When seed from the clonal polycross nursery is thoroughly tested, the original clones that show the best polycross performance in disease resistance, yield, et cetera, are increased vegetatively and paired in all combinations to produce single crosses. Single crosses are made by planting the two relatively self-sterile clones in alternate rows in an isolated field where they can be naturally pollinated without being contaminated with pollen from other sources. Very little alfalfa seed is produced without the tripping of the flowers by pollinating insects. During this process the pollen is carried by the insects from one clone to another. Thus, with relatively self-sterile clones, usually over 95 percent crossing is obtained.

The possibility of commercial production of hybrid alfalfa was first proposed in November 1941. Since then a number of experimental hybrids have been produced in natural field crossing blocks. These have been tested in both forage and seed yield studies in comparison with the best standard varieties. In a 2-year test conducted at the Nebraska Agricultural Experiment Station, the better hybrids and polycrosses yielded from 4.16 to 4.34 tons an acre. In the same test, Grimm yielded 3.43 tons of cured hay to the acre. The hybrids showed a superiority of from 20 to 27 percent over Grimm. In a seed production test at the Utah Agricultural Experiment Station some of the hybrids yielded 660 pounds of seed to the acre; while Grimm produced 450 pounds an acre.

As I have said, many of these hybrids are resistant to bacterial wilt. Some also show resistance to attacks by the potato leaf hopper (Empoasca fabae). It has been found that alfalfa subjected to leaf hopper attacks (alfalfa yellows) is greatly reduced in carotene content. Carotene content is directly related to the vitamin A value of alfalfa hay. Tests under leaf hopper infestation have shown that more resistant hybrids have twice as much carotene (and thus vitamin A) as standard varieties. There is promise of producing hybrids more resistant to the leaf spot diseases. This would be important from the standpoint of increased quality of forage because the leaves of alfalfa contain approximately 66 percent of the protein and 75 percent of the carotene content of the plant. Leaf diseases destroy the leaves, and cause great losses in feeding value. Besides the usual hay types, the possibility of producing rhizomatous hybrids for grazing, for hay, and for erosion control is promising.

It may be possible to produce a double-cross hybrid alfalfa by planting seed of two high-combining single crosses in alternate rows for natural crossing. In that case the result would not be a 100-percent first-generation, or F1, cross because of some pollination within each single cross. Nevertheless, by using two single crosses that would produce an exceptionally good hybrid between them, the resulting progenies would prove high yielding and certainly much better than present standard varieties. As a matter of fact, yield tests have shown that as much as 25-percent selfed seed planted with the hybrid reduces the yield of the Mixture very little below that of the pure hybrid. No doubt this is because in any field planting there are a large number of young plants that die from competition with more vigorous plants. In that event, the strong hybrids would crowd out the plants from selfed seed. In addition to the production of hybrid alfalfa in the above manner, the possible use of male sterility (sterile pollen) should be investigated as a means of producing hybrid alfalfa.

The same procedure outlined for the selection of high-combining clones for use in producing hybrids would be used for the development of superior synthetic varieties. A synthetic variety may be defined as a variety that is developed by crossing, compositing, or planting together two or more unrelated strains or clones, the bulk seed being harvested and replanted in successive generations. By natural intercrossing the unrelated strains or clones are "synthesized" into a new variety. A synthetic variety can be increased through successive seed generations so long as the desired characteristics of the variety are retained. Some synthetic combinations have been tested in a preliminary way and have demonstrated their superiority over standard varieties. Thus if hybrids do not show a sufficient superiority to merit the extra labor involved, the same clones can be utilized in the production of synthetic varieties. The original clones would be maintained or increased vegetatively to form a source of pure foundation seed. The optimum number of clones to be used to produce a synthetic variety is being investigated.

Field tests have shown clearly that hybrid alfalfa has a definite place in alfalfa improvement. For the production of either hybrid or superior synthetic alfalfas one of the essential features is the use of foundation materials that have high-combining value and therefore maximum hybrid vigor. This must be taken into consideration if maximum improvement in alfalfa is to be obtained.

The production of hybrid and synthetic alfalfas is still in the experimental stages, but their utilization promises more certain production of higher quality forage and pasture. Since legumes might well be the keystone of a permanent and profitable agriculture, improved varieties of alfalfa, an important legume crop, would be of tremendous value for the needed expansion in acreage, for protection against soil erosion, and for furthering the livestock industries at lower production costs.

THE AUTHOR

H. M. Tysdal is an agronomist in the Bureau of Plant Industry, Soils, and Agricultural Engineering. He is a graduate of the University of Saskatchewan, Kansas State College, and the University of Minnesota. Dr. Tysdal, as a fellow of the American Scandinavian Foundation, studied plant breeding and plant physiology in Sweden and on the continent in 1927-28; he has also taught at the University of Minnesota, Michigan State College, and the University of Nebraska. He has written numerous papers on the culture and improvement of alfalfa. In an address before the American Society of Agronomy in 1941, he was the first to suggest the possibility of producing hybrid alfalfa commercially.