All varieties of crops have some desirable characteristics or they would not be used. Nevertheless, each of these varieties is known to possess one or more undesirable traits which, if eliminated, would result in higher yields and better quality. The aim of the plants breeder is to develop superior varieties by eliminating the undesirable characteristics and combining the desirable ones in the same variety.
Plant improvement is based on principles or laws of heredity which are included in the science known as genetics. Many of the principles and techniques used in intensive study and training are required.
Selection is a simple, but important method of improving plants. As the name suggests this method consists of selecting the outstanding types and discarding those that are undesirable because of certain characteristics being possessed by them. For example, in small grains, plants resistant to lodging may be selected; and with alfalfa those capable of surviving in severe winters are to be retained. After a period of testing, during which plants are selected for certain desired traits or characteristics, a superior strain may be developed. Improving by selection cannot be accomplished, however, unless the variety from which the selections are being made possesses some plants containing the characteristics desired.
Selection is not a new method of improving plants. Actually this process is as old as plants themselves. For many thousands of years plants have been subjected to the stern and relentless forces of nature, and only the fittest is left entirely to nature, the process is extremely slow. Man cannot wait for nature alone to improve plants for him. By selecting superior plants, he is able to bring about improvements in a few years that would require thousands of years of time if left to nature alone.
Two procedures are commonly used when new varieties are developed by the process of selection. They are referred to as mass selection and individual selection.
Mass selection consists of selecting a fairy large number of individual plants possessing the desired characteristics. The seed from such plants is then mixed and sown together, and the better individuals are again selected or the poor ones discarded. This process of selection is to be repeated for a few years until the plants proved to be reasonably uniform for the qualities desired.
Individual plant selection is commonly referred to as pedigree or pure-line selection. When this method is used, individual plants are selected that are superior for certain characters but instead of mixing the seed as in mass selection, the seed of each head or individual is planted in a row of its own in such a manner as to keep the progeny of each parent separate. The progeny of each plant are then carefully observed, a record being made of their appearance and performance. Comparisons between the different progenies are made, those with undesirable characters being discarded.
Records of performance are carefully checked and compared each year with those of standard varieties which are also grown under the same conditions. If after a testing for a number of years, the strain proves to be superior to the standard varieties, it is then grown in larger plots to increase the supply of seed.
A period of several years may be required for sufficient seed to be obtained for general distribution to farmers. As a rule, 8 to 14 years are usually required for making the selection, testing it and increasing it to the point where the new variety can be released to farmers.