| Summary: | The thesis investigated the interactions that occur between perennial ryegrass cultivars in seeds mixtures on sward composition, OM production and sward quality. The effect of companion white clover, different managements and animal grazing were also examined. These factors were assessed accessed across experiments using perennial ryegrass cultivars. The cultivars were combined in binary mixtures formulated using different ratios of the cultivars. There was no consistent advantage to mixing cultivars on OM production. Mixtures tended to dilute the performance of the highest yielding cultivar. The OMO values of mixtures tended to lie closer to the most digestible cultivar, indicating that sward quality may be less compromised than OM production when cultivars are sown in mixtures. White clover was found to have no effect on the competitive hierarchies between perennial ryegrass cultivars. Adding white clover to perennial ryegrass mixtures had no effect on OM production, suggesting there was insufficient diversity between ryegrass and white clover to generate a synergistic response between the two species. Examination of the sward composition using starch gel electrophoresis found that tetraploid cultivars significantly increased their proportions in the sward when sown with a diploid cultivar due to different growth characteristics of the cultivars. These increases occurred under both simulated grazing and conservation managements (P<0.05). The increases were dependant on the sown proportions as they occurred when the tetraploid cultivars were sown at less than 50 % of the seed mixtures. Similar increases did not occur when tetraploid cultivars were sown at greater than 50 % of the seed mixtures, therefore, they never completely dominated the sward. This suggests that a competitive equilibrium developed within the swards. The study established that canopy structural differences between ploidies are a likely factor affecting competitive ability and mixture construction should account for these hierarchies to achieve the desired sward composition. As cultivar performance in mixtures remained largely consistent with that in monoculture, national and recommended list trials can provide a reliable guide to the agronomic performance of seed mixtures.
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