This information reports on seed persistence of key weeds of the Northern Region (barnyard grass, bladder ketmia, climbing buckwheat, common sowthistle, fleabane, liverseed grass, paradoxa grass, turnip weed, and wild oat). It also compares the impact of cultivation and burial depth on persistence, together with implications for weed management.

The seed persistence of these nine weeds was examined in either pot experiments, with seed sown in large black plastic pots sunk into the ground, or in field experiments, with natural infestations or sown seed. All pots and field plots were managed to prevent any new weed seed replenishment after commencement of the experiments. Pots were exhumed and soil cores were collected from the field at regular intervals. The contents of the pots or cores were washed through a series of wire sieves of suitable size to capture the remaining weed seeds. Viable seed, defined as firm seed with no evidence of decay, were then counted. Exponential curves were fitted to the viable seed data for each weed and treatment in all experiments, apart from Experiment I, to estimate the percentage remaining for longer intervals than measured.

Details of experiments

Experiment I: climbing buckwheat, liverseed grass, paradoxa grass, turnip weed, and wild oat

Seeds were sown in pots at 0-8 cm depth, and soil was mixed manually 4 times per year to simulate tillage. Pots were exhumed after 4 years. This experiment was duplicated with a second series sown a year later with new seed in different pots.

No liverseed grass or paradoxa grass seeds were detected after 4 years of burial (Table 1).  As well, no wild oat seed or very few seeds were detected.  However, 2-11% of the sown seed of climbing buckwheat, turnip weed and wireweed persisted for 4 years.

Experiment II: barnyard grass, bladder ketmia, and liverseed grass

Seeds were sown in pots at 0-2 and 10 cm depths in soil that was left undisturbed.

After 2 years of burial, only 1-2% of barnyard grass and liverseed grass seed remained in the top 2 cm, whereas 19-21% remained when buried at 10 cm depth (Table 2).  In contrast, a substantial portion of bladder ketmia seed (38-64%) remained after 2 years irrespective of burial depth. The estimated portions that remained after 4 years were <0.1% in the surface soil for the grasses and 17% for bladder ketmia.

Experiment III: common sowthistle

Seeds were sown in pots at 0, 2 and 10 cm depths in soil that was left undisturbed.

Less than 1% of common sowthistle seed remained on the soil surface after 2 years, whereas 2 and 10% remained at 2 and 10 cm depths respectively (Table 3). No seeds were estimated to remain on the soil surface after 4 years, in contrast to 2% at 10 cm.

Experiment IV: paradoxa grass

Seeds were sown in pots at 0, 2 and 10 cm depths in soil that was left undisturbed. Seed were also mixed in the top 10 cm in separate pots, and this soil was manually mixed annually.

After 2 years of burial, only 1-4% of paradoxa grass seed remained irrespective of depth or soil disturbance (Table 3).  The estimated portions after 4 years of burial were <1% on soil surface and <0.1% for other depths and for tilled soil.  After the first year, there was a trend of seed persisting longer on soil surface than when buried.

Experiment V: paradoxa grass

Soil cores were collected over 3 years within a defined zero-tilled field plot artificially infested with paradoxa grass. This experiment was duplicated a year later, with half of the area undisturbed and other half cultivated annually, with cores collected over 2 years.

In the first experiment, paradoxa grass seed-bank declined from 6990 to 0 seed m^-2 in 3 years. In the second experiment, the seed-bank declined from 61600 to 153 seed m^-2 in non-disturbed plot and to 4 seed m^-2 in the disturbed plot in 2 years.  The estimated persistence after 4 years in both experiments was <0.1% (Table 4).

Experiment VI: turnip weed

Soil cores were collected over 4.5 years from a defined zero-tilled field plot artificially infested with turnip weed.

Thirty-six percent of turnip weed seed persisted after 2 years, and 7% remained after 4 years in the top 10 cm of soil (Table 4). 

Experiment VII: wild oats

Soil cores were collected over 2 years from 8 defined areas within a zero-tilled paddock naturally infested with wild oat.

