Response to divergent selection for ease of particle size reduction of dried leaves of smooth bromegrass ( Bromus inermis Leyss) and correlated effects on nutritive value indicators and plant fitness

The rate at which forage can be broken down and cleared from the rumen is a major constraint on voluntary intake by ruminants. Responsiveness to selection has been demonstrated for physical characters possibly related to the ease of breakdown of forage grasses and is a prerequisite for the exploitation of these characters in a breeding program. The objective of this study was to evaluate at two locations the response to one cycle of divergent selection for the ease of breakdown, or particle size reduction index (PSRI), of dried leaf blades in four populations of smooth bromegrass (Bromus inermis Leyss.) grown as spaced plants and the correlated response in neutral detergent fibre (NDF), in vitro dry matter digestibility (IVDMD) and on seedling performance and second year spread as indicators of plant fitness. Divergent selection for PSRI resulted in significant differences between high and low groups in all populations. The contrast for asymmetry of response was not significant, but response tended to be higher in the downwards direction relative to an unselected group. Realised heritabilities ranged from 10–21% when the unselected group was used to represent the parental generation. In the most responsive population, WB19e, seedling growth was poorer at both sites and seedling mortality higher at one site in plants selected for increased PSRI. Selection for PSRI had no effect on IVDMD but there were closely correlated changes in NDF (r = –0.73 across populations, p<0.01). However, comparisons between populations suggested that differences in PSRI independent of NDF also existed. PSRI can be exploited in a breeding program if gains in forage intake by ruminants, independent of changes in NDF, are demonstrated. This revised version was published online in July 2006 with corrections to the Cover Date.