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I'm Gwen Out with the VOA Special English Agriculture Report.

A listener in Vietnam has a question about corn breeding. Ho Lan Phuong has read about "pure-line development" and "broad-line development." She wants to know the difference.

Since ancient times, farmers have chosen the best examples from each crop to provide seed for the next year. This method is called mass selection. It may be what our listener has seen described as broad-line development. Mass selection produces plants with similar genetic qualities over time.

The pure-line method of breeding is similar. But it is more systematic. In pure-line breeding, the best plants are chosen from a crop that has many different genetic qualities.

The seeds from these plants are grown. Then the best plants are chosen from the new crop. This process can go on for many years, until the seeds produce plants with measurable similarities and desirable qualities. Seed companies may use this method to produce seed for some crops.

But the pure-line method is not often used with widely traded crops. Today major crops like corn or wheat are developed as hybrids.

About one hundred years ago, a scientist in the United States, G.H. Shull, made important discoveries about corn hybrids.

He mated corn with itself. Corn does not normally fertilize itself in nature. If corn is inbred, the seeds will produce a plant that clearly shows the qualities of the parent. But this inbreeding does not produce a strong plant.

Shull found that if he mated two inbred corn plants, they would produce a strong line with the good qualities of the parent plants. This is called crossbreeding.

Researchers soon recognized that they could crossbreed four inbred lines of corn. This "double cross" results in stronger corn with the best qualities of the parent plants. This is the way most hybrid corn is developed.

Modern hybrid corn produces much more grain than its ancestors. But success can create its own problems. For example, there is very little genetic difference in the corn grown across the United States. Experts estimate that current hybrids use less than five percent of the genetic diversity that exists.

There is a project called Germplasm Enhancement of Maize, or GEM. It is a cooperative effort to increase the genetic diversity in corn. The project involves the Department of Agriculture, sixteen universities and twenty international companies.

This VOA Special English Agriculture Report was written by Mario Ritter. I'm Gwen Outen.