Divergent selection for two laboratory predictors of voluntary intake: relationships among the predictors and leaf morphology variables

Voluntary intake of forage by ruminants is limited by forage bulk density and the rate at which forage is degraded to particles that can pass from the rumen. Two indirect laboratory measures of intake, neutral detergent fiber (NDF) concentration and particle-size reduction index (PSRI), are being used in plant breeding programs. The objective of this study was to determine the relationship among NDF, PSRI, and several morphological variables of smooth bromegrass (Bromus inermis Leyss.) leaves. Four populations of smooth bromegrass were selected for divergent PSRI or NDF. Two clones per group (high vs. low PSRI and high vs. low NDF) and population were evaluated for 2 years in a replicated field experiment. Selection for PSRI led to divergence between high-PSRI and low-PSRI groups of 6.6–8.6 percentage units, which was generally consistent across sampling dates. Selection for NDF led to divergence between high-NDF and low-NDF groups of 4–31 g kg−1, which was highly inconsistent among sampling dates. Selection based largely on an August sampling date created divergence in NDF concentration that was probably temperature-dependent, with the greatest differences observed during June, July, and September harvest dates. While there was an overall trend toward a negative correlation between PSRI and NDF, little variation in PSRI could be explained by NDF concentration. Selection for high PSRI led to smaller and lighter leaf blades, indicating that selection for leaf size and/or plant vigor should probably be an active part of the selection program. Paired clones with divergent PSRI and similar NDF concentration, or vice versa, should have future value in determining the role of these plant characteristics in regulating voluntary intake by ruminants.