Chromosome doubling and top-crossing as a means of exploiting heterosis in perennial ryegrass

Recurrent combined phenotypic and family selection within restricted diploid breeding populations continues to be effective in combining improved dry matter yield (DMY) with increased water-soluble carbohydrate (WSC) content and good persistency in perennial ryegrass. But population improvement inevitably leads to the accidental fixation of some deleterious genes, and thus there will always be potential to make further gains in DMY by hybridising plants developed from different breeding populations. Current schemes for producing F1 hybrids of forage grasses are unlikely to be practical because they all significantly increase the cost of seed production. Since the ending of subsidies for grass seed crops in the EU, such crops must generate more income than other (more reliable) combinable crops if farmers are to be willing to grow them. Tetraploidy offers a simple means of preserving some heterosis over several generations of seed multiplication without reducing seed yield. However, chromosome doubling tends to reduce persistency and there is little evidence for heterosis with WSC. We generated top cross families using a recently chromosome-doubled perennial ryegrass population as the male parent and selected female parents from different tetraploid cultivars. Nine families selected for improved dry matter yield combined with good WSC were multiplied and the second generation seed was evaluated for plot performance and uniformity, together with three control varieties. Eight of the nine families were sufficiently uniform in heading date and habit for commercial development. One uniform family had significantly higher DMY than all three control varieties under both conservation and simulated grazing managements and adequate WSC, although it also had significantly lower ground cover at the end of the third harvest year.