Using Hybrid Vigor

In corn, hybrid vigor is obtained in first-generation crosses by detasseling one of the parents to prevent any self-pollination from occurring. In alfalfa it is not practical to prevent self-pollination artificially. About 15 percent of the alfalfa plants, however, are self-sterile. In these self-sterile plants, pollen from a given plant will not fertilize flowers on the same plant but may be fully fertile on another plant. Hence it is possible to obtain practically 100-percent crossing by planting two such plants in an isolated block. The pollen of each plant readily fertilizes the flowers of the other plant, but not its own.

One might raise the question that two such plants would not produce enough seed for a very large hay acreage. Here is where another characteristic of alfalfa comes into use. Alfalfa can be very readily propagated by vegetative cuttings, much in the manner of making geranium slips; a single desirable plant can be increased to any number of plants. Because alfalfa is a perennial, moreover, these plants will live many years and frequent replantings will not be necessary.

Plants selected for hybrid-seed production must be relatively self-sterile. They must also be resistant to diseases, cold, and insect pests. They must combine well with each other. This combining ability is a rather abstract thing. The plant breeder cannot say by looking at a plant whether it will combine well (or "nick" well, as livestock breeders say) with another plant or not. Combining ability depends upon how the genes, or inheritance carriers, of one plant complement those in the other plant. The only way to determine this point definitely is to make the cross. However, a procedure called the polycross method has been developed that helps to pick out the best combiners just as the top-cross in corn helps corn breeders choose the best inbred lines in corn.

In the polycross method, stocks used in the polycross nursery are highly selected, desirable plants. They are selected either from the better varieties of nursery-bred alfalfa or from old fields that have had natural elimination. Highly self-fertile, or autogamous, plants are not selected, because self-fertilization reduces progeny yields. It is not necessary to use selfed lines, as corn breeders do. The plant breeder chooses self-sterile plants and they are propagated vegetatively. These plants are subjected to various diseases to test their resistance. Those that resist diseases and, as far as possible, insect pests, and have the other desirable characteristics are selected. They are increased vegetatively and planted in the clonal polycross nursery. In this nursery each clone (that is, each vegetatively propagated plant) is pollinated by natural methods. Pollen from the same clone or other clones in the nursery is carried to the flowers by insects. Because these plants are relatively self-sterile, it has been found that most of the seed so produced is out-crossed. Thus the seed from one clone or female parent derives its pollen or male parentage from many different clones. The progenies resulting from seed produced in this manner are known as polycrosses.

Tests comparing the combining ability of clones by the polycross method with the performance of the same clones in single crosses, or with their performance in a top cross, have shown that the polycross performance gives a reliable indication of their combining ability. It is much easier and cheaper to produce the polycross seed than to make all possible single crosses or to use a separate isolation block for each top-cross. By vegetative propagation, the original plants can be increased as much as is necessary to produce enough seed for thorough testing under various conditions in different parts of the country. The Eastern States depend largely upon the West and Midwest for their seed supply. It is, therefore, of prime importance to select plants whose progenies are adapted to relatively wide regional areas.

Besides showing the combining ability of clones, the polycross method makes it possible to obtain new superior combinations by natural crosses between selected clones in the polycross nursery. The term cumulative improvement is used to designate this phase of the program. In cumulative improvement it has been found that second-cycle material ( that is, plants selected from the polycross progenies) is often better than the original parents. It should be remembered, however, that in cumulative improvement, precautions should be taken to prevent the basic stocks from becoming inter-related. This can be done by separating the various types in different polycross nurseries. It is also advisable to introduce new, unrelated selections into the program as often as possible.