Identifying potential heterosis in perennial ryegrass

Abstract

Plant breeders aim to develop cultivars of perennial ryegrass Lolium perenne L., (PRG) that better support New Zealand farmers through improved genetic potential for key traits such as seasonal dry matter yield. Hybrid vigour (heterosis) may be one way plant breeders can increase dry matter yields, and potentially lift the long term rate of genetic gain. We report evaluation of PRG full-sibling progeny for heterosis expressed under grazing. Parents were randomly sampled from sixteen cultivars, and for each cultivar combination within mid-season and late season classes, six pair crosses were made. In 2013, replicated single row trials of parent cultivar samples and progeny were sown at two sites near Palmerston North. For 2 years, prior to each grazing, growth score (GS) data were recorded to estimate dry matter yield of each plot. Progeny GS relative to parent average GS (mid-parent heterosis, MPH), and better parent GS (high-parent heterosis, HPH) were calculated on the basis of the mean GS over years and sites. Significant (P<0.05) line effects were detected, as were site, season, and year interactions. For the medium flowering date progeny, MPH and HPH GS ranged from -19 to 23% and -22 to18%, respectively. For the late flowering date progeny MPH and HPH for GS ranged from -9 to 25% and -15 to 21%, respectively. Some progeny exhibited significant (P<0.05) MPH and HPH. The five progeny with highest MPH values ranged from 14 to 23% and 10 to 25% for crosses within medium and late flowering classes, respectively. The same top progeny exhibited HPH ranging from 12 to 18% and 7 to 21% among medium and late flowering classes, respectively. A moderate, significant (P<0.05) negative correlation was observed between high GS of the better parent and the progeny's level of HPH. However there were notable exceptions to the trend. The GS of the best progeny entry was significantly (P<0.05) higher than the GS of the best parent entry, indicating the potential of hybrid vigour to lift the yield barrier in PRG. Investigation is needed of the genetic, wider biological, and economic context under which a hybrid breeding system may be justified.