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 a nd 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 us ed in plant breeding programs. The objective of this study was to determine the relationship among NDF, PSRI, and several morphological variables of s mooth bromegrass (Bromus inermis Leyss.) leaves. Four populations of smooth bromegrass were selected for divergent PSRI or NDF Two clones per group (h igh 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 divergenc e 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 di vergence between high-NDF and low-NDF groups of 4-31 g kg(-1), which was hi ghly inconsistent among sampling dates. Selection based largely on an Augus t sampling date created divergence in NDF concentration that was probably t emperature-dependent, with the greatest differences observed during June, J uly, and September harvest dates. While there was an overall trend toward a negative correlation between PSRI and NDE little variation in PSRI could b e explained by NDF concentration. Selection for high PSRI led to smaller an d Lighter leaf blades, indicating that selection for leaf size and/or plant vigor should probably be an active part of the selection pro,sram. Paired clones with divergent PSRI and similar NDF concentration, or vice versa, sh ould have future value in determining the role of these plant characteristi cs in regulating voluntary intake by ruminants. (C) 2000 Elsevier Science B .V. All rights reserved.