Seedbank life of emerging problem weeds – implications for management

Take home messages

• Assessment of seedling emergence at three sites with varying rainfall showed that bifora and bedstraw can persist in the soil for at least three years.
• Brome grass and barley grass showed longer seedbank persistence at Karoonda, which experiences low growing season rainfall, and has a soil with a low organic carbon content and low microbial biomass.
• Cumulative recruitment from the seedbank for several weed species tended to be greater at the lower rainfall site with low soil organic carbon. These results suggest that greater seed decay occurs on heavier textured soils in areas of high rainfall.
• Small seeded weed species, such as common sowthistle and Indian hedge mustard, showed seedling recruitment for 2-3 years at the different study sites.
• The mesh bag method of investigating seedbank persistence appears to underestimate the life of the seedbank of some weed species.

Background

Information on the life of the weed seedbank (persistence) is critical for the development of management programs for herbicide resistant weed populations or for difficult to control weeds. Some weed species, such as brome grass, are easier to manage in oilseed and legume break crops than in cereals. Management decisions, such as changes to cropping sequences, can be made to drive down weed populations to low levels provided the rate of seedbank decline for the weed species in question is known. However, information on the rate of decline of seedbanks under field conditions is not available at this stage, even for some of the major weed species in Australia.

Our recent research on barley grass and brome grass (Fleet and Gill, 2012; Kleemann and Gill, 2013) has clearly established that seed dormancy in these species can vary considerably between populations present on the same farm. Weed populations from intensively cropped paddocks can have much greater seed dormancy than those found in non-cropped situations. Weed populations with greater seed dormancy are not only more difficult to control during the growing season, but their seeds may also persist in the soil for a longer period of time and pose management problems in the future. There is some anecdotal evidence that seed dormancy in populations of many weed species has increased over time. Early research from New Zealand by Popay (1981) showed that there is little seedbank persistence, if any, in barley grass seeds and very few seeds are likely to be present after one year. Later Australian research by Powles et al. (1992) showed that only about 2% of barley grass persisted from one year to the next when new seed set was completely prevented. However, total eradication of barley grass in this study required three years. In a more recent study at the University of Adelaide (UA), some populations of barley grass collected from cropped fields in South Australia showed 10%-20% seedbank persistence from one year to the next (Shergill et al. 2015). There is some evidence that weed populations can evolve in response to management practices and some populations within the same species can possess much greater seedbank persistence than others. Therefore, single year management interventions are likely to be much less effective on weed populations with higher levels of seedbank persistence. Instead, multi-year management programs are needed to deplete the seedbank to more controllable levels. In this paper, we discuss major findings of the project UA00156 on weed seed persistence in five emerging weed species in southern Australia.

Method

Exclusion rings (non-crop)

Field sites were established at three locations with contrasting rainfall - Karoonda (low), Roseworthy (medium) and Tarlee (high). A known number of seeds of two populations of each weed species were introduced into replicated exclusion rings (37cm diam.) in the field in the summer of 2016. Rings containing weed species known to be wind dispersed were covered with fine aluminium mesh to prevent escape and contamination from background weed populations. At the start of the growing season, soil within the rings was cultivated to simulate tillage or a seeding operation. At regular intervals, census of germination was undertaken, and emerged seedlings were removed to prevent seed set. At the end of the study, soil within these rings will be retrieved and weed seeds exhumed and checked for viability.

Mesh bags to determine the effect of seed burial depth

Previous research has consistently shown that the depth of seed burial by tillage can have a large effect on the persistence of weed seedbanks. Even though most growers in Australia now practice no-till, weed seeds can still be buried to varying depths during the seeding operation. A known number of seeds of each weed species were placed in nylon mesh bags and buried at 0, 2cm and 10cm depths. Bags will be exhumed at 3, 6, 12, 18, 24, 30 and 36 months and weed seed germination, dormancy and viability assessed. Seeds were exhumed over shorter periods (1, 2, 3, 6 and 12 months) for species prone to rapid seed decay, such as common sowthistle, prickly lettuce and statice.

