Use of First‐ and Advanced‐Generation Two‐Clone Synthetics as Parents of Four‐Clone Synthetics of Alfalfa 1

T HE search for superior germ plasm and investigations on methods for i.ts utilization in forage crop improvement have been intensified in recent years. Although interest in inbreeding for the production of relatively homozygous materials with respect particularly to disease and insect reactions has continued, inbred material has not been promising as commercial varieties per se or in experimental hybrid or synthetic combinations. Neither singlecross nor doubtecross production has been achieved commercially. Selected non-inbred plants continue to provide good source material for incorporation into improved varieties in .the form of synthetics, several of which have achieved commercial importance. Much of the information on synthetic varieties has been obtained from research on corn wher.e inbred lines were used as parental material and controlled crossing was used at least in the initial phases. Synthetics of alfalfa and other perennial forage crops have been developed by intercrossing non-inbred plants without control of pollination except for restricting parentage in number and by isolation. The term synthetic variety as used herein includes the firstand advanced-generation progeny of two or more selected inbred or non-inbred strains or clones, their crosses or other progeny, grown together in isolation under openpollinated conditions. The first generation of a synthetic variety is designated Syn-1 in contrast ’to Fj where crossing can be completely controlled. Intercrosses of 2 non-inbred clones are considered 2-clone synthetics rather than singlecrosses. Combinations involving more than 2 clones are considered multiple-clone synthetics, although specific designations such as 4-clone, 5-clone, etc., to indicate the number of parents involved are appropriate. Doublecross hybrid corn production through the use of inbred lines as originally described by Jones (2) has been successful commercially. Tysdal et al. (8) outlined a possible commercial method of producing F1 seed of an alfalfa doublecross. Their plan involved seed production from 4 self-sterile clones; vegetative increases of 2 such clones in each of 2 isolated natural crossing blocks. The use of par~tially fertile clones was also considered. Seed of the 2 resulting singlecrosses would be mixed and planted in isolation for the production of F1 doublecross seed intended for sowing in hay fields. It was pointed out that if noninbred clones were used, "singlecrosses" in alfalfa might be equivalent to doublecrosses in corn and similarly that "doublecrosses" in alfalfa might be .equivalent to an 8-line or a multiple cross in corn. Bolton (1) proposed an alternative to the Tysdal et al. plan, suggesting the possible use .of non-self-tripping self-fertile plants to reduce to a considerable extent the necessity for vegetative propagation.