992):
Dent corn (so named because the seed has a depression, or dent, in the crown) is the most widely grown type of corn in the U.S. (Jugenheimer, 1992). The characteristic denting is caused by rapid drying and shrinkage of the soft starch at the crown (Jugenheimer, 1992). It is thought that Corn Belt dents of the United States originated from hybridization of New England flints and gourdseed, a variety grown by the Indians of southeastern North America (Jugenheimer, 1992). Dent corn is used extensively for industrial use and for livestock feed (Jugenheimer, 1992).
The kernels of flint corn are smooth and hard and contain little soft starch (Jugenheimer, 1992). Flint corn was probably the type first seen by Europeans (Jugenheimer, 1992). Flint corn is more widely grown in Europe, Asia, Central America, and South America than in the U.S. (Jugenheimer, 1992). In temperate zones, flint corn often matures earlier, germinates better, and has earlier plant vigor than dent corn (Jugenheimer, 1992).
Popcorn, an extreme form of flint, contains only a small proportion of soft starch (Jugenheimer, 1992). It is grown primarily for human consumption as freshly popped corn and popcorn confections (Jugenheimer, 1992). The ability to pop seems to be conditioned by the unique quality of the endosperm which resists the steam pressure generated within the heated kernel until it reaches explosive force (Jugenheimer, 1992).
The kernels of flour corn are composed largely of soft starch and have little or no dent (Jugenheimer, 1992). Because of the softness of the kernels, the American Indians ground them for flour (Jugenheimer, 1992). Flour corn has been widely grown in the drier sections of the U.S. and in the Andean region of South America (Jugenheimer, 1992).
The kernels of sweet corn have a translucent, horny appearance when immature and are wrinkled when dry (Jugenheimer, 1992). The immature ears are eaten fresh or are canned or frozen (Jugenheimer, 1992). Sweet corn differs from dent corn primarily for only one recessive gene (Jugenheimer, 1992). Some sweet corn varieties are unable to convert sugar to starch (Jugenheimer, 1992). Others accumulate a water-soluble polysaccharide (WSP), phytoglycogen, at the expense of starch (Jugenheimer, 1992). A considerable quantity of sweet corn is grown as a winter crop in the southern part of the U.S., primarily Florida (Jugenheimer, 1992).
The kernels of this corn are waxy in appearance (Jugenheimer, 1992). Its starch differs chemically from common corn starch in that it lacks amylose (Jugenheimer, 1992; Watson, 1988). Waxy mutations (wx) are well characterized in American dents (Jugenheimer, 1992; Watson, 1988). Waxy hybrids are grown on a relatively small scale to produce a starch similar to tapioca starch (Watson, 1988; Jugenheimer, 1992).
Although not grown commercially, podcorn is of interest in terms of the origin of corn, since it is thought to resemble varieties of primitive corn (Jugenheimer, 1992). In addition to the entire ear being enclosed by a husk, each kernel is enclosed in a pod or husk in podcorn (Jugenheimer, 1992).
Sources of information:
"Specialty Corns" (ed. A.R. Hallauer), CRC Press, Boca Raton, FL (1994).
Jugenheimer, R.W. Corn. In "The Software Toolworks Multimedia Encyclopedia", Version 1.5, Grolier, Inc. (1992).
"Quality-Protein Maize" (ed. F.R. Ruskin), National Aacdemy Press, Washington, D.C. (1988).
"The Maize Handbook" (ed. M. Freeling, V. Walbot), Springer-Verlag, NY (1994).
M.G. Neuffer, E.H. Coe, S.R. Wessler, "Mutants of Maize", Cold Spring Harbor Laboratory Press (1997).
Galinat, W.C. The origin of corn. In "Corn and Corn Improvement" Third Edition, (ed. G.F. Sprague, J.W. Dudley), American Society of Agronomy, Madison, WI, pp. 1-31 (1988).
Wych, R.D. Production of hybrid seed corn. In "Corn and Corn Improvement" Third Edition, (ed. G.F. Sprague, J.W. Dudley), American Society of Agronomy, Madison, WI, pp. 565-607 (1988).
Shaw, R.H. Climate requirement. In "Corn and Corn Improvement" Third Edition, (ed. G.F. Sprague, J.W. Dudley), American Society of Agronomy, Madison, WI, pp. 609-638 (1988).
Olson, R.A., Sander, D.H. Corn production. In "Corn and Corn Improvement" Third Edition, (ed. G.F. Sprague, J.W. Dudley), American Society of Agronomy, Madison, WI, pp. 639-686 (1988).
Alexander, D.E. Breeding special nutritional and industrial types. In "Corn and Corn Improvement" Third Edition, (ed. G.F. Sprague, J.W. Dudley), American Society of Agronomy, Madison, WI, pp. 869-880 (1988).
Watson, S.A. Corn marketing, processing, and utilization. In "Corn and Corn Improvement" Third Edition, (ed. G.F. Sprague, J.W. Dudley), American Society of Agronomy, Madison, WI, pp. 881-940 (1988).