Abstract
Ley farming
The ley farming system of southern Australia emerged in the early 20th century as a flexible, resilient, sustainable and profitable form of agriculture. Ley farming relied upon a self-regenerating annual legume pasture (ley) to emerge without the intervention of machinery after a cereal crop phase. One key to the success of this system was that the pasture legume produced seed with physical dormancy (PY) to form a “seed bank” that would persist in the soil for several seasons. When the field was “rested” from cropping, the legume would germinate, feed grazing animals, and fix biological nitrogen.
Intensification of agriculture
However, agriculture intensified in the late 20th century with farm managers regularly cropping land in three successive seasons. Weed control in the intensified cropping system relied increasingly upon selective herbicides applied in-crop. Together this strongly mitigated against seed set from the conventional ley legumes. Other profitable non-leguminous crops emerged such as Brassica napus (Canola), which delivered a disease break, and inexpensive N fertilisers delivered abundant N; both formerly provided by the ley legume. These developments collectively pushed ley farming towards obsolescence in southern Australia. The practise, with the common legumes Trifolium subterraneum and Medicago spp., was in terminal decline by 1990.
A new legume ideotype
A decision was taken in the 1990s to remodel ley farming, to accommodate contemporary agriculture. The research comprised two phases: In the first, after analysing the changing economic, ecological and biological challenges to ley farming, the ideotype of an ideal ley legume was redefined. An immediate requirement was to seek new species whose seed could be harvested conventionally (rather than by suction), and which possessed increased PY, with suitable release patterns to survive through three successive crops. From this, a second generation of ley legumes was domesticated (the G2HSLs).
Overcoming barriers to adoption
In a rare departure for researchers, a substantial part of the science in the second phase was undertaken to overcome sociological and economic barriers to pasture renovation. Research focussed on the selection of G2HSL genotypes with PY seed breaking in a particular pattern that would facilitate adoption without expensive processing. The preferred pattern allowed the legume to be sown in summer, with environmental cues breaking a high proportion of the PY by early winter. This innovation was termed “dormant summer sowing (DSS)”.
To support the adoption, 28 experiments were conducted across southern Australia to illustrate how PY seed could be sown into dry soil in summer (DSS), relying upon the fluctuating temperature and moisture levels in the soil to soften the dormant seed in time for an appropriate rainfall event in late autumn.
Outcomes
DSS with G2HSLs reduced establishment costs of renovated pastures by up to 90 % and moved the operation forward to a period of the year when labour availability was greater. DSS of G2HSLs resulted in a 25–50 % increase in seed yield compared to conventional sowing of scarified seed. Under severe drought, seed yield of subterranean clover did not exceed 10 % of that of the G2HSLs established using DSS. The increased seed production ensured a sufficient seed bank to preserve ley farming through multiple crops. Post-legume crop yields exceeded 3 tonnes/ha without the need for fertiliser N, and cereal seed protein levels were maintained or increased. Over five decades, more than 20 cultivars of G2HSLs were commercialised, mainly from newly domesticated species in four genera Ornithopus, Trifolium, Trigonella and Biserrula. Overall, the use of these G2HSLs in mixed farming systems has the capacity to considerably lower the level of cost and risk in the farming enterprise, while increasing its sustainability.