Corn Breeding: Lessons From the Past

Overview

This is the first in a series of lessons specifically designed to instruct individuals without any formal training in genetics or statistics about the science of corn breeding. Individuals with formal training in genetics or statistics but without any training in plant breeding also may benefit from these lessons.

 

Corn Breeding: Lessons From the Past - Overview and Objectives

Ken Russell
Associate Professor, Department of Agronomy and Horticulture at University of Nebraska Lincoln (UNL), USA
Leah Sandall
Graduate Student, Department of Agronomy and Horticulture at University of Nebraska Lincoln, USA
 

JNRLSE Seal of approval


This is the first in a series of lessons specifically designed to instruct individuals without any formal training in genetics or statistics about the science of corn breeding. Individuals with formal training in genetics or statistics but without any training in plant breeding also may find these lessons beneficial.

To learn more about the science of corn breeding, a reasonable starting place is a review of the history of corn breeding. You can learn much by considering the failures and successes of past breeding efforts and the causes behind these. In subsequent lessons, many of the concepts introduced in this lesson will be examined in more detail.

Objectives

At the completion of this lesson you will be able to

What are the Origins of Corn?

Corn (the scientific name is Zea mays L., from the Greek word “zea” for a kind of grain and the West Indian word “mahis” for corn) is the most productive grain crop in the world. Grain yields higher than 400 bushels per acre (27 tons per hectare) have been reported. As impressive as corn’s productivity is its adaptability and variability. It is grown successfully in every continent but Antarctica, from equatorial lowlands to the Matnahuska Valley in southern Alaska to Andean highlands that are 12,000 feet (3600 meters) above sea level. How did this wonderfully productive and adaptable crop come into existence?

Corn is native to the Americas. Until Columbus introduced corn to Europe, it was not being grown outside the Americas. How corn evolved is still a matter of controversy among scientists, but probably the most prevalent theory is that corn was domesticated from teosinte. Tripsacum also may have been an ancestor. Both teosinte and tripsacum are grasses that can be found growing wild in various parts of the Americas (Figures 1a and 1b).


Note: Click once on any figure to view an enlarged version.

Fig. 1a: Tripsacum dactyloides (Eastern gamma grass) (University of Nebraska-Lincoln, 2004)

Fig. 1b: Two types of teosinte (University of Nebraska-Lincoln, 2004)
To understand the remainder of this lesson and subsequent lessons in this series, knowledge of the primary parts of a corn plant and of its reproductive biology is necessary. If you are familiar with the basic anatomy and reproduction of corn, then you can skip the next section and proceed directly to “Races of Corn”.

Anatomy and Reproduction of Corn

Most corn plants have a single stem, called a stalk, which grows vertically upward from the ground (Figure 2a). The height of the stalk depends both on the variety of the corn and the environment in which a corn plant is grown. As the stalk grows, leaves emerge. A typical corn plant grown by a farmer in the central United States will have a stalk that is 7 to 10 feet tall and has 16 to 22 leaves. The lower part of each leaf wraps around the stalk and is attached to the stalk at a juncture called a node. Typically the lowest four nodes are below ground. Roots develop from each of these nodes. Sometimes, roots develop from the first aboveground node, and these are known as brace roots (Figure 2b). Some varieties of corn in certain environments produce secondary stalks, known as tillers, which grow outward from near the base of the main stalk.

Fig. 2a: The primary parts of a mature corn plant. (University of Nebraska-Lincoln, 2005)

Fig. 2b: The primary parts of mature corn roots. (University of Nebraska-Lincoln, 2005)
Every corn plant has both male and female parts. The male part, which is known as the tassel, emerges from the top of the plant after all the leaves have emerged. The tassel usually consists of several branches, along which many small male flowers are situated. Each male flower releases a large number of pollen grains, each of which contains the male sex cell.

