Weed ecology: the key to weed control

Key messages

  • All weed species had their greatest seedling emergence in the first year after seed set, but species with small seeds had a higher proportion of seedlings emerge in year one than species with larger seeds. Species with large seeds had higher emergence in years two and three.
  • Burial of seeds extends the life of the seed bank in most weed species. Seed on the soil surface will germinate (and get sprayed) or degrade more rapidly than buried seeds.
  • Weed-crop competition is related to weed emergence time.

Background

A five-year national GRDC project entitled ‘Seed bank ecology of emerging weeds’ was completed in 2020. The project investigated the seed bank ecology, growth and competitive ability of ten different weed species. The species included Afghan melon (Citrullus amarus Schrad.), barley grass (Hordeum leporinum Link), great brome (Bromus diandrus Roth), button grass (Dactyloctenium radulans (R.Br.) P.Beauv.), caltrop (Tribulus terrestris L.), double gee (Rumex hypogaeus T.M.Schust. & Reveal, syn. Emex australis Steinh.), roly poly (Salsola australis R.Br.), sowthistle (Sonchus oleraceus L.), windmill grass (Chloris truncata R.Br.) and wireweed (Polygonum aviculare L.). These species do not have much in common; they are grass or broad leaf species, erect or prostrate, winter or summer annuals. However, the comparison of these species within a single project has highlighted broad ecological trends that can be used to develop integrated weed management (IWM) strategies for different species.

Method

Emergence per year

Annual emergence of the ten weed species was monitored under irrigated conditions at Northam Western Australia, with the full methods and results reported in Borger and Hashem (2020). The seed size for each of the investigated species was reported in Borger et al (2020). Simple linear regression analysis was used to compare seed weight to total seedling emergence for each species over three years.

Burial and seed bank longevity

Seeds of two populations of each weed species were buried in mesh bags at 0cm (soil surface), 2cm and 10cm for three years. Bags were retrieved after 3, 6, 12, 24 and 36 months and seeds were removed. However, after the
three-month exhumation, it was noted that the populations of caltrop, roly poly and button grass had low viability. For each of these species, new seed was buried, and retrieved at 3 to 24 months. To assess viability, seeds were placed in petri dishes on moist filter paper and maintained in a germination cabinet at 10-20oC for winter or winter-spring weed species (barley grass, doublegee, great brome, sowthistle and wireweed) or 20-30oC for summer weeds. After two weeks seedlings were counted and removed. Small seeds (sowthistle, windmill grass, roly poly and button grass) were manually squashed with tweezers to assess viability. The other, larger seeds were tested for viability using a tetrazolium chloride test. An ANOVA was applied to the data, to investigate the impact of species, population, depth and retrieval time on seed viability. Roly poly and button grass data were removed from the analysis due to low viability.

Competitive ability and time of emergence

Emergence time and competitive ability of every weed species against wheat was tested at one trial site over three years at Wongan Hills (2016-2019), with full details of methods and results in Borger et al (2020). Competitive ability of great brome, barley grass, sowthistle, doublegee and wireweed at varying densities in wheat was tested in field trials over four years (2016-2019) at Wongan Hills, with full methods and results in Hashem et al (2019) and Dhammu et al (2020). In each of these trials, weeds were allowed to grow unrestrained. The only chemical application was a non-selective herbicide before sowing. Crop yield was compared to a weed free control, to determine the impact of each species in each year. The impact of summer weeds on crop growth was determined by comparing yield of the subsequent wheat crop to yield of the weed-free control plots. In each trial, the earliest time of weed emergence was recorded for each species, in each year. The range in competitive ability of each weed species over three or seven trials was presented next to the range in initial weed emergence time (Table 2).

Results

Emergence per year

Emergence in year 1 decreased as seed size increased (Table 1). Weed species with seed less than 1mg, like sowthistle, button grass, roly poly and windmill grass had emergence percentages of 89-99% in year 1. Species with larger seeds like wireweed, barley grass, great brome had 63-75% emergence in year 1. Species with the largest seed, like doublegee, Afghan melon and caltrop had emergence percentages of 56-59% in year 1. In year 2, the trend was reversed, with greatest emergence from those species with largest seeds, i.e. 30%, 21% and 23% emergence of Afghan melon, caltrop and doublegee. However, barley grass also had high emergence in year 2 (26%). In year 3, the greatest emergence was again observed in Afghan melon, caltrop and doublegee (9-10%).

