Capturing heterosis in forage crop cultivar development

Despite the widespread use of hybrid cultivars in many crop species, most f orage 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 p olyploidy, limit the ability to produce conventional single- or double-cros s hybrids, but the viable alternative of developing semihybrid populations has been relatively unexplored. Both theoretical and empirical evidence sup port 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 o f heterotic groups within a species. Progeny formed by crossing. population s 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 t he feasibility of using semihybrids to capture partial heterotic gains. Dev elopment of pure hybrids is more problematic but could be realized by using transgenic selectable markers, such as herbicide resistances, to rogue non hybrid plants in production fields. Molecular markers may aid in the develo pment of the heterotic groups and in choosing parents to be crossed. Potent ial heterotic groups in several forage species are suggested. By requiring attention to germplasm resources, this method will also be useful for broad ening and improving the genetic bases of most cultivated forage crops.