Variation in digestibility in nine populations of perennial ryegrass (Lolium perenne) and its genetic basis

Summary Thirty five individuals from each of nine perennial ryegrass populations were sampled from set-stocked and zero-grazed swards and from an old permanent pasture. The swards were 6, 18, 30 months and 11 years old when sampled. The permanent pasture was at least 40 years old.Dry matter digestibility (DMD) and organic matter digestibility (OMD) were determined for green leaf material harvested 4 days after ear emergence. Significant differences were found between the populations, but genotypic differences were found only for OMD. The largest proportion of the total variation was due to differences between blocks. In the zero-grazed populations, DMD and OMD declined significantly with increasing sward age. This was not found for the set-stocked swards. Two sets of pair crosses, from which mid-parent, progeny regressions could be determined, were carried out.The regressions of offspring on mid-parent for set-stocked and zero-grazed swards were used to assess the presence of additive genetic variation and to estimate narrow sense heritability of DMD. The coefficients of correlation, 0.122 and 0.226 respectively, were non-significant, indicating an absence of additive genetic variation.