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Inbreeding Medicago sativa L. by Sib-mating. II. Agronomic Traits¹

In two alfalfa (Medicago sativa L.) populations, (a) random population of noninbred ‘Vernal’ and (b) a composite S₁ population derived from Vernal, the effects of breeding by self-fertilization and sib-mating were compared for certain agronomic traits. In general, as the degree of inbreeding increased, there was a reduction in spring vigor, forage yield, seed production, plant height aud width, and an increase in days to flower after first harvest. Also there was a trend toward more upright growth habit. Because the rate of inbreeding was less with sib-mating than with selfing, the sib-mated progenies were superior in performance to the selfed progenies.

Inbreeding coefficients were computed for the several levels of inbreeding attained, and in general, the reduction observed in forage yield, spring vigor, and seed production was much greater than that expected in an autotetraploid species. The reduction in summer plant height and midsummer flowering was not as great. The use of sib-mating in a breeding program and reasons for the rapid reduction in performance was discussed.

Key Words: autotetraploid • self-fertilization • generations • inbreeding depression

² Assistant Professor, Agronomy Dept., University of Maryland, College Park, Md. (formerly Research Associate, Iowa State University and Professor, Dept. of Agronomy, Iowa State University, Ames, Iowa 50010.

Received for publication February 21, 1968.