Divergent Selection for Two Measures of Intake Potential in Smooth Bromegrass

Abstract

Voluntary intake is often the greatest limitation to animal performance. Two different laboratory measures of intake potential were used as selection criteria in four smooth bromegrass (Bromus inermis Leyss) populations. The objective of this study was to measure the direct and correlated responses to one cycle of divergent selection for neutral detergent fiber (NDF) concentration or particle-size reduction index (PSRI). Sixteen progeny populations were created by divergent selection (high or low) for NDF or PSRI of smooth bromegrass leaf blades within four populations. Divergent selection for NDF resulted in changes of 4 to 8 g kg -1 (0.7-1.3% of the mean) for NDF and -2.2 to -3.0% (6.6-9.4% of the mean) for PSRI. Divergent selection for PSRI resulted in changes of 1.9 to 3.9% (5.4-12.9% of the mean) for PSRI and inconsistent changes for NDF. Although realized heritabilities were low, there was additive genetic variation for both traits within each population. Divergent selection for NDF resulted in strong changes in PSRI, in the opposite direction, and largely consistent across populations, suggestive of possible pleiotropic relationships. Conversely, divergent selection for PSRI resulted in weak andlor inconsistent changes in NDF, suggesting that there are multiple mechanisms by which PSRI is regulated in smooth bromegrass leaves. Decreased NDF concentration uniformly results in increased PSRI, but PSRI can probably be increased by alterations to the cell wall or to changes in cell types without changes to NDF concentration of herbage.