This is Steve Ember with the VOA Special English Agriculture Report.

Since ancient times, farmers have chosen the seeds for the coming year from the biggest and best plants in their crop. The hope is that these seeds will have the same good qualities as their parent plants. This method is calledinbreeding（近親繁殖）. But experts say it is not the best way to develop seed with strong, healthy qualities over time.

In nineteen-oh-six, the genetic researcher G.H. Shull started work on breeding corn in New York State. He found that if he mated two inbred groups of corn plants, he could create a stronger new line of corn. This process is called crossbreeding（雜交）. It produces hybrids from putting together different kinds of related plants.

Researchers soon recognized that they could crossbreed four inbred lines of corn. The result is stronger than corn crossbred only once. Hybrid corn first appeared in nineteen-twenty-one. Today, almost all corn planted in the United States is hybrid. And farmers harvest about seven times more corn from each hectare than they did seventy years ago.

Corn is not the only hybrid crop. Yuan Longping is called the Father of Hybrid Rice. He and other Chinese scientists worked on this idea in the nineteen-sixties and seventies. The first hybrid rice appeared in nineteen-seventy-four. Mister Yuan used three lines of parent seed that produced fifteen to twenty percent more grain. By nineteen-ninety-five, half of all the rice grown in China was hybrid.

There are also hybrid animals. Long ago, farmers discovered that a female horse mated with a male donkey produces a mule. This animal is strong and good for work, although it cannot reproduce.

In the early nineteen-eighties, American fish farmers wanted to raise striped bass. This fish had almost disappeared from the wild. So researchers created a fast-growing hybrid bass. By two-thousand, fish farmers harvested almost seven-million kilograms of the new sunshine bass.

Hybrids are not the answer to every problem in agriculture. New hybrid seeds must be bought each year. They also cost more than other seed. Hybrids can take many years to develop. And not all crops can be crossbred successfully. But hybrids have been an important development for productivity.

This VOA Special English Agriculture Report was written by Mario Ritter. This is Steve Ember.