Swathing subterranean clover (Trifolium subterraneum) does not suck: a promising alternative to vacuum seed harvesting

Context
Buried seeds of subterranean clover (Trifolium subterraneum) are typically harvested using the suction-based Horwood-Bagshaw Clover Harvester (HB) in late summer after preparatory tillage. This method relies on outdated equipment and causes significant soil disturbance. Swathing offers a promising alternative, whereby roots are cut and the sward windrowed before senescence for subsequent pickup and threshing once dried.
Aims
This study compared seed extraction efficiency and quality between swathing using a modified peanut digger as a first harvesting step and conventional HB harvesting.
Methods
Trials on a commercial subterranean clover seed crop in Capel, Western Australia, compared swathing using a modified peanut digger on 25 November and 1 December 2022 followed by hand collection of seeds to a single HB pass following standard tillage (14 February 2023). Measurements included canopy greenness, shoot biomass, soil moisture, seed extraction efficiency, seed weight, germination and seedling vigour.
Key results
Potential seed yield did not differ among treatments. As a first harvesting step, swathing achieved 100% (25 November) and 84.2% (1 December) seed extraction, compared to 61.3% for harvesting with the HB. November-swathed seeds had 82.6% germination, while December swathed (98.0%) and HB harvested (97.5%) had comparable germination. Seedling emergence followed the same trend. Swathed seeds weighed 17.1% (25 November) and 12.0% (1 December) less than HB-harvested seeds; however, seedling dry weights were mostly equivalent.
Conclusions
Although early swathing slightly reduced seed quality, swathing as a first harvesting step outperformed HB harvesting in extracting viable seed.
Implications
Swathing subterranean clover is a viable alternative to HB harvesting, addressing efficiency and environmental concerns associated with the traditional method.