FOCUS ON: HYBRID TECHNOLOGY
HYBRID CORN
Harnessing Genetics for Higher Yields
Introduction
Corn is a versatile crop. It's a food staple for humans – from fresh corn on the cob, to corn tortillas, to popcorn. Corn can be made into cornstarch, a thickener in foods; corn oil, used for cooking; and corn syrup, a sweetener in so many of our food and drink products. But corn doesn't just feed us; livestock eat feed corn and corn can be processed to make both bio-diesel fuel and ethanol that can be mixed with the gasoline that we use in our cars. The market for corn is enormous.

Corn can be eaten or processed into other products.
In the late 1800s and early 1900s, US corn farmers faced a significant problem. Farmers wanted to produce more corn, but other farmland was already in use for other crops. Instead, farmers needed to figure out how to grow more corn on the land they already had. Despite farmers' best efforts, however, the amount of corn produced per acre - the yield - was stable.

How could corn farmers harvest more corn from the same amount of land? The key to higher yields is understanding how desirable traits – like corn cob size and plant height -- pass from generation to generation. Read on to learn about how the emerging science of inheritance and genetics in the early 1900s transformed corn plant breeding, addressed the corn yield problem, and shaped the corn industry into what it is today.
About the Organism
Corn, also known as maize, is a type of grass. Corn was domesticated about 8,700 years ago from teosinte, a large wild grass that grows in Mexico and Central America. Today's corn plants grow 3-13 ft (1-4 m) high, with a tall central stalk, and long leaf blades.



What is corn's scientific name?

NEEDS TO BE REDRAWN!
A modern maize or corn plant. The male and female flowers are separate. The ear, or cob, contains the female flowers that, after fertilization, develop into kernels.
Unlike most grasses, corn plants have separate male and female flowers. The tassel at the top of the plant contains the male reproductive organs that produce the pollen. Lower, where the leaf blades attach to the stem, the plant forms female flowers that contain the ovules. Each ovule has a long, thin silk that grows up to the top of the cob. After the silks at the top of the cob capture the pollen grains on the sticky stigmas, the male reproductive cells travel down the long styles to pollinate the ovules.

Corn typically cross-pollinates as the wind blows the pollen from one plant to the silks of other corn plants. Corn can pollinate itself, though self-pollination is not common. Instead, cross-pollination helps maintain the genetic diversity of corn because genetic information from two different parents mixes during fertilization of the ovule.
Corn? or Maize?
Corn means grain in Old English. In the United States, corn refers specifically to the maize plant; we use the names corn and maize interchangeably. In England, corn is often used for all types of grain.
Cross Pollination
In cross pollination, the pollen from one plant is transferred to the stigma of a different plant.
Self-Pollination
In self-pollination, the pollen from a plant in transferred to the stigma of the same plant.
As long as the plant gets the right amount of light, water, and nutrients, each fertilized ovule grows into what we call a corn kernel or seed. Hundreds of kernels cover each cob. The kernels are the support packages for the next generation. Each kernel contains a plant embryo and the starchy energy reserves to sustain it until it can start doing photosynthesis. The energy, or calories, we eat from corn come largely from the kernels' starches! Although each plant produces a few cobs, usually only the uppermost cob produces a large, complete ear.



Corn kernels come in a wide variety of colors, depending upon the traits of the parent plants.
Embryo
A plant embryo contains the tissue that will become the plant's stem, roots and leaves. Corn plant embryos contain one embryonic leaf or cotyledon.
Photosynthesis
In photosynthesis, plants use energy captured from sunlight to form sugars.
A field of corn plants with male flowers at the top.
What kinds of corn have you eaten? Baby corn? Popcorn? Sweet corn on the cob? Corn is planted from seed and harvested after the plants have matured to the point appropriate for their use. If the plant is grown for baby corn, it is harvested immediately after the female flowers develop and silk emerge, but before pollination. Sweet corn, often eaten as corn-on-the-cob, is harvested about 15-20 days after pollination and 2-3 months after planting, when the kernels have a high level of sugar. Field corn is left to mature further and dry in the field so that it's easy to store and process. Field corn, often a type called dent corn, is used as animal feed or made into cornmeal, corn syrup, ethanol, and many other products. Corn grown for popcorn is also harvested later in the season.
The "discovery" of the Americas by the Europeans in the 1400s and 1500s has also been called the discovery of corn. Since then, corn has traveled global trade routes and now is grown in temperate and tropical climates (between 58°North latitude to 40°South latitude) around the world. Worldwide, one billion metric tons of corn were grown in 2016. The US grows the most corn; other top growing countries include China, Brazil, Mexico, and Argentina.

