Herramienta para el diagnóstico de los mecanismos existentes

Maize originated in the Americas, having been first devel- oped by native peoples in the highlands of Mexico. It was quickly adopted by the European settlers, Spanish, English, and French. The first English settlers found the native peoples of the northern regions growing a hard- kerneled, early-maturing flint variety that kept well, though its yield was low. Indigenous peoples in the south- central area of English settlement grew a soft-kerneled, high-yielding, late-maturing dent corn. There were doubt- less many haphazard crosses of the two varieties. In 1812, however, John Lorain, a farmer living near Philipsburg, Pa., consciously mixed the two and demonstrated that certain mixtures would result in a yield much greater than that of the flint, yet with many of the flint’s desirable qualities. Other farmers and breeders followed Lorain’s example, some aware of his pioneer work, some not. The most widely grown variety of the Corn Belt for many years was Reid’s Yellow Dent, which originated from a fortu- itous mixture of a dent and a flint variety.

At the same time, other scientists besides Mendel were conducting experiments and developing theories that were to lead directly to hybrid maize. In 1876 Charles Darwin published the results of experiments on cross- and self-fertilization in plants. Carrying out his work in a small greenhouse in his native England, the man who is best known for his theory of evolution found that inbreed- ing usually reduced plant vigour and that crossbreeding restored it.

Darwin’s work was studied by a young American bota- nist, William James Beal, who probably made the first controlled crosses between varieties of maize for the sole purpose of increasing yields through hybrid vigour. Beal worked successfully without knowledge of the genetic

principle involved. In 1908 George Harrison Shull con- cluded that self-fertilization tended to separate and purify strains while weakening the plants but that vigour could be restored by crossbreeding the inbred strains. Another scientist found that inbreeding could increase the pro- tein content of maize, but with a marked decline in yield. With knowledge of inbreeding and hybridization at hand, scientists had yet to develop a technique whereby hybrid maize with the desired characteristics of the inbred lines and hybrid vigour could be combined in a practi- cal manner. In 1917 Donald F. Jones of the Connecticut Agricultural Experiment Station discovered the answer, the “double cross.”

The double cross was the basic technique used in developing modern hybrid maize and has been used by commercial firms since. Jones’s invention was to use four inbred lines instead of two in crossing. Simply, inbred lines A and B made one cross, lines C and D another. Then AB and CD were crossed, and a double-cross hybrid, ABCD, was the result. This hybrid became the seed that changed much of American agriculture. Each inbred line was con- stant both for certain desirable and for certain undesirable traits, but the practical breeder could balance his four or more inbred lines in such a way that the desirable traits outweighed the undesirable. Foundation inbred lines were developed to meet the needs of varying climates, growing seasons, soils, and other factors. The large hybrid seed- corn companies undertook complex applied-research programs, while state experiment stations and the U.S. Department of Agriculture tended to concentrate on basic research.

The first hybrid maize involving inbred lines to be produced commercially was sold by the Connecticut Agricultural Experiment Station in 1921. The second was developed by Henry A. Wallace, a future secretary of

agriculture and vice president of the United States. He sold a small quantity in 1924 and, in 1926, organized the first seed company devoted to the commercial production of hybrid maize.

Many Midwestern farmers began growing hybrid maize in the late 1920s and 1930s, but it did not domi- nate corn production until World War II. In 1933, 1 percent of the total maize acreage was planted with hybrid seed. By 1939 the figure was 15 percent, and in 1946 it rose to 69. The percentage was 96 in 1960. The average per acre yield of maize rose from 23 bushels (2,000 litres per hectare) in 1933, to 83 bushels (7,220 litres per hectare) in 1980, to about 165 bushels (about 14,400 litres per hectare) in 2009.

The techniques used in breeding hybrid maize have been successfully applied to grain sorghum and several other crops. New strains of most major crops are devel- oped through plant introductions, crossbreeding, and selection, however, because hybridization in the sense used with maize and grain sorghums has not been success- ful with several other crops.

whEat

Advances in wheat production during the 20th century included improvements through the introduction of new varieties and strains; careful selection by farmers and seeds- men, as well as by scientists; and crossbreeding to combine desirable characteristics. The adaptability of wheat enables it to be grown in almost every country of the world. In most of the developed countries producing wheat, endeav- ours of both government and wheat growers have been directed toward scientific wheat breeding.

The development of the world-famous Marquis wheat in Canada, released to farmers in 1900, came about

through sustained scientific effort. Sir Charles Saunders, its discoverer, followed five principles of plant breeding: (1) the use of plant introductions; (2) a planned cross- breeding program; (3) the rigid selection of material; (4) evaluation of all characteristics in replicated trials; and (5) testing varieties for local use. Marquis was the result of crossing a wheat long grown in Canada with a variety introduced from India. For 50 years, Marquis and variet- ies crossbred from Marquis dominated hard red spring wheat growing in the high plains of Canada and the United States and were used in other parts of the world.

In the late 1940s a short-stemmed wheat was intro- duced from Japan into a more favourable wheat-growing region of the U.S. Pacific Northwest. The potential advantage of the short, heavy-stemmed plant was that it could carry a heavy head of grain, generated by the use of fertilizer, without falling over or “lodging” (being knocked down). Early work with the variety was unsuc- cessful; it was not adaptable directly into U.S. fields. Finally, by crossing the Japanese wheat with accept- able varieties in the Palouse Valley in Washington, there resulted the first true semidwarf wheat in the United States to be commercially grown under irrigation and heavy applications of fertilizer. This first variety, Gaines, was introduced in 1962, followed by Nugaines in 1966. The varieties grown in the United States have produced 100 bushels per acre (8,700 litres per hectare), and world records of more than 200 bushels per acre have been established.

The Rockefeller Foundation in 1943 entered into a cooperative agricultural research program with the gov- ernment of Mexico, where wheat yields were well below the world average. By 1956 per acre yield had doubled, mainly because of newly developed varieties sown in the fall instead of spring and the use of fertilizers and