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Phenotypic Recurrent Selection for Stem Length in ‘Kenstar’ Red Clover¹

The effect of six cycles of phenotypic recurrent selection for stem length in red clover (Trifolium pratense L.) was studied in spaced plantings and in drilled plots. The objectives were to evaluate the genetic and agronomic changes that occurred during selection and to determine if further increases in stem length are possible. To determine if inbreeding occurred during selection of these populations, F₁ crosses between the selected populations and clones from the base population also were tested in spaced plantings. Over cycles of selection, stem length increased linearly 3.7 and 2.9 cm/cycle at first and second harvests, respectively, as evaluated in spaced plantings. Internode counts and histological examinations indicated that the lengthened stems were due to more cells per internode. No appreciable changes were detected in internode numbers or maturity. Estimates of the weighted average change in allelic frequency indicated inbreeding depression was not an important factor in the advanced cycles. A linear increase in stem length, lack of inbreeding depression, and maintenance of phenotypic variance over cycles of selection indicated that further increases in stem length would be possible if selection were continued. Selection for increased stem length was accompanied by decreases in stem number per plant and persistence. Herbage yield in spaced plantings increased over cycles at first harvest but declined over cycles in the regrowth. Redistribution of photosynthates from normal root and crown development or changes in root reserves were suggested as reasons for the initial yield increase and subsequent decline. Selection among plants after regrowth or in the 2nd year were proposed as methods of avoiding similar undesirable changes in breeding red clover.

Key Words: Trifolium pratense L. • Correlated response • Herbage yield • Persistence • Inbreeding • Cell length

² Former graduate assistant, (now assistant professor, Crop Sci. Dep., Univ. of Guelph, Guelph, Ontario Canada NIG 2W1), professor of agronomy, and professor of agronomy and statistics, respectively, Univ. of Kentucky, Lexington, KY 40546-0091.

Received for publication July 18, 1983.