Redefining the link between rainfall and crop establishment in dryland cropping systems
Bonnie M. Flohr, Jackie Ouzman, Therese M. McBeath, G. J. Rebetzke, John A. Kirkegaard, Rick Llewellyn
Abstract
The seasonal break is a rainfall event that has traditionally initiated sowing and enabled plant establishment in Mediterranean-type environments. Despite its importance to crop production, neither the pattern of the seasonal break, nor the impact of recent autumn rainfall decline in eastern Australia on the pattern has been quantified at a national scale. Adaptive strategies to remove reliance on the seasonal break for crop establishment can include combinations of novel genotypes and innovative crop management. This paper (i) shows how seasonal changes may be altering the pattern of sowing opportunities across southern Australia, and (ii) evaluates the potential for adaptive management to deal with these changes using a case-study in southern Australia involving early, deep-sown wheat cultivars with long coleoptiles to reduce the dependence on the seasonal break for successful crop establishment. In the first part of the study, the timing of the seasonal break across the southern Australian cropping regions was determined using the sum of a 7-day rolling mean rainfall to evaporation ratio > 1 to trigger a sowing opportunity between 1 March and 30 July using long-term (1971–2018) gridded (0.05° × 0.05°) historical climate data. The relationship between early-season climatic conditions and adaptive management options was then evaluated using a growth chamber experiment and a field experiment in a Mediterranean-type environment to evaluate the factors leading to successful establishment of earlier deeply sown wheat cultivars with long coleoptiles. The analysis revealed spatial and seasonal variability with the earliest median seasonal break (27-March) in New South Wales (NSW) and Victoria, and the latest (3-June) in Western Australia (WA). The largest shift in seasonal break was a 17-day advance in the Mallee and Sandplain of WA, and an 11-day delay in Central NSW during the period 1990–2018. Evaluation of the factors controlling establishment of a long coleoptile wheat cultivar suggested that soil temperatures in the early sowing windows were unlikely to reduce coleoptile length, but seeding depth, soil type and water availability will play an important role on emergence. In summary, the shifts in the climatic conditions at the break of the growing season have not been the same across the southern cropping region. Adaptations for this climatic shift such as long-coleoptile, deep-sowing strategies for wheat show significant promise but careful consideration of soil factors is required to ensure success and to improve on-farm decision making regarding its use.