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The Science of Hybrids

   
The scientific basis for today's amazing hybrid crops goes back more than 150 years, but it was not until the 1930s – just as the Depression was beginning – that hybrids came to the attention of Midwestern farmers. Corn was the first hybrid seed crop to be marketed extensively, and it is still the most important economic crop grown in the U.S. So, examining the breeding of corn gives us a good insight into the science of hybridization.

Charles Darwin began the hybrid revolution by proposing that species of plants and animals will change over time. The changes or mutations that helped a species survive were the traits in a species that were then more likely to be passed on to the next generation. Later on in the prehistoric period, man helped out in the process. That's what happened, for instance, when Native Americans picked out the best ears of maize or corn to plant the next year, thereby promoting the best qualities in the species.

In the 1860s, about the same time as Darwin, Gregor Mendel discovered he could cross breed different strains of pea plants and predict the traits of the offspring. He proposed that there was a genetic basis for inherited traits and demonstrated that he could control them. But his work languished until after the turn of the century. After his work was rediscovered, the science of plant breeding took off.

Corn is a remarkable plant. It efficiently transforms energy from the sun into stored chemical energy in its seeds. Some varieties are no more than two feet tall. Others grow to over 20 feet. Most of the corn now grown in the central Corn Belt of the U.S. grows about eight feet tall. What all varieties share is a basic arrangement of roots, stalk, leaves, seed structure and the arrangement of the reproductive parts of the plant. On normal modern varieties, the seed will germinate and grow quickly. About 55 days after it begins to grow, the plant will produce a tassel at the top that has five to 20 branches. Each branch has hundreds of little spikelets – the flowers of the plant. Within the spikelets are anthers that produce pollen – the male reproductive part of the plant.

At about 60 days after the plant germinates, it produces the female part of the plant – the ear. At this stage, each ear contains around 1,000 ovules or potential kernels of corn. These are "potential" kernels because they require the male part (pollen) to fertilize them before they can grow into a full kernel. Attached to the top of each ovule is a strand of "silk" that grows out of the protecting husk to the top of the ear.

Sometime after the 60th day, millions of grains of pollen are released and float down to the silks. When a grain of pollen hits a silk, it divides and travels down the silk to fertilize the ovule or egg. An entire field can pollinate within 24 hours, and within a couple of days, the farmer will know fertilization has occurred when the silks turn from a golden color to brown.

It's in the time between when the tassels develop and when the silks develop that the opportunity to produce hybrid corn occurs. Scientists realized they could plant two pure or "inbred" varieties in a field, alternating rows. Then they could remove or cover the tassels of one variety so that the only pollen released into the field would come from the second variety. The ears harvested from the first variety would have the hybrid seeds.

One of the first things the plant scientists noticed when they began crossing different pure lines was that hybrid plants were usually more vigorous than their parents. The simple act of crossing different strains resulted in higher yields and stronger plants. They had discovered "hybrid vigor."

Hybrid vigor is so powerful that scientists began "double-crossing." Two inbred parents, A and B, are crossed at the same time that two other inbred parents, C and D, are crossed. The resulting hybrids, AB and CD are then crossed together to produce a hybrid with the characteristics of all four parents, ABCD. In addition, the new ABCD variety has two generations of hybrid vigor bred into it. This method of double crossing hybrids was used almost exclusively from 1926 until the 1960s. Then scientists developed better inbred lines and found they could get a good result with a single cross.

The first commercial seed corn company to try to take advantage of hybrid vigor was Pioneer Hi-Breds, founded in Des Moines, Iowa, in 1926. The company was founded by an influential farmer and editor of a national farm journal, Henry A. Wallace. Despite his prominence, the company lost money in 1927 and again in 1932 as the Depression grew worse. But hybrid varieties proved to be so much better than open-pollinated – those pollinated by pollen blowing in the wind – varieties that Pioneer and a host of other seed companies thrived despite the Depression. Some of the other Nebraska companies included Robinson Seed, Lauber Hybrids, Hoegemeyer Hybrids, Prairie Valley, Nem-Co Hybrids, Robin Hill Seeds and Steckley Hybrid Corn. Many of these companies worked Stan Jensenwith University of Nebraska agronomists to develop hybrids.

Stan Jensen is still amazed by the increase in yields that hybrids produced even though he became a plant scientist and worked for Pioneer Seeds most of his career.

At the beginning the 1930s, almost no hybrid corn was planted in Nebraska. By the end of the decade, hybrids were being planted in well over half of the fields in the central Corn Belt. By the end of the 1930s, kids in the Corn Belt had a whole new industry for summer employment – hybrid seed companies started hiring young workers for in-field detasseling operations.

Today, somewhere around 99 percent of U.S. corn is grown from hybrid seed. The same is true for wheat, soybeans, grain sorghum, cotton, peanuts, and many other crops.

Then & NowIn recent years, scientists have begun using biotechnology techniques to identify and splice in genes into plants to help them resist certain pests or produce other desireable traits. But biotechnology has also set off a firestorm of controversy. Some consumer groups, particularly in Europe, believe that biotechnology is not safe. The science has produced a political and cultural debate that could threaten U.S. agricultural exports.

Written by Claudia Reinhardt and Bill Ganzel, the Ganzel Group. First written and published in 2003.

 

Henry A. Wallace


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