Table 1. Average seed size and emergence of ten weed species in year 1, 2 or 3. The equation (and R2 and P values) indicate the results of the regression analysis relating emergence in each year to seed size.

Table 1_seed_size_borger

Burial and seed bank longevity

Seeds of multiple weed species had greater viability after 24 or 36 months when buried at 2-10cm, compared to seed left on the soil surface (0cm, Table 2). For example, Afghan melon had greater seed viability on the soil surface over 3 to 24 months, but at 36 months there was 2% viable seed at 0cm compared to 15% or 10% viable seed at 2-10cm. Likewise, great brome seed at 0cm had 0% viability after 36 months, compared to 8-13% viability at 2-10cm. Barley grass, doublegee and sowthistle had 0-0.5% viable seed at 0cm by 24 months, significantly lower than viability of seed at 2-10cm (although by 36 months the difference in seed viability at 0-10cm was not significantly different). Caltrop at 24 months had 3% viable seed at 0cm compared to 10-11% viable seed at 2-10cm. The difference between windmill grass seed viability at 24 months was not significant between depths (0.2, 4 and 3% at 0-10cm). There was no significant difference between wireweed seed viability after 24 or 36 months.

Table 2. Viable seed (%) of each weed species following burial at 0, 2 or 10cm, for a duration of 3 to 36 months (with a significant interaction of species, duration of burial and burial depth, P<0.001, LSD: 6.09).

Table 2_viable_seed_borger

Competitive ability and time of emergence

Reduction in wheat yield was highly variable for all winter weed species (Table 3). The greatest reduction to yield resulted from great brome grass. This species also had the earliest emergence times, with the first cohort emerging in June, at a similar time to the crop, in each trial. The second greatest yield reduction resulted from barley grass, with emergence in late June or July. Sowthistle, windmill grass, wireweed and doublegee also reduced yield in most seasons, but had a smaller impact than great brome or barley grass. These weeds also frequently had delayed emergence. Summer weed species emerging in spring or summer had no impact on yield of the following wheat crop in all trials and years.

Table 3. The yield reduction to wheat from weed species in multiple weed competition trials, where the weeds were allowed to grow in the absence of chemical control. Direct yield reduction results from weeds growing in crop, and indirect yield reduction from weeds growing over the summer fallow. Emergence time indicates the range in time of earliest emergence for each weed species. Number of years indicates the number of years/trials in which impact on yield and emergence time were investigated for each species.

Table 4_yield_reduction_borger

Conclusion

A comparison of multiple weed species has allowed us to highlight some ecological ‘rules of thumb’. They will not be consistent for every single weed species in Australia. However, for those species where research on biology and seed bank ecology has not yet been conducted, these rules of thumb may give a starting point for developing an IWM program. These rules of thumb include:

  1. Weed species with small seed may have the greatest emergence in year one. Therefore, one year of seed set control will dramatically reduce the population.
  2. Burial of seeds extends the life of the seed bank in most weed species. If seeds are left on the soil surface, (i.e. zero tillage), they will germinate (and get sprayed) or degrade more rapidly than seeds at depth.
  3. Weed-crop competition is related to weed emergence time, with a greater competitive impact from those species that emerge at a similar time to the crop.

It is valuable to know that integrated and sustained management plans will quickly control the seed bank of those species with small seed, and longer management programs (over several years) are required for species with large seeds. However, those species with small seeds often have broad scale dispersal, so it is also important to monitor neighbouring areas and check for new seed moving into the paddock (Rees, 1993).

A zero tillage system using disc seeding will cultivate approximately 5% of the soil surface (with seed burial parameter summaries in the Weed Seed Wizard decision support tool, Agriculture and Food Western Australia, 2020). As a result, 95% of weed seed remains on the soil surface and 5% is buried between 0-5cm. By comparison, a no tillage system with knifepoint seeding disturbs 5-20% of the soil surface (depending on row spacing) but further soil throw generally results in an average of 10% of seed left on the soil surface and 90% of seed buried between 0-5cm. This research highlighted that even those weed species with hard, woody seed coats (i.e. doublegee, caltrop, Afghan melon) have a greater reduction in viability (due to germination or degradation) on the soil surface than at depth. It is clear that IWM programs will require fewer years of intensive weed control in a zero-tillage system.

