Potential of lucerne sowing rate to influence root development and its implications for field stand productivity

The relationship between root morphology and productivity in lucerne (Medicago sativa L.) has been well documented, but the effect of sowing rate on root development is unclear. Results from three field experiments were analysed to address these objectives: (a) to investigate lucerne root development as a function of distance among plants, (b) to evaluate the effect of sowing rates on lucerne root traits and (c) to investigate the relationship between root traits and forage yield and yield components. A 4-year experiment that compared spaced plants (50 x 50 cm, four plants m(2)) versus row seeding (12.5 cm, 700 germinated seeds m(-2)) revealed fast annual tap-root diameter (TD) development in spaced plants, with about 12 mm, and three times greater TD and five times the lateral root number compared with row-spaced plants. Three-year field plot experiments at one site compared sowing rates of 400 versus 700 germinated seeds m(-2) in less favourable soil for lucerne, and 700 versus 900 seeds m(-2) in a better soil. Across experiments, higher plant density and root potential index were positively related to forage yield only when in association with better soil conditions and optimal water supply. Lower plant density offered advantages for faster root development. An effect of reduction in the sowing rate was increased stand height and increased yield in the third cut in a year with drought stress. Results show the need for investigation of effects of more precise spatial arrangement of lucerne plants to ensure optimization of root development and to enable yield benefits and drought resistance.