Differential productivity and persistency responses to simulated and animal grazing of perennial ryegrass genotypes

The objective of this thesis was to investigate changes within perennial ryegrass cultivars over time. The effect of animal grazing on cultivar performance and the effect of cultivar on animal performance were also investigated. These factors were examined in a number of experiments. Cultivars were managed under a number of sward conditions, before plants were extracted and compared to the original seed lines. Directional selection within perennial ryegrass was detected, however, the occurrence and magnitude of this selection was influenced by both cultivar and sward management. Directional selection was detected in 8 and 10 cultivars under simulated grazing and conservation, respectively. No such change was observed under sheep grazing in a contrasting climate. Phenotypic measurement from the Distinctness, Uniformity and Stability protocol proved to be affected by directional selection within cultivars under mechanical defoliation. When directional selection did occur, the plants that survived had a smaller mean phenotype in comparison to the original seed lines, suggesting plants with a lower yield potential survived. Simulated grazing was found to be a good predictor of the DM yield and digestibility of cultivars under animal grazing supporting the reliability of information generated during recommended list trials to identify elite cultivars for utilisation by grazing animals Sward chemical and structural attributes were found to influence animal performance of lactating dairy cows. A cultivar that produces large leaves in conjunction with a low sheath height and can maintain a high digestibility across the season will likely optimise animal performance from grazed pasture. Cultivars that exhibit declining sward digestibility in the autumn period can significantly inhibit animal performance. Phenotypic measurement used in the current study can be used in future studies to allow a greater understanding of how varying management and environmental factors may Influence directional selection.