Only 1% of wild oat persisted in top 10 cm of soil after 2 years (Table 4), and an estimated <0.1% would remain after 4 years.

Experiment VIII: fleabane

Seeds were sown in pots at 0-2, 5, and 10 cm in soil that was left undisturbed. Pots were exhumed over 3 years, and the soil layer with the sown seed was placed in the glasshouse under ideal conditions for fleabane emergence, which was recorded regularly.

After 3 years of burial, only 1% of fleabane seed remained viable in the surface soil, although 8-10% remained at 5-10 cm depths (Table 5).

Implications

Persistence of these weed seeds differed substantially between groups of weed species.  The annual grasses, irrespective whether they were summer or winter growing species, had only a short persistence in the surface soil layer. Less than 5% remained after 2 years and between 0 and 1% remained, or was predicted to remain, after 4 years. Common sowthistle and fleabane also had a similar short persistence.

However, seed persistence of barnyard grass, liverseed grass, fleabane and common sowthistle increased markedly when seed were buried at lower depths.  For example, 10-21% of the seed remained after 2 years compared with 1-6% in the top 2 cm.

Seed persistence of bladder ketmia, climbing buckwheat, and turnip weed, weeds with a hard seed coat, was much longer than for the annual grasses. For example, 36-64% of bladder ketmia and turnip weed remained after 2 years, whilst 5-11% remained after 4 years for climbing buckwheat and turnip weed in the top 8-10 cm of soil.  Burial depth also influenced persistence of bladder ketmia, with 38% remaining in the top 2 cm after 2 years compared with 64% at 10 cm.

Soil disturbance appeared to have only minimal impact on weed seed persistence. For paradoxa grass, the rate of seed loss tended to be slightly greater in cultivated soil compared with zero-tilled treatments.  This was due to cultivation stimulating significantly greater emergence of paradoxa grass seedlings, which were then effectively controlled.  The measured persistence of liverseed grass, turnip weed, and wild oat seed after 4 years of tilled soil in Experiment I was very similar to that measured and/or predicted in zero-tilled soil in Experiments II, VI and VII.  However, a tillage operation that inverts soil and buries weed seed is likely to have a much greater impact on seed persistence in contrast to tillage operations that just mix the weed seed within the top layer of soil.

The differences among weed species in seed persistence and the impact of seed burial have implications with regard to their weed management strategies. For all weeds, it is critical to leave weed seed on, or near, the soil surface, as this will lead to more rapid decline of viable seed in the soil seed-bank.

For annual grasses, fleabane and common sowthistle, management strategies without weed seed burial or seed rain will lead to ³99% reduction in the seed-bank in the surface soil within 2-3 years.  This indicates that paddocks heavily infested with these weeds could be managed to substantially minimize the impact of weeds within a relatively short period. For paddocks with resistant annual grasses and common sowthistle, management strategies that prevent weed seed burial and seed rain will lead to a rapid decline in these herbicide resistant populations.

Whilst the same approach could be applied to weeds, such as bladder ketmia, climbing buckwheat, and turnip weed, the management strategy will need to be in place for at least 4-5 years to reduce the seed-bank by ³95%.  For these weeds, it is much more critical to prevent any seed rain, given that their seeds are much more persistent, and therefore more costly to manage than annual grasses and common sowthistle.  It is also important to implement preventive strategies for herbicide resistance prior to these weeds developing resistance, due to the long persistence of these weed seeds in the soil seed-bank.

Acknowledgement

Authors: Steve Walker¹, Bruce Wilson², Hanwen Wu³, Michael Widderick¹, Ian Taylor⁴

¹Department of Primary Industries & Fisheries, ²Department of Natural Resources, Mines and Water, ³NSW Department of Primary Industries, ⁴Cotton Research & Development Corporation

This research was funded by CRC for Australian Weed Management, CRC for Australian Cotton, Grains Research & Development Corporation, and Cotton Research & Development Corporation. The report was compiled for a 2007 GRDC Grains Research Update.

For more information, contact:

Steve Walker
Ph: 07 4639 8838
E-mail: steve.r.walker@dpi.qld.gov.au