The female floral organ is called an ear. The ear develops at the tip of a shank, which is a small, stalk-like structure that grows out from a leaf node located approximately midway between the ground and the tassel (Figure 3). Occasionally, a plant will produce an ear at several consecutive nodes, but the one that is located uppermost on the stalk becomes the largest ear. The immature ear consists of a cob, eggs that develop into kernels after pollination, and silks. The cob is a cylindrical structure upon which kernel development occurs. The kernels are arranged on the cob in pairs of rows. From each egg, a hair-like structure called a silk grows and eventually emerges from the tip of the husk, which is a group of leaves attached to the shank that encloses the entire ear. Pollination occurs when pollen falls on the exposed silks. Following pollination, a male sex cell grows down each silk to a single egg and fertilization (the union of the male and female sex cells) occurs. The fertilized egg develops into a kernel and inside each kernel is a single embryo (a new plant). A vigorous corn plant may have 500 to 1000 kernels on a single ear.

Fig. 3: An ear of corn with shank and husk. (University of Nebraska-Lincoln, 2005)

Races of Corn

Types of corn are classified into races. Over 100 distinct races of corn have been described. However, when Europeans started to settle along the Atlantic Coast of North America, two races of corn pre-dominated in this region – the Northern Flints and the Southern Dents. Longfellow is a typical variety of the Northern Flint race and Gourdseed is a typical variety of the Southern Dent race (Figures 4a and 4b).

Fig. 4a: Northern Flint (Longfellow, left) and Southern Dent (Gourdseed, right) plants (University of Nebraska-Lincoln, 2004)

Fig. 4b: Northern Flint (Longfellow, top) and Southern Dent (Gourdseed, bottom) ears (University of Nebraska-Lincoln, 2004)
Northern Flints

Plant characteristics:

Ear characteristics: Southern Dents

Plant characteristics: Ear characteristics:

A New Race of Corn Is Born

Fig. 5: Northern Flint (Longfellow, left), Southern Dent (Gourdseed, right), and an example of a Corn Belt Dent (middle) (University of Nebraska-Lincoln, 2004)

Both by accident and by design, the settlers crossed the Northern Flints and the Southern Dents. From these crosses emerged an entirely new race of corn called the Corn Belt Dents. This race is the ancestor of nearly all the corn hybrids currently produced in the United States. The Corn Belt Dent race is variable in appearance, but ears are cylindrical or slightly tapered and have 14 to 22 rows of kernels that are characteristically dented at the crown at maturity (Figure 5).

As the new American farmers spread westward, they took their corn seeds with them and continued to make crosses between different types of corn, most often Northern Flints and Southern Dents. The result of this activity was the development of many open-pollinated varieties (OPV’s). (Figures 6a-6e)

Fig. 6a: Open-pollinated varieties (Lancaster Sure Crop, left and Reid Yellow Dent, right) (University of Nebraska-Lincoln, 2004)


Fig. 6b: Open-pollinated variety, Lancaster Sure Crop (University of Nebraska-Lincoln, 2004)

Fig. 6c: Open-pollinated variety, Reid Yellow Dent (University of Nebraska-Lincoln, 2004)



Fig. 6d: Open-pollinated variety, Midland (University of Nebraska-Lincoln, 2004)

Fig. 6e: Open-pollinated variety, Jarvis (University of Nebraska-Lincoln, 2004)

What is an open-pollinated variety?

The name, open-pollinated variety, refers to how the farmers replenished their seed stock. After a new variety was produced by crossing two varieties, a farmer propagated this new variety by saving the seed from the most desirable ears from the most desirable plants each fall. These ears were open-pollinated ears. That is, there was no effort to control the source of the pollen. The pollen that fell on the silks of these ears was dispersed from tassels of nearby plants by wind and insects. The result of this open-pollination was that every plant grown from saved seed was genetically unique. However, all the plants shared certain characteristics that were desirable to the farmer. Grain productivity was certainly one of these characteristics, but not the only one. For example, James Reid, who developed the Reid open-pollinated variety with his father in central Illinois, was an artist. He selected for a corn with a small shank that could be easily hand-harvested without spraining his artist’s wrist.

All the open-pollinated varieties grown in the United States in the 19th and early 20th centuries were developed by farmers. There were no professional corn breeders. The farmers were the corn breeders.

