Divergent Phenotypic Selection Response in Smooth Bromegrass for Forage Yield and Nutritive Value

There has been increasing interest in breeding smooth bromegrass (Bromus inermis Leyss.) for improved nutritive value. Several studies have shown improved animal performance in forages bred for increased nutritive value. The objective of this study was to determine if phenotypic selection for improved nutritive value was possible without sacrificing dry matter yield. The study evaluated eight potential measures of nutritive value, dry matter yield, and five base selection indices. Divergent phenotypic grid selection was practiced in a population of 625 plants for neutral detergent fiber (NDF), acid detergent fiber (ADF), acid detergent lignin (ADL), (ASH), cellulose (CEL), hemicellulose (HEM), crude protein (CP), in vitro matter digestibility (IVDMD), and dry matter yield. The highest lowest 75 plants for each selection criterion were crossed in semiisolation. A set of 75 randomly selected plants were bulked to form the unselected check synthetic. The synthetics were evaluated in a forage yield trial with three replicates at three locations in 2 yr. Synthetics differed significantly (P < 0.05) for IVDMD, CP, and all cell wall components except ASH. There were few genotype ✕ environment interactions for the nutritive value traits. Direct divergent selection responses were significant (P < 0.05) for IVDMD, NDF, ADF, and ADL. The divergence between synthetics appeared to be asymmetrical with progress greater in the direction of higher nutritive value. The ADL synthetic had the largest relative gain from selection. Correlated responses with ADL indicated that a 2 g kg -I decrease in ADL caused a 14 g kg⁻¹ increase in IVDMD and a decrease of 10 g kg⁻¹ NDF and 9 g kg⁻¹ ADF. Acid detergent lignin content was an excellent chemical predictor of IVDMD in this smooth bromegrass population and was superior to selecting for IVDMD directly. Research supported by the College of agric. and Life Sci., Univ. of Wisconsin Madison, Madison, WI 53706-1597.