Production quantities of maize by country, average 1994-2016
Think and Apply
Examine the two pie charts and describe how the regional production of corn has changed between 1967 and 2017.
Answer:
Most corn is produced in the Americas, although that proportion decreased to 51.6% in 2016 from 56.7% in 1961. Asia has increased the proportion of corn it produces relative to other regions. The proportion of corn produced in Europe, on the other hand, decreased from nearly 20% in 1961 to only 11% in 1960. The proportion of corn produced in Africa and Oceania relative to the rest of the world remained almost the same.
How Nutritious?


Sweet corn is a delicious and nutritious vegetable. A one cup serving of sweet corn provides 125 calories of energy, with much of the energy coming from carbohydrates in the sweet and starchy kernels. Low in fat and sodium, corn provides some protein and a substantial amount of dietary fiber as well. A serving of sweet corn also provides good amounts of potassium, magnesium, and Vitamin C.




Dietary Fiber
Dietary fiber is the parts of plants that your body cannot digest. Although it passes undigested and intact out of your body, it helps maintain bowel health and healthy cholesterol and blood sugar levels.
Potassium
Potassium is a mineral that acts as an electrolyte, helping our bodies maintain the right amount of fluid and promoting proper muscle and nerve function.
Vitamin C
Vitamin C is a water-soluble vitamin that acts as an antioxidant in our bodies.
Think and Apply
What types of corn products provide the most nutrition? Compare the nutrition information of sweet corn, above, with that of other products made from corn. Note that the serving sizes are different than the 1 cup of sweet corn, but for each serving size, compare the calories, where those calories come from (protein, fat, carbohydrates), the amount of dietary fiber, and the amount of various nutrients in each food.
Answer:
Corn syrup's calories are from carbohydrates; it's all carbohydrates, with no fat and no protein. Corn syrup provides no dietary fiber and no other nutrients.

A serving of tortilla chips contains more fat and sodium, as well as fewer carbohydrates and proteins, than a serving of sweet corn. Chips also have lower amounts of vitamins and minerals and provides a low level of dietary fiber.

While low in vitamins and minerals, air popped popcorn is also low in fat, sodium and calories as well. Most of the calories come from carbohydrates. Popcorn is low in dietary fiber.
The Challenge
In the late 1800s and 1900s, the demand for corn in the United States was high. People ate corn as food, canned it to have in the winter, and ground it into flour. Additionally, livestock farmers were becoming more interested in corn as feed for cows. Farmers needed to grow more corn, but the farmland available for growing corn was already in use, so farmers' only option was to grow more corn per acre. Sometimes with the help of state agricultural extension workers, farmers grew different varieties of corn, selected the best seed, and grew plants from that seed the next year. Unfortunately, the next year's crop was often variable, and yields still didn't increase. Despite farmers' best efforts to develop the highest quality seed, yields were flat. Farmers were stuck.
Think and Apply
Examine the graph of US Corn Yields between 1860 and 1930. Describe the trend in corn yields during that time.
Answer:
Corn yields stayed stable during that time period. The average yield was only about 25-30 bushels / acre.
Despite the stagnant yields, thousands of years of farming knowledge offered a possibility for the corn farmers. Farmers knew that crossing different species or varieties sometimes resulted in more vigorous offspring. The mule, useful because of its strength and endurance, was a cross between a horse and a donkey. There were also examples of crossing donkeys with other closely related species. A similar concept was observed in plants. Archeological evidence shows that Native Americans regularly planted two different strains of corn together, in some areas, so that they would cross-pollinate and produce more vigorous offspring. That traditional knowledge was supported and expanded upon by scientific experiments through the 1800s and into the 1900s.
Several scientists found greater positive benefits in cross-pollinated plants than in self-pollinated ones. One such scientist was Gregor Mendel, who published his results in the late 1800s. Mendel investigated the inheritance of traits in pea plants, which he chose because he could easily control their pollination. In addition to outlining the basic principles of inheritance, Mendel noted that hybrid plants – those that were the result of a cross between two different inbred lines of "true-breeding" parents - tended to grow much more robustly than the self-pollinated plants.

