Direct and indirect responses to divergent phenotypic selection for fiber traits in timothy (Phleum pratense L.)

Abstract

Genetic improvement of timothy digestibility by breeding requires an effective and reliable selection procedure. The general objective of this study was to evaluate the concentrations of four fiber components (neutral detergent fiber, acid detergent fiber, acid detergent lignin, hemicellulose, and cellulose) and four ratios involving these concentrations (acid detergent lignin over hemicellulose, acid detergent lignin over cellulose, acid detergent lignin over the sum of hemicellulose and cellulose and hemicellulose over cellulose) as selection criteria to improve timothy digestibility without affecting plant biomass. Selected genotypes and populations derived from them were evaluated in a field experiment. Significant variability was observed among genotypes for all nine traits. Divergent phenotypic selection for seven of the traits (all except hemicellulose and hemicellulose over cellulose) produced groups of genotypes that consistently differed for the trait used as the selection criterion. Selection criteria that involved acid detergent lignin were the most effective in identifying genotypes with consistent differences in in vitro true digestibility. Genotypes selected for high or low values of each criterion were intercrossed to produce progeny populations. Five of the nine pairs of divergent populations were consistently different for the trait used as the selection criterion. Selection for acid detergent lignin over cellulose was the most effective in producing populations with consistent differences (22 to 32 g kg-1 DM) in in vitro true digestibility, with reduced values of this ratio associated with increased digestibility. This was mainly due to a reduction of the concentrations of lignin and neutral detergent fiber, particularly in the stem fraction, resulting in greater stem digestibility. Among the criteria examined here, the ratio of acid detergent lignin over cellulose therefore seems to be the most promising for phenotypi