Corn Grain Yields, 1870 to 1930

From 1870 until about 1930 there was no increase in the U.S. national average corn grain yield (Figure 7). The average yield in the decade of the 1920s was 26.4 bushels per acre (1.8 tons/hectare), which actually was a bushel per acre less than occurred 50 years earlier. What were the reasons for this lack of response?

Average corn grain yields in the U.S. from 1870 to 2002 (University of Nebraska-Lincoln, 2004); shows level production until about 1940, then continuous increase from about 25 bushels per acre in 1940 to nearly 140 bushels per acre in 2002.
Fig. 7: Average corn grain yields in the U.S. from 1870 to 2002 (University of Nebraska-Lincoln, 2004)

Corn Shows, A Social Phenomenon

Corn shows were largely a social phenomenon that became popular in the U.S. Corn Belt beginning about 1900. In these shows, which were held at county and state fairs in the fall, a variety would be judged based on the appearance of a 10-ear sample. Uniform appearance of both ears and kernels reigned supreme. Thus, selection for uniformity became of paramount importance to many farmers from 1900 to 1920.

Some evidence suggests that this selection for uniformity actually caused grain yield potential to decline in open-pollinated varieties. But the prestige of the corn shows was so great, that few of the judges—many of whom were trained at reputable agricultural colleges—ever thought of testing the best of the show corns against similar open-pollinated varieties that were more variable.

Inbreeding, Hybrid Vigor, and Hybrid Corn

At about the same time the corn shows were peaking in popularity, in the northeast United States two researchers were experimenting with controlled pollinations in corn. Their findings would soon cause a remarkable change in the corn culture of North America. Both Edward East at the Connecticut Experiment Station and George Shull at the Carnegie Experiment Station on Long Island had begun to self-pollinate corn.

Under self-pollination, the silks of an ear are pollinated by pollen from the same plant. Typically, little self-pollination occurs in a field of corn. Most silks of a given plant are pollinated by pollen from surrounding plants. This is known as cross-pollination (Figure 8).

Fig. 8: Cross pollination in corn


When a plant from an open-pollinated variety is self-pollinated, all the progeny resemble that plant, although they all differ from each other and from the parent plant to some extent. If one of the progeny plants is self-pollinated, the new progeny again differ from each other and from the parent plant, but the degree of the difference is not as great as occurred after the first self-pollination. If this process is repeated about seven times, then a plant known as an inbred is produced. An inbred is a pure-breeding strain of corn. This means that if an inbred is self-pollinated, all of the progeny will be genetically identical to each other and to the inbred parent.

Fig. 9: Inbreeding in a variety of corn. (University of Nebraska-Lincoln, 2004)


This process of repeated self-pollinations is known as inbreeding. Inbreeding corn results in loss of vigor (Figure 9).

n figure 9, the plant at the far left is non-inbred, the plant second from left was produced by one generation of self-pollination, and the two plants on the right were produced by two generations of self-pollination. Inbred plants developed from open-pollinated varieties of corn are not as vigorous or high yielding as the non-inbred plants of the open-pollinated varieties. But what East and Shull observed was that when the self-pollinated plants were cross-pollinated to produce hybrid progeny, these plants sometimes were even more vigorous than the plants from which the inbreds had been developed. This phenomenon is called hybrid vigor (Figures 10a and 10b).





 


Fig. 10a: Inbred plant B73 (left), inbred plant Mo17 (middle), and hybrid plant B73 x Mo17 (right). (University of Nebraska-Lincoln, 2004)

Fig. 10b: B73 ear (left), B73 x Mo17 hybrid ear (middle), and Mo17 ear (right) (University of Nebraska-Lincoln, 2004)

A hybrid developed by crossing two inbreds is known as a single-cross hybrid. As is true for an inbred, all plants of a single-cross hybrid are genetically identical to each other. The productivity of a hybrid depends on the relationship between the two inbreds. A hybrid produced by crossing two inbreds developed from different but equally productive open-pollinated varieties usually will produce a more vigorous hybrid than one produced by crossing two inbreds from the same open-pollinated variety.

