Interpopulational and intervarietal hybridization inLolium perennebreeding: heterosis under non-competitive conditions

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Abstract
The current status of herbage grass breeding is considered in relation to the utilization of first-generation hybrid vigour. A breeding scheme is outlined by which it is suggested that interpopulational F1 hybrid varieties could be developed to provide a simple and effective means of exploiting heterotic potential in herbage grass species. A survey of interpopulational F1 heterosis expressed, under two levels of fertility in non-competitive spaced-plants conditions, by the hybrids of a group of six adapted perennial ryegrass varieties, is reported here.Although the F1 hybrid populations studied were not on average superior to the parental varieties there were large variations between specific hybrids. Two hybrids were particularly promising, exhibiting 25% and 31% heterosis over the higheryielding of their parents for total green-matter yield over two harvest years and two fertility levels. The bulk of this heterosis arose under high-fertility conditions, when the mean yield response of the interpopulational F1 hybrid was twice that of the parental varieties.The effects of inherent relationships between the parental varieties on the observed levels of heterosis are considered, as are the possibilities for the practice of recurrent selection procedures for the enhancement of heterotic expression in some of these hybrids. It is concluded that there is sufficient evidence for the existence of interpopulational heterosis in this species to merit an extension and application of this approach to include a wider range of varieties and. populations, both adapted and introduced.