It is well known that direct crop-weed competition is most heavily influenced by the initial time of weed emergence compared to crop emergence, and the total duration of the weed-crop interaction (Mokhtassi-Bidgoli et al 2013). In both Western Australia and South Australia, some barley grass populations have evolved late emergence strategies to avoid pre-seeding and in-crop herbicide applications (Borger & Hashem 2018; Kleemann et al 2018). In South Australia, great brome populations also frequently have delated emergence, although this is less common in Western Australia (Borger & Hashem 2018; Kleemann et al 2018). However, delayed emergence is an expensive strategy for a weed species, in terms of competitive ability. In this set of experiments, the summer weed species had no impact on yield of the subsequent wheat crop. However, the trials were all conducted in Wongan Hills, which has a very sandy soil type (Hunt & Kirkegaard 2011). In other locations and soil types in Australia, summer weed species have a larger impact on crop yield (Hunt & Kirkegaard 2011).

Acknowledgments

The research undertaken as part of this project is made possible by the significant contributions of growers through both trial cooperation and GRDC investment; the author would like to thank them for their continued support. We would also like to thank project staff Glen Riethmuller, Harmohinder Dhammu, Nerys Wilkins, Dave Nicholson and Peter Gray, as well as Shari Dougall and Bruce Thorpe at the Wongan Hills Research station and Gurjeet Gill, the national project leader. Thanks are due to Alex Douglas for reviewing this paper.

Contact details

Dr Catherine Borger
Department of Primary Industries and Regional Development
75 York Rd (PO Box 483) Northam WA 6401
Ph: 0467 816 082
Fx: 08 9622 1902

References

Agriculture and Food Western Australia (2020) Weed Seed Wizard. Version 7.1.7. Available at: https://www.agric.wa.gov.au/weed-seed-wizard-0 [Accessed 29 April 2020].

Borger, C. and Hashem, A. (2020) Weed seed banks – how long do they last? In: GRDC grains Research Updates, Perth Western Australia. Grains Industry Association of Western Australia.

Borger, C.P.D. and Hashem, A. (2018) Emergence of great brome grass and barley grass. In: Proceedings Weed Biosecurity - Protecting our Future. 21st Australasian Weeds Conference. (eds Johnson, S., Weston, L., Wu, H. and Auld, B.). 9-13 September, Manly, Australia. 341-344. The Weed Society of NSW, Sydney, Australia.

Borger, C.P.D., Hashem, A., Van Burgel, A. and Gill, G. (2020) Invasiveness of agronomic weed species in wheat in Western Australia. Weed Research 60, 251-258. https://doi.org/10.1111/wre.12419

Dhammu, H., Hashem, A., Borger, C., Riethmuller, G., Wilkins, N. and Gray, P. (2020) Competition of five weed species in wheat. In: GRDC grains Research Updates, Perth Western Australia. Grains Industry Association of Western Australia.

Hashem, A., Riethmuller, G. and Borger, C. (2019) Competitiveness of emerging weed species in a wheat crop. In: Proceedings Grains Research Updates. 25-26 February 2019, Perth, Western Australia. Grains Industry Association of Western Australia, Grains Research & Development Corporation.

Hunt, J.R. and Kirkegaard, J.A. (2011) Re-evaluating the contribution of summer fallow rain to wheat yield in southern Australia. Crop & Pasture Science 62, 915-929. http://dx.doi.org/10.1071/CP11268

Kleemann, S., Fleet, B., Preston, C. and Gill, G. (2018) Latest research on brome grass and susceptibility of emerging weed species to harvest weed seed capture and control. In: Proceedings GRDC Research Updates. 20 Feb 2018, Adelaide, South Australia. Grains Research and Development Corporation.

Mokhtassi-Bidgoli, A., Navarrete, L., Aghaalikhani, M. and Gonzalex-Andujar, J.L. (2013) Modelling the population dynamic and management of Bromus diandrus in a non-tillage system. Crop Protection 43, 128-133.

Rees, M. (1993) Trade-offs among dispersal strategies in the British flora. Nature 366, 150-152.

GRDC Project Code: UA00156,