"It must be noted that the longer of the two parental axes is usually even exceeded by the hybrid, which perhaps can only be ascribed to the great luxuriance that appears in all plant parts once axes of very different length are joined. Thus, for example, in repeated experiments, axes of 1' and 6' length in hybrid union yielded axes whose length fluctuated between 6 and 7½' without exception." (Mendel, 1866, Versuche über Pflanzen-Hybriden, p. 11)


This Punnett square shows the cross between an inbred line of small pea plants (tt) and an inbred line of tall pea plants (TT). Mendel observed that all the offspring tended to be as tall as or taller than even the taller parent.
Charles Darwin, best known for his ideas about evolution by natural selection put forth in On the Origin of Species, also published a book on plant breeding in 1876 – The Effects of Cross and Self Fertilization in the Vegetable Kingdom. In this book, he shared data from plant crosses and provided more support for the idea that self-pollinated plants tend to make less vigorous, less pest-resistant, lower-yielding offspring than plants that are hybrids, or crosses between two strains. In short, there was growing evidence that for some plants, hybrids tend to be more robust. Darwin's work had a strong influence on many scientists, including William Beal, who, in the 1880s, studied pollination control as a way to minimize the negative effects of inbreeding in corn.

Recall that when Mendel and Darwin were working, DNA had not yet been discovered. Mendel was just working out patterns of inheritance; scientists didn't yet use the terms alleles or genes. The late 1800s into the early 1900s were times of great change in our understanding of traits and inheritance.

At this same time, corn farmers in the US had a problem. They had tried to increase yield by saving their best seeds, but they weren't able to maintain the increased quality and yield they sometimes found. They had a challenge that careful experimentation in the emerging areas of inheritance could possibly address: how could farmers maximize and sustain the effects of cross-pollination to increase their corn yields?

The Solution
Darwin, Beale, and others noted both the negative impacts of inbreeding in corn plants and the positive impacts of outbreeding due to cross-pollination. Creating corn varieties with desired traits, however, required connecting the pollination work findings with the recently rediscovered work of Gregor Mendel - the fundamentals of inheritance and ideas about what we now call alleles. In the early 1900s, George Shull applied Mendel's ideas to understanding inbreeding and outbreeding in corn and conducted experiments that led to a hybrid revolution. At the same time, Edwin Murray East, a young chemist, conducted a similar set of experiments and developed a parallel set of ideas.
Cornell plant breeder Professor Margaret Smith demonstrates making crosses with corn. Crosses between inbred lines produce hybrids and a boost in yield.
In 1908, George Shull was the first to clearly describe the heterosis concept. This idea changed the quest for desirable corn varieties. Corn breeders, as Shull suggested, gradually shifted their search from existing lines to looking for the best hybrid combinations.
Single cross hybrids: a promising concept, but not a practical one

In a 1909 paper, Shull outlined procedures for corn breeding to make vigorous, uniform single cross hybrids. At that time, however, the idea didn't really catch on. Why? It was too difficult and expensive to produce hybrid seeds. The inbred lines of corn, which are necessary to make F1 hybrids, were not very strong or vigorous. They had low yield because they do not make much seed. They were susceptible to pests. The biggest problem, however, was that inbred corn plants can be outcompeted by weeds. Remember that there were no pesticides in the early 1900s. As a result, it was very difficult and thus very expensive to produce hybrid seed!

Crossing two inbred lines produces a single-cross (F1) hybrid.
Double-cross hybrids offer a high-yield solution

However, in 1918, D.F. Jones, a student of East, developed a different type of hybrid: the double-cross hybrid. He crossed single-cross hybrids (also called F1 or first-generation hybrids) to produce another hybrid, the double-cross hybrid, or F2 hybrid. Those double-cross hybrids were developed from four different inbred varieties. Jones' experiments determined that the yields of the double-cross hybrids were only slightly lower than those of the single-cross hybrids and other desirable traits were also maintained.

Double-cross hybrids were a significant turning point in hybrid seed production. Because they were made from F1 hybrids, which were stronger, competed better against weeds, and were more uniform than the inbred lines, the double-cross hybrid seed was easier and cheaper to produce. Double-cross hybrids were commercially available by 1921.

