Morphological and Chemical Responses to Selection for in Vitro Dry Matter Digestibility in Smooth Bromegrass

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Abstract
Improved nutritional value of forage crops, through selection and breeding, can be due to changes in chemical composition or the relative amounts of tissues or organs with differing nutritional value, or both. The objective of this study was to evaluate morphological and chemical changes associated with divergent selection for in vitro dry matter digestibility (IVDMD) in smooth bromegrass (Bromus inermis Leyss.). Parents and progeny, selected for high or low IVDNID, werevaluated in replicated field trials for two harvests in each of 2 yr. Cell wall composition and IVDMD of stems (stem plus leaf sheath), leaves (leaf blade), and whole plants were determined. Several morphological traits of leaves and stems were also determined. Whole plant variation in IVDMD was due mainly to differences in stem IVDMD. Increased stem IVDMD was associated with reduced lignin and cellulose, but increased hemicellulose contefits, with overall reduced cell wall content. Increased leaf IVDMD was due to similar causes, but without reduced cell wall content. These results provide further evidence that IVDMD can be improved by genetic modification of cell wall composition, without reducing total cell wall content. Although differences were not always consistent, high IVDMD genotypes tended to have larger and heavier, but fewer leaves. Leaf/stem ratio changed slightly but not enough to explain differences in whole plant IVDMD. Selection for IVDMD did not appreciably affect plant growth and development in these environments.