When a single-cross hybrid is allowed to open-pollinate (as happens in a farmer’s field), approximately half the hybrid vigor is lost. This is the basis of the hybrid corn seed industry. The crop produced from open-pollinated seed harvested from a single-cross hybrid will not be as productive as the original single cross.

Corn Grain Yields, 1930 to Today

The national average corn grain yield in the United States began to increase steadily in the 1940s (Figure 11). In the most recent decade, the average yield was 125 bushels per acre, nearly five times greater than 70 years before. Several studies conducted by universities have indicated that much of this improved yield was the result of improved genetics; that is, it occurred because farmers were planting improved varieties of corn developed through plant breeding. Greater use of fertilizer, more and better herbicides, improved soil tillage, and other altered production practices also contributed to the increased yields.
Fig. 11: Corn Grain Yields (University of Nebraska-Lincoln, 2004)
The beginning of the yield increase coincides with the beginning of the transition by farmers from planting open-pollinated varieties to planting hybrids. However, not all of the yield increase that occurred during the past 70 years can be explained by hybrid vigor.

The yield advantage of a single-cross hybrid produced by crossing two inbreds developed from two different open-pollinated varieties over the average of the two open-pollinated varieties varies greatly depending upon the open-pollinated varieties that are chosen. It may be as great as 100%, but in many instances will be less. But improvement in average yields from 1930 to 2002 was 400%. In addition to hybrid vigor, genetic improvements were made. Today’s single-cross hybrids yield more than the single-cross hybrids of 70 years ago. Also, public corn breeders have developed many varieties (often called populations or synthetics) that are superior to the open-pollinated varieties that were popular before the introduction of hybrids.

Why were corn breeders in the mid- and late-20th century able to make such substantial genetic improvements for grain yield, whereas no increase in yields was realized from 1870 to 1930? The development of single-cross hybrids was partly the answer. But two other factors contributed.

Corn Breeding: Lessons From the Past - Summary and Definitions of Key Words

Although corn is grown across the world, it originated in the Americas. Types of corn are called races. Nearly all modern corn grown in the United States belongs to the Corn Belt Dent race, which largely was developed from two other races, the Northern Flints and the Southern Dents.

Early American farmers developed and grew open-pollinated varieties, but from 1870 until 1930 the annual average corn grain yield in the United States did not increase. In the 1930s, open-pollinated varieties were gradually replaced by hybrids that were produced by crossing inbreds, and corn yields started to increase. Today, the average corn yield in the United States is approximately five times greater than it was 70 years ago. This increase is partly attributable to new breeding and testing methods that have resulted in genetically superior hybrids.

 Key Words

Zea mays L.
– the scientific name of corn.

Race – a class of corn in which all plants of that class share certain characteristics, such as ear shape and number of kernel rows.

Open-pollinated variety – a variety of corn that is named for the manner in which seed of the variety is propagated across generations.

Self-pollination – the type of pollination that occurs when pollen from a single plant falls on the silks of that same plant.

Cross-pollination – the type of pollination that occurs when pollen from one plant falls on the silks of a different plant.

Inbreeding – a system of mating in which mates are more likely to be related than would occur if mating was random. Self-pollination is an extreme type of inbreeding.

Inbred – a pure-breeding strain of corn.

Single-cross hybrid – the type of hybrid that is produced when two different inbreds are cross-pollinated.

Hybrid vigor – the phenomenon of a hybrid plant having greater vigor than its parents.

Corn Breeding: Lessons From the Past - Further Reading

Eubanks, M.W. 2001. Mysterious origin of maize. Econ. Bot. 55:492-514.

Goodman, M.M. and W. L. Brown. 1988. Races of corn. p. 33-79. In G.F. Sprague and J.W. Dudley (eds.) Corn and Corn Improvement. ASA, Madison, WI.

Troyer, A. F. 1999. Background of U.S. hybrid corn. Crop Sci. 39:601-626.

Wallace, H.A. and W.L. Brown. 1988. Corn and its early fathers. Iowa State Univ. Press, Ames.