Crossing two hybrids produces a double-cross (F2) hybrid.
Hybrid varieties + Farming Technology = Higher Yields

Hybrid corn varieties– both single- and double-crosses – have significant benefits. Plant breeders intentionally create varieties with particular traits; they might be fast-growing, or able to tolerate drought, or particularly resistant to a pest like European corn borer. In addition to key desirable traits, hybrid yields are higher and fields planted with hybrid varieties are genetically uniform.

Genetic uniformity has some advantages. Genetic variability means that plants are often different heights and that they usually have different timing for flowering, pollination, and harvest, as well as different grain characteristics. The uniformity of a field of hybrids made it easier to use farming technology like mechanical harvesting. Doing so reduced the need for manual labor and reduced crop losses, thus further increasing yield. Since the 1940s, there have been many other changes to corn farming that contributed to increased yields and reduced production costs, including increased use of fertilizers, increased use of herbicides, increased irrigation in arid climates, and additional mechanization of the farming process.

Return to Single-Cross Hybrids

Single-cross hybrids have less genetic variation than double-cross hybrids. Therefore, yields are higher and desirable characteristics like pest resistance, are more uniformly expressed in all plants. With better weed control from herbicide use, plant breeders were able to grow inbred lines with less competition from weeds. Beginning in the 1960s, single cross hybrids, which are higher-yielding as well as faster and simpler to create, became the norm.

Think and Apply
Examine the graph of US Corn Yields between 1930 and 2000. Describe the trend in corn yields during that time. Connect the changes in yields in the figure to hybrid varieties and farming technology.
Answer:
Corn yields increased from 20-30 bushels per acre in 1930 to nearly 60 bushels per acre in 1965. During that time, primarily double-cross hybrids were grown. Due to changes in farming technology, in the 1960s, farmers began using single-cross hybrids; yield has continued to increase to between 120-130 bushels per acre in 2000.
Genetic Vulnerability: A Downside of Hybrids

Yield increases of the 20thcentury were tied to genetic uniformity. Genetic uniformity brought uniformly higher yield, consistent pest resistance, and fields of similarly-sized plants that matured at the same time, which enabled mechanization.

However, genetic uniformity can have a cost: vulnerability. If a "predator" can exploit an unforeseen weakness in one plant, all the genetically uniform plants are vulnerable. In 1970-1971, a disease outbreak destroyed 15% of the US corn harvest at the cost of more than one billion dollars. The "predator" was a fungus called southern corn leaf blight (SCLB). SCLB had a naturally-occurring, but rare, mutation that allowed it to infect more than 85% of the corn grown in the United States in those years. The vulnerable genes were found in so many varieties of 1970 and 1971 because of their value in hybrid production. One of the most expensive, labor-intensive parts of producing hybrid seed is removing male flowers, or tassels, from all the plants that will eventually bear the ears of seed corn. The genes, through specific genetic interactions, made particular male flowers sterile, so seed producers didn't have to remove millions of tassels by hand, while producing fertile offspring that farmers could use as seed.

The epidemic was a significant lesson in the dangers of genetic uniformity. Since then, plant breeders have worked to protect genetic diversity between varieties while preserving the advantages of genetic uniformity within a variety.
Conclusion
In the early 1900s, US corn farmers produced 20 -30 bushels per acre. Today, the United States produces more than 120 bushels of corn per acre. Advances in scientific understanding were part of the key to higher corn yields. Researchers in the early 1900s began to understand how desirable traits – like corn cob size and plant height – passed from generation to generation. They also realized that hybrid plants often have higher yield and growth rate than that of inbred parents. The work of scientists like Shull, East, and Jones, who built off the ideas of Mendel, Darwin and others, led to huge changes in agriculture. With a new understanding of heterosis, plant breeders worked to combine inbred lines into hybrid varieties. The hybrids had higher yields and their genetic uniformity made mechanization - and even higher yields - possible.

The discovery of heterosis – hybrid vigor – in corn led to its use in many other crop species as well. Other crops like rice, sorghum and sunflowers, as well as various vegetables and many ornamental flowers, now rely on hybrid varieties for their yield boost and uniformity.
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