Identifying potential approaches to improve the reliability of terminating a lucerne pasture before cropping: a review

In the mixed cropping regions of Australia, the perennial fodder legume lucerne ( Medicago sativa L.) is being utilised to improve the sustainability of farming systems through improving soil fertility and reducing groundwater recharge. However, the full rotational benefits of lucerne cannot be realised without a reliable approach to terminate the lucerne phase before cropping. Farmer surveys have identified difficulties in successfully removing lucerne before cropping as a significant problem with lucerne-based phase-farming systems. In 1 survey up to 40% of the respondents were not satisfied with the level of lucerne control they were achieving, while in another survey 49% indicated that lucerne removal issues were a constraint to further adoption of lucerne on their farms. Most growers used herbicides to kill lucerne, usually in conjunction with cultivation or grazing. Respondents estimated that herbicides were capable of killing 80% of the lucerne, but most regarded this level of control unsatisfactory. Lucerne removal using herbicides is ineffective when lucerne is able to regenerate from buds in the crown that have not received a lethal herbicide dose. Systemic herbicides such as glyphosate and the auxinic herbicides are predominantly translocated to those parts of the plant with a high demand for photosynthate, often the apical meristems. Hence, effective control of lucerne can only be achieved through targeted application of systemic herbicides to lucerne plants that are actively translocating photosynthate to the crown and taproots. Evidence presented in this review suggests that this is most likely to occur when the storage reserves in the crown and taproot are being replenished several weeks after defoliation. The importance of timing of removal and the potential for intercropping are also discussed.