Carbon Isotope Discrimination, Selection Response, and Forage Production of Tall Fescue in Contrasting Environments

Carbon isotope discrimination (Δ) usually correlates with the dry matter to transpiration ratio (transpiration efficiency) in C 3 species, but its heritability and relationship to forage production are less clear. In a 4‐yr study of tall fescue ( Festuca arundinacea Schreb.) at Pullman, WA (relatively cool with low humidity), and Ardmore, OK (relatively hot with high humidity). we determined (i) if Δ differences in divergently selected populations made on single plants were maintained in solid seeded plots, and (ii) how Δ in selected populations and a set of four cultivars was related to forage production. Differences in Δ for low and high Δ populations selected on spaced plants were maintained in solid seeded plots at both Pullman and Ardmore. At Pullman, the low Δ selection had higher production than the high Δ selection with the base population intermediate. Partial correlations with all entries between Δ and forage production, controlling for harvest date effects, were not significant. However, partial correlation between Δ and forage production on the selected and base populations was significant ( r = −0.59, P < 0.05, n = 12) at Pullman, although not at Ardmore. The data show selection for low Δ may improve forage production in some environments, although not consistently. For breeding tall fescue, one cycle of phenotypic selection for low Δ in advanced material is recommended.