Recurrent selection of new breeding lines and yield potential, nutrient profile and in vitro rumen characteristics of Melilotus officinalis

Melilotus officinalis is an important forage and green manure crop. There is a high degree of genetic diversity within species of M. officinalis. Despite the forage potential and the ability of nitrogen fixation, it has not been propagated in recent years because its high coumarin content. As a forage crop, the breeding goal of M. officinalis was mainly focused on improving forage yield and quality and reducing coumarin content. In this study, 25 superior half sib (HS) families of M. officinalis were used to carry out the second cycle recurrent selection, and genotypic variation of key traits (yield, quality and coumarin content) at two locations during 2018 and 2019 seasons were evaluated. Significant genotypic variation existed for the key traits: plant height (PH), dry biomass (DB), crude protein (CP), acid detergent fiber, neutral detergent fiber and coumarin. Also, significant genotype × location interactions were observed for the traits: DB, PH, CP, stem diameter, acid detergent lignin and acid insoluble ash (P < 0.05). Coumarin was negatively correlated with DB and PH, and positively correlated with CP. Genetic gain of all the traits of the second cycle recurrent selection were lower than that of the first cycle recurrent selection, except for PH and leaf/stem ratio (LSR). After the second cycle recurrent selection, the genetic gains of coumarin and LSR were higher than for other traits. Furthermore, new breeding lines generated by polycrossing superior HS families were evaluated. The polycrossing improved the yield and in vitro rumen fermentation characteristics and reduced the coumarin content. Our work provides new breeding lines of M. officinalis, beneficial for a sustainable grassland-based livestock industry.