Browsing IJAFR volume 50, no. 1, 2011 (Special Issue) by Title
Now showing items 7-8 of 8
Studies into the dynamics of perennial ryegrass (Lolium perenne L.) seed mixturesThe dynamic interactions of four perennial ryegrass seed mixtures sold in Northern Ireland were studied under simulated grazing and conservation managements. Mixture composition was determined as changes in phosphoglucoisomerase isozyme frequencies by calculation from known isozyme frequencies of the component varieties. Mixture productivity was measured over 4 growing seasons and compared with yields predicted from those of the components in monoculture, weighted for their actual proportion in the mixture. No significant differences were found between actual yields for mixtures and their predicted yields, but when these differences were regressed against the heading date range among the varieties in each mixture, a significant relationship was observed. A wide range in heading date among the components of the mixtures was associated with increased yield stability over years and with a declining yield advantage for the mixture compared to its components grown as monocultures. In this aspect, the mixtures showed a more rapid decline under conservation management than under simulated grazing. Mixtures also had a flatter seasonal yield-production profile than their component varieties. Tetraploid components were more aggressive than diploids, though a more open-growing diploid maintained its proportion in the sward better than a dense-growing type and manipulating the sowing ratios could be used to influence final sward composition after 2 years. It was concluded that the differences in heading date range within mixtures had a significant impact on mixture dynamics, with the tetraploid component being the most aggressive.
A theoretical and practical analysis of the optimum breeding system for perennial ryegrassThe goal of plant breeding is to effectively and efficiently select for the best phenotypes leading to the development of improved cultivars. The objectives for this review are to describe and critically evaluate breeding methods appropriate to the improvement of perennial ryegrass (Lolium perenne L.) in a long-term breeding programme. The optimum breeding system is dependent on the traits for improvement, and the available physical and human resources. Forage dry matter yield, persistency, disease resistance, nutritional value and seed yield are considered among the most important traits for improvement. Careful consideration should be given to the expression of the trait under the management regime imposed in the breeding programme and under real-world sward conditions in the target sowing region. Recurrent selection programmes for intrapopulation improvement are most appropriate for breeding perennial ryegrass. Three distinct types of recurrent selection may be implemented: (i) phenotypic recurrent selection, (ii) genotypic recurrent selection and (iii) marker-assisted selection. Genotypic recurrent selection will be a necessary part of the breeding system if forage yield is a trait for improvement. Genotypic recurrent selection may be practiced using full-sib or half-sib families, each with their own advantages and disadvantages. Phenotypic recurrent selection in tandem (i.e., within-family selection) or in succession with genotypic recurrent selection should be used to improve traits that have a high-correlation between performance from spaced plants and from sward plots. Genome-wide selection represents the most interesting and exciting potential application of marker-assisted selection, although it remains to be seen how beneficial it will be in practice.