Heritability of ‘Ease of Tripping’ in Alfalfa and Its Possible Relation to Seed Setting 1

well as to local lines, as examples. To the R line(s) various genes or chromosome segments would be added by backcrossing, for example; Transec for leaf rust and mildew resistance. Transec and genes for stem rust resistance also would be part of the input to the A line. The consequent homozygosity of genes at certain loci in the Fj must be considered a necessary genetic load for the population to carry and any reducton in heterosis would be more than compensated by the added protection against diseases. Two restrictions affecting the commercial hybrid crop must be imposed in this scheme: 1) a relatively narrow range of maturity to facilitate harvesting and seed handling and 2) attention to quality of the harvested crop to meet accepted standards. Concerning maturity, a range of 7 to 10 days from first ripe to all ripe might be feasible. Concerning quality, the intended use would influence the range and degree of heterogeneity permissible in the harvested seeds; designing a broadbase hybrid to meet specific quality requirements (e.g., milling and baking) would logically involve the analysis of the products of experimental hybrids made up using combinations of different genotypes in the B line composite or in the single (or seed blend of two or more) R linc(s); wheats for feed or other purposes might have less stringent design requirements. A word of explanation for the absence of visual population uniformity as a requirement for the commercial crop: in a 1952 paper (2) I specified phenotypic uniformity as a requisite feature of multiline varieties. I have changed my opinion. Observation and recent investigation (3, 4, 5) suggest that phenotypic nonuniformity of individuals in a population can add significantly to the productive yield potential through over-compensatory responses in competitive genotypeenvironment interactions, where other genotypes are considered as a part of the environment. Such bonus situations may be erroneously credited to heterosis in hybrid wheat; rather, they are competitive responses attributable to population structure and, in fact, cannot occur in the purest form of heterosis, the single cross, where unvarying uniformity precludes individual phenotypical differences in such things as height, maturity, tillering, leaf angle, awn or head type and so forth. There are several ways of insuring crop uniformity but little attention has been given to design problems associated with productive nonuniformity; the formation of the latter type of population is a principal objective of this method. If a significant productivity advantage can be gained from phenotypic nonuniformity it is time to educate growers to judge such nonpure line varieties on merit. HERITABILITY OF 'EASE OF TRIPPING' IN ALFALFA AND ITS POSSIBLE RELATION TO SEED SETTING