Development and application of a selection criterion for particle size breakdown of smooth bromegrass leaves

The rate of particle size breakdown in the reticulo-rumen is an important determinant of voluntary intake by ruminants. The objective of this study was to develop a selection criterion to measure particle size breakdown and to study its suitability for creating unique germplasm. Dried leaf tissue of smooth bromegrass (Bromus inermis Leyss.) was weighed and placed in a rubber drum, with 200 steel balls. The drum was rotated at constant speed, ball-milling the sample, after which the contents were sieved through a 1-mm screen. The particle size reduction index (PSRI) was determined as the weight of tissue passing through the 1-mm screen as a percentage of total sample weight. Two experiments focused on the optimal milling time, leading to the conclusion that a 30-s time was optimal. Five experiments focused on sample size and showed that sample sizes of ≥ 2.0 g adequately controlled precision. Repeatability of duplicate determinations was judged to be moderate in one experiment, suggesting that a single determination could characterize a sample of dried leaf tissue with a 24% reduction in precision compared to duplicate determinations. Another experiment indicated that genotype × harvest interaction was very important, but could be partly controlled by using two-stage selection. Finally, PSRI was not correlated with fiber digestibility, but was moderately and negatively correlated with fiber concentration. The results suggest that genetic variation exists for this selection criterion, it is repeatable, and it characterizes a physical property of grass leaves distinct from fiber digestibility or concentration.