Results and discussion

Effect of seed burial on seedbank persistence

Soil disturbance associated with pre-sowing tillage and seeding operations is known to influence patterns of recruitment and persistence of weed seedbanks. Small-seeded species, such as common sowthistle, have a preference for seedling recruitment at shallow burial depths, because exposure to light stimulates germination. As expected, the common sowthistle seedbank was short-lived on the soil surface (Figure 1). Most of the common sowthistle seed had germinated in the field in the first three months (90%-95%). However, a small fraction of the common sowthistle seedbank (10%) in one of the two populations remained viable at the surface after 12 months. Similarly, the barley grass seedbank was rapidly depleted on the soil surface, as the seed has a low level of innate dormancy. However, one of the barley grass populations showed a high level of persistence (60%) up to 12 months. The brome grass seedbank was reduced by 37%-55% after 12 months by seedling recruitment and seed decay, while the seedbank was completely exhausted after 18 months. Seed germination of brome grass is strongly inhibited by light, which leads to greater seedbank persistence on the soil surface (Kleemann and Gill, 2006). This trend was also observed in the current study. Therefore, the seedbank persistence of brome grass is likely to increase in zero-till systems because more seeds remain on the soil surface. Persistence of viable seeds present on the soil surface after 18 months was also clear for both populations of bedstraw (16%-31%) and bifora (27%-75%). Inhibition of seed germination in these species by light exposure has been reported (Chauhan et al. 2006; Mennan, 2003).

Increasing the burial depth can increase the rate of depletion of the seedbank of species that prefer dark conditions for germination. The entire brome grass seedbank germinated within three months after burial at 2cm, while persistence was also low (<20%) following burial at 10cm over the same period of time (Figure 1). These results suggest rapid germination of brome grass occurs after seed burial in conventional tillage systems. Barley grass seeds buried at a shallow depth (2cm) retained 20%-26% viability until 12 months. However, seed burial of barley grass at a depth of 10cm resulted in all seeds becoming non-viable within 6 to 12 months.

A considerable amount of the bedstraw seedbank remained dormant and viable (20%) at the three burial depths after 18 months (Figure 1). The longevity of the bifora seedbank was also preserved with increasing burial depth. A very high proportion (>60%) of the bifora seedbank persisted at 2cm and 10cm of burial, which could be related to the high levels of seed dormancy in this species. These results suggest that bedstraw and bifora are likely to be long term problems (>3 years) in infested paddocks and care should be taken in selecting crop types that allow for the use of effective herbicides each year. The persistence of common sowthistle seeds buried at 2cm for 12 months was much greater (20%-58%) than seeds buried at 10cm. Soil conditions at 10cm are likely to remain moist over a longer period than closer to the surface (2cm), which may have caused greater decay of common sowthistle seeds.

Figure 1. The effect of seed burial depth on the seedbank persistence of five emerging weed species at Roseworthy, SA. Seed was buried at: (A) 0cm; (B) 2cm; and (C) 10cm. The viability of exhumed seed was assessed at three-month intervals for two populations of barley grass (◯ Ba 8 and ● Ba 7); brome grass (◇ Br 8 and ◆ Br 4); bedstraw (□ Be 5 and ■ Be 14); bifora (▽ Bi 3 and ▼ Bi 8), and common sowthistle (△ CST 12 and ▲ CST 7). Each point represents the mean of four replicates. Vertical bars are the standard error of the mean.

Seedling emergence as an indicator of seedbank persistence

Seedbank depletion in the field occurs through seedling recruitment, seed decay, or a combination of these two factors. The results from this study demonstrate that there are clear differences between weed species in seedbank persistence. Furthermore, climatic and soil conditions at the three sites had a large effect on the patterns of seedling recruitment of different weed species.

For many species, cumulative seedling recruitment over the three years was much greater at Karoonda than at Roseworthy or Tarlee. For example, barley grass at Karoonda showed greater (>70%) seedling recruitment in the first year of the study compared to Roseworthy (44%) and Tarlee (27%). Furthermore, seedling recruitment in barley grass in the second year was only observed at Karoonda and not at the other two sites. These results suggest that barley grass seeds that did not germinate in the first year at Roseworthy or Tarlee just decayed. It is likely that greater wetting of the soil at Tarlee and Roseworthy during autumn and winter contributed to the higher levels of seed decay at these sites, as barley grass is known to exhibit a low level of innate dormancy.

Brome grass was the only species to show similar and consistently high levels (57%-60%) of seedling recruitment at all three sites in the first year of the study (Figure 2). However, a much smaller proportion (P≤0.05) of the seedbank emerged in 2017 (second year) at Tarlee relative to Roseworthy and Karoonda. Therefore, the brome grass seedbank persisted for at least two years at two out of the three sites. Similarly, barley grass, prickly lettuce and bedstraw showed higher levels (P≤0.05) of seedling recruitment at Karoonda in 2017 (second year) relative to the other sites. Statice only showed seedling recruitment in the second year at Karoonda. Therefore, the seedbank persistence of these species was much greater on a sandier-textured soil in the low rainfall zone. Seed decay in statice has been previously associated with high soil organic carbon, annual rainfall and soil microbial biomass (Kleemann and Gill, 2018). Greater soil organic carbon content and soil microbial biomass, as well as rainfall received at the Roseworthy and Tarlee sites, is likely to have promoted seed decay in these species.

Bedstraw, bifora and marshmallow showed some seedling establishment in all three years of this study at all three sites, which confirms their long-term seedbank persistence. Changes in the residual seedbank of marshmallow were also slow to occur, as no differences (P>0.05) in recruitment were observed across the three years at the Tarlee and Roseworthy sites. The high levels of dormancy expressed by marshmallow allow for significant seedbank carryover (Michael et al. 2007). Brome grass showed a low level of seedling emergence in the third year at Karoonda, but no emergence was observed at Roseworthy and Tarlee. The observed trend is also consistent with field observations of brome grass being a dominant weed in the Mallee areas around Karoonda. Small seeded species, such as common sowthistle, Indian hedge mustard and wild turnip showed persistence into the third year at Roseworthy, but not at Tarlee.

Figure 2. Cumulative seedling recruitment for 11 emerging weed species from 2016 to 2018 at Karoonda (low rainfall), Roseworthy (medium rainfall) and Tarlee (high annual rainfall). Each bar represents the mean of three populations pooled with three replicates. Vertical bars are the standard error of the mean.

Conclusion

An investigation of weed seed persistence by two different methods in SA showed large differences between species in the rate of seed decay. Several species including bedstraw, bifora, and marshmallow persisted for at least three years. The experimental site had a large effect on the persistence of the seedbank. Most weed species persisted for a shorter time at the site with the highest rainfall and a heavier textured soil. In contrast, more species persisted for at least three years at Karoonda, which has low growing season rainfall, a sandy textured soil and low soil microbial biomass. Control programs for weed management need to consider the variation between weed species in seedbank persistence.

References

Chauhan, B. S., Gill, G., and Preston, C. (2006). Seedling recruitment pattern and depth of recruitment of 10 weed species in minimum tillage and no-till seeding systems. Weed Science 54, 658-668.

Fleet, B. and Gill, G.S. (2012). Seed dormancy and seedling recruitment in smooth Barley (Hordeum murinum ssp. glaucum) populations in southern Australia. Weed Science, 60, 394-400.

Kleemann, S. G. L., and Gill, G. S. (2006). Differences in the distribution and seed germination behaviour of populations of Bromus rigidus and Bromus diandrus in South Australia: adaptations to habitat and implications for weed management. Australian Journal of Agricultural Research 57, 213-219.

Kleemann, S. G., and Gill, G. (2018). Seed Germination and Seedling Recruitment Behavior of Winged Sea Lavender (Limonium lobatum) in Southern Australia. Weed Science, 66, 485-493.

Mennan, H. (2003). The effects of depth and duration of burial on seasonal germination, dormancy and viability of Galium aparine and Bifora radians seeds. Journal of Agronomy and Crop Science 189, 304-309.

Michael, P. J., Steadman, K. J., and Plummer, J. A. (2007). Seed development in Malva parviflora: onset of germinability, dormancy and desiccation tolerance. Australian Journal of Experimental Agriculture 47, 683-688.

Popay, A.I. (1981). Germination of seeds of five annual species of barley grass. Journal of Applied Ecology, 18, 547-558.

Powles S.B., Tucker E.S., and Morgan T.R. (1992). Eradication of paraquat‐resistant Hordeum glaucum Steud. by prevention of seed production for 3 years. Weed Research 32, 207-211.

Shergill LS, Fleet B, Preston C, Gill G (2015). Incidence of herbicide resistance, seedling emergence and seed persistence of smooth barley (Hordeum glaucum) in South Australia. Weed Technology 29, 782-792.

Acknowledgements

The research undertaken as part of this project is made possible by the significant contributions of growers through both trial cooperation and the support of the GRDC – the author would like to thank them for their continued support. The authors would also like to thank Dr Sam Kleemann for his contribution to research undertaken in this project, as well as Jerome Martin for providing technical support.

Contact details

Gurjeet Gill
University of Adelaide
08 8313 7744
gurjeet.gill@adelaide.edu.au