Exploiting heterosis of semi-hybrids and heterogeneity of cultivar mixtures to enhance alfalfa crop performance

Alfalfa semi-hybrids (alias free-hybrids or chance-hybrids) of genetically-distant populations may display heterosis, but their actual value is substantially unknown. Higher biomass yield may also derive from heterogeneity in cultivar mixtures. This study aimed to compare three two-way and one three-way semi-hybrids and three two-way physical mixtures issued from putative heterotic populations selected from elite Italian germplasm, Egyptian germplasm and a semi-erect pool from Eastern Europe, Canada and Spanish Mielga, their source populations, and a reference synthetic variety selected from the same pool as the Italian population. We assessed three-year dry-matter biomass yield, final persistence, adaptability and yield stability across eight densely-planted managed environments obtained by the factorial combination of two summer water amounts by two soil types by pure stand (PS) or mixed stand (MS) with forage grasses. Heterogeneity increased yield by 17.0% on average while affecting persistence inconsistently. The advantage over mid-parent value averaged 35.1% for yield and 16.3% for persistence of two-way semi-hybrids, while being 35.6% for yield and 21.1% for persistence of the three-way semi-hybrid. The top-yielding semi-hybrid, issued from Italian and Egyptian material, out-yielded both parent populations, and exhibited 7.4% higher mean yield and greater competitive ability than the reference synthetic variety. PS or MS condition affected plant material × environment interaction for yield or persistence more than summer water availability or soil type, but interaction patterns were mainly quantitative with increasing variation in MS. No manifest plant material variation emerged for yield stability. Based on our results, a semi-hybrid variety type can be valuable for alfalfa.