Capturing heterosis in forage crop cultivar development

Despite the widespread use of hybrid cultivars in many crop species, most forage crop cultivars are synthetic populations that do not express maximum heterosis for important agronomic traits such as yield. Several aspects of these crops, including severe inbreeding depression, perfect flowers, and polyploidy, limit the ability to produce conventional single- or double-cross hybrids, but the viable alternative of developing semihybrid populations has been relatively unexplored. Both theoretical and empirical evidence support the notion that hybrids, developed by crossing divergent populations, can outyield the better performing parental population. A method to produce semi hybrids is discussed, necessitating the development and maintenance of heterotic groups within a species. Progeny formed by crossing. populations are semihybrids because in a cross of two populations, 1/2 of the progeny result from interpopulation crosses and 1/2 from intrapopulation crosses. Expected genotype proportions are presented for several scenarios to show the feasibility of using semihybrids to capture partial heterotic gains. Development of pure hybrids is more problematic but could be realized by using transgenic selectable markers, such as herbicide resistances, to rogue nonhybrid plants in production fields. Molecular markers may aid in the development of the heterotic groups and in choosing parents to be crossed. Potential heterotic groups in several forage species are suggested. By requiring attention to germplasm resources, this method will also be useful for broadening and improving the genetic bases of most cultivated forage crops.