Persistence and Yield Stability of Intersubspecific Alfalfa Hybrids

Long-term persistence and sustained biomass yields of alfalfa (Medicago sativa L.) stands are important to producers. Yield performance and persistence of intrasubspecific crosses between alfalfa subspecies sativa and falcata after the first post-establishment year are unknown. The objectives of this study were to measure biomass yields, persistence, and biomass yield stability in interand intrasubspecific alfalfa crosses derived by mating nine elite sativa clones and five falcata clones in a half-diallel. Progeny were space planted in 1998 at Ames and Nashua, IA, and evaluated for persistence, biomass yield, and biomass yield stability from 1998 through 2002. Medicago sativa subsp. sativa (sativa) 3 M. sativa subsp. falcata (falcata) hybrids produced substantial biomass yield and exhibited heterosis through the first post-establishment year, but declined in subsequent years. From 1998 through 2002 the intersubspecific hybrids had persistence equivalent to the more persistent parental subspecies, with the effect becoming more apparent as time progressed. Intrasubspecific hybrids had less biomass yield stability than the more stable intrasubspecific sativa crosses. The results indicate that selection of improved falcata germplasm for long term persistence and higher yield is needed. BREEDING METHODS to capitalize on heterosis, such as semihybrids (Brummer, 1999) or hybrids using a male sterility system (Wagner et al., 2003), have been developed but are not widely used in the alfalfa industry. We have reported previously thatM. sativa subsp. sativa (hereafter sativa) and M. sativa subsp. falcata (hereafter falcata) represent heterotic breeding pools for biomass yield (Westgate, 1910; Waldron, 1920; Sriwatanapongse andWilsie, 1968; Riday and Brummer, 2002a, 2005). We have also investigated the heterotic effects of sativa 3 falcata alfalfa hybrids for nutritive value and agronomic traits (Riday et al., 2002; Riday and Brummer, 2002b, 2004). However, both our results and those of the other experiments were based on data obtained in the establishment and first production years, when little time had elapsed for the cumulative effects of stress to impact stand persistence. Long-term stand persistence is important for the economical production of alfalfa because it allows the costs of seeding to be amortized over a longer period. Persistence is a complex trait affected by a large number of factors, including grazing, mechanical harvesting equipment, intensity of harvest management, diseases and pests, cold weather, inadequate dormancy, and interand intraspecies plant competition. Numerous breeding programs have used artificial and natural selection to develop improved alfalfa germplasm able to withstand various plant stresses, thereby increasing stand life (Brummer and Moore, 2000; Kallenbach et al., 2002). Most breeders agree that to truly select for persistent alfalfa requires long-term evaluation of plants and selection among survivors in advanced stand years (Gallepp, 1997). Commercial alfalfa breeding programs typically select plants after 3 to 5 yr in the field. In addition to yield per se throughout the life of the stand, consistently superior performance over environments is also desirable. Cultivar stability can be assessed by testing germplasm inmany different environments and selecting genotypes that perform well in most environments (Fehr, 1991). Studies have shown that increased heterozygosity of individual plants and increased heterogeneity of plant mixtures often lead to increased environmental stability (Allard and Bradshaw, 1964). These findings led us to suspect that sativa 3 falcata hybrids should be more stable than intrasubspecific synthetics. The purpose of this study was to examine biomass yield, plant persistence, and biomass yield stability in intersubspecific hybrids of sativa and falcata over the 5 yr of the stand. The first objective was to determine biomass yields in advanced years of production and to determine if the hybrid yield advantage identified in the establishment and first production years (Riday and Brummer, 2002a) held in subsequent production years as the stand aged. Our second objective was to determine if sativa3 falcata hybrids had superior persistence. Our final objective was to determine if intersubspecific hybrids had more stable yields than intrasubspecific crosses in Iowa environments. MATERIALS AND METHODS