Genetic Variation for Rate of Cell Wall Digestion and Related Traits in First Cut Smooth Bromegrass1

Abstract

Genetic improvement and an increased understanding of forage quality as it relates to ruminant nutrition will depend upon identification of appropriate selection criteria. The objectives of this study were to investigate genetic variation for rate of cell wall digestion and the length of the initial digestion lag phase (lag time) in smooth bromegrass (Bromus inermis Leyss.) and to relate variability for these traits to chemical composition. Eighteen clones were grown at Arilington, WI, and sampled in 1982 and 1984. Forage samples were analyzed for cell wall constituents and in vitro cell wall digestibility (IVCWD) after 6, 12, 24, 48, and 72 h of incubation. Rate of cell wall digestion and lag time were computed from the log‐linear regressions of potentially digestible cell wall remaining on incubation time. Five regression models were used, based on various combinations of incubation times, to evaluate the influence of each incubation time on the rate and lag estimates. clones differed significantly for all traits evaluated. Cloanl variation for rate of cell wall digestion and lag time could only be detected when data from the 48‐h incubation time was excluded from the regression models. The low repeatability of 2‐yr clone means (0.35 for rate and 0.26 for lag time) and the low or moderate interyear rank correlations (0.19 for rate and 0.52 for lag time) suggest that replicated genotypic selection will be necessary for successful genetic improvement of these traits. Both rate and lag time were positively correlated wih in vitro dry matter and cell wall digestibility. Lag time appeared to be influenced by the concentration of cellulose and lignin, whereas variation in rate of cell wall digestion appeared to be a function of other plant factors.