Heterosis of Agronomic Traits between
Alfalfa ssp. sativa and ssp. falcata
Heathcliffe Riday and E. Charles Brummer
Forage Breeding and Genetics, Iowa State University, IA 50011
Increasing forage yields remains a top priority of most alfalfa breeding programs. Long-term yield data suggests that forage yields have not been improving over the past 20 years (Riday and Brummer, 1999). Brummer (1999) suggests a semi-hybrid breeding system as a possible solution to overcome forage yield stagnation. To develop a semi-hybrid system in alfalfa it is necessary to identify heterotic groups. Studies of crosses between dormant M. sativa ssp. sativa and M. sativa ssp. falcata suggest a heterotic pattern exists between the two subspiecies (Riday and Brummer, 1999). Yield, however, is not the only consideration in a breeding program; agronomic traits and forage quality need to be considered, especially when trying to develop breeding material from non-adapted sources.
In this study we attempted to gain a better understanding of the performance of elite sativa x falcata crosses (SFC) in relation to elite sativa x elite sativa crosses (SSC) and falcata x falcata crosses (FFC). Nine elite sativa clones and five falcata clones were crossed in a diallel mating design. Progeny were space planted in 1998 at two locations in Iowa (Nashua and Ames). Three cuts were made in 1999 for yield evaluation; in addition spring regrowth, regrowth after cutting, fall regrowth, spring vigor, 1st cut vigor, plant height before cutting, maturity before cutting, and growth habit were measured. Forage quality was determined from fall cut 1998 Ames and 1st cut 1999 Ames and Nashua dry matter samples. Leaf-stem rations (LSR) were calculated and IVDMD, NDF, ADF, ADL, Ash, and CP were determined from stem samples.
Yield, spring vigor, 1st cut vigor, and height showed specific combining ability (SCA), while all traits showed general combining ability (GCA). Mean comparisons between SSC and FFC showed that SSC have superior field traits, while FFC have slightly superior forage quality. SFC were superior to their mid parent population mean (MPPM) for yield, spring regrowth, spring vigor, 1st cut vigor, height and maturity. SFC were inferior to their MPPM for IVDMD, NDF, ADF, CP, LSR, and regrowth. This study suggests a heterotic advantage in SFC for field traits compared to their MPPM but a slight decline in forage quality compared to their MPPM.
Reference
Brummer, E.C. 1999. Capturing heterosis in forage crop cultivar development. Crop Sci. 39:943-954.
Riday, H. and E.C. Brummer. 1999. Heterosis in Alfalfa: Medicago sativa subsp. sativa x subsp. falcata. (Online) http://genes.alfalfa.ksu.edu/TAG/TAGpapers/riday/riday.html