Impact of chaff lining on the seed persistence and emergence of weeds

Take home message

• Burial of weed seeds under chaff can reduce seedling emergence, but the amount of suppression depends on weed type, chaff type, and amount of chaff cover.
• There is no evidence as yet that weed seeds rot more quickly in chaff tramlines, but it is likely environmental conditions and weed seed type could make a difference.
• Stripper fronts and conventional harvest fronts can capture similar percentages of weed seeds providing the header is set up and used appropriately.
• Chaff lining and chaff tramlining will not totally prevent weed emergence – be prepared to use other measures (e.g. spraying the tramlines with a shielded sprayer).
• The best reason to use chaff lining or chaff tramlining is to concentrate weed seeds into a narrow area for easy follow up.

Background

Herbicide resistance is a major concern for crop production due to the increasing frequency of resistance in key weeds. Non-herbicide weed management alternatives are needed to delay the spread and onset of further herbicide resistance (Walsh et al. 2013). One such alternative is harvest weed seed control (HWSC). HWSC refers to a suite of management practices which target the seed of weeds present at harvest time and borne at harvestable height (typically about 15cm or more above ground height, depending on header set up).

Current HWSC systems include narrow windrow burning, chaff tramlining, chaff lining, chaff carts, bale direct and seed destruction (Walshet al. 2013). Chaff lining and chaff tramlining have potential for widespread adoption in Australia owing to their low cost and ease of implementation relative to some other HWSC practices. Chaff tramlining is the practice of concentrating the weed seed bearing chaff material on dedicated tramlines in controlled traffic farming (CTF) systems, typically using a chaff deck to deposit chaff into two lines (one per wheel track). Chaff lining is a similar concept, where the chaff material is concentrated using a chute into a single narrow row between stubble rows directly behind the harvester (i.e. not specifically onto tramlines).

Weed seed collection

The proportion of weed seed entering the front of the harvester is key to the efficacy of HWSC methods. An increasing number of Australian grain growers are adopting stripper harvester fronts. These fronts use rows of fingers on a spinning rotor to pluck grain heads and pods from mature crop plants. Conventional (known as Draper) header fronts cut and collect the entire plant above the harvest height. Stripper fronts leave more stubble standing. By reducing the quantity of material being processed, stripper fronts increase the speed and efficiency of harvesting. Because HWSC relies on weed seeds entering the harvester, it is important to know whether stripper fronts can achieve weed seed collection proportions similar to conventional fronts.

The weed seed collection effectiveness of stripper and conