Earlier swathing opens a new pathway for subterranean clover seed harvest

Subterranean clover’s ability to bury its seed supports pasture persistence but makes commercial seed harvesting difficult. The industry relies on ageing Horwood-Bagshaw (HB) suction harvesters, which collect seed from fully dry crops after repeated tillage and raking.

Two experimental studies investigated a very different approach: cutting and lifting plants with modified peanut-harvesting equipment while they are still green, then drying them in windrows for later collection and threshing.

The research demonstrates that viable seed can be recovered well before complete crop desiccation, creating a potential pathway towards harvesting with fewer machinery passes and less soil disturbance.

Testing a different harvest system

A commercial-scale trial at Capel, Western Australia, compared swathing a subterranean clover seed crop on 25 November and 1 December 2022 with conventional HB harvesting in February 2023.

A modified peanut digger cut the roots below ground, lifted the plants and placed them in windrows with the burrs attached, representing the first stage of a proposed two-stage system. Seed was collected from the windrows by hand because the second stage, mechanical pickup and threshing, was not tested.

The equipment successfully lifted and windrowed the crop, although soil attached to the plants and weeds became tangled in the machinery. These issues indicate that substantial equipment refinement and changes to paddock and harvest management would be needed.

Green plants retained viable seed

The most important finding was that subterranean clover could be harvested while plants still retained green material. Potential seed yield did not differ between either swathing date and the much later HB harvest, indicating that maximum seed yield had largely been reached before the crop was completely dry.

Swathing on 25 November moved almost 100% of the available seed into the windrow. This fell to 84.2% on 1 December as the crop became more senescent but remained higher than the 61.3% recovered by a single HB pass following three preparatory tillage operations.

The earliest harvested seed was lighter and had lower and more variable germination. However, seed swathed on 1 December achieved 98.0% germination, comparable with 97.5% for seed harvested by the HB in February. Seedlings that emerged from the earlier harvested seed generally showed similar vigour to those produced by later harvested seed.

These results suggest there is a harvest window in which plants remain intact enough to retain their burrs while the seed has developed sufficiently to maintain yield and quality.

Timing will be critical

Supporting research examined changes in burr attachment and harvest loss during seed development in six subterranean clover cultivars. Burr attachment strength declined over time in every cultivar. Harvest loss also increased over time in four of the six cultivars, supporting the value of harvesting before plants become dry and brittle.

Responses varied between cultivars and within the trial site. Burr attachment strength was only a useful indicator of loss for some greener, less mature plants. Soil moisture, crop biomass, burr burial, soil conditions and seasonal weather are also likely to influence the timing and effectiveness of swathing.

The commercial trial reinforced how quickly conditions can change. During the six days between swathing treatments, crop senescence advanced and seed extraction efficiency declined by almost 16 percentage points. This means the practical harvest window may be relatively narrow and will vary between cultivars, paddocks and seasons.

Future-proofing subterranean clover seed production

Industry management priorities and commercial conditions will continue to change, but investigating alternative harvest systems remains important. HB harvesters are no longer manufactured and conventional harvesting requires repeated soil disturbance, creates dust and depends on crops being completely dry.

This research shows that complete desiccation is not essential for recovering viable subterranean clover seed. Earlier swathing could potentially reduce burr losses, machinery passes, soil disturbance and reliance on ageing equipment.

The trials established that harvesting green subterranean clover is technically possible and worthy of further development, providing industry with another option to investigate as it prepares for future changes in machinery availability, production costs and environmental expectations.