Hybrid is the offspring of parents of different breeds, varieties, or species. Hybrids occur naturally and contribute to the great variety of living things. However, the term hybrid is used most often to refer to crops and ornamental plants bred by human beings.
Hybrid plants rank among the most important agricultural products. Most commercial varieties of corn, sorghum, sugar beets, and burley tobacco are hybrids, as are some varieties of alfalfa, barley, rice, and wheat. Vegetables produced from hybrid seed include broccoli, carrots, cauliflower, onions, pumpkins, and tomatoes. Hybrids of such fruits as grapes, pears, and plums are also grown. Many marigolds, orchids, roses, and other flowers that beautify homes and parks are hybrids.
Hybrid animals have also become important in agriculture. People commonly produce hybrid livestock by mating different breeds. Such hybrids are called crossbreds. Cattle crossbreds rank among the most common animal hybrids worldwide. In the United States, most chickens, hogs, and turkeys, as well as many beef cattle, are crossbreds.
People develop crossbreds to produce animals with desirable traits. For example, crossbred cattle can yield leaner, better quality meat than do other cattle. Hybrid plants are developed to improve the quality and productivity of cultivated plants. The following section deals with how people produce such plants.
Producing hybrid plants.
Plants selected for hybrid production have hereditary characteristics that growers want to continue into future generations. For example, one variety of corn may resist disease better than another variety does. But the second variety may be hardier in cold weather than the first. By crossing (mating) the two varieties, growers can obtain hybrid seeds. These seeds develop into plants that have the traits of both parents. Seeds from hybrid offspring, however, produce plants of varying quality. As a result, growers plant new hybrid seed each year to maintain the quality of their crops.
Most flowering plants have both male and female sex parts, contained either in separate flowers or in the same flower. To ensure that only certain plants cross, growers select some to serve as males and some as females. They generally remove the male sex parts, which produce pollen, from the plants that will act as females, thereby preventing the plants from pollinating themselves or other females. In some cases, they use female plants that have been specially bred to produce infertile pollen. The male plants are untouched.
The hybrid seed produced from the crossing of selected parents is often the result of a long and complicated process. Corn growers, for example, first cause selected corn plants of the same variety to breed. This procedure is called inbreeding. The seeds produced from the inbred plants are planted, and the plants that grow from them are in turn forced to inbreed. This process continues through several generations until plants having pure hereditary lines are established.
Growers then cross two inbred varieties. They remove the tassels (male corn flowers) from the plants chosen as females, leaving the ears (female corn flowers) for pollination by the males. Seed resulting from this breeding produces single-cross hybrids. These plants combine the hereditary characteristics of two inbred varieties.
History.
People have produced such hybrids as mules for centuries. Mules result from the mating of a male donkey with a female horse. Many different kinds of hybrid plants were described by naturalists during the 1700’s and 1800’s. In 1922, hybrid seed corn was sold to farmers in the United States for the first time. By the mid-1940’s, nearly all corn planted in the major U.S. corn-growing areas was hybrid corn. Hybrids became important for other kinds of crops during the 1960’s. At the end of the 1900’s, most vegetables and many kinds of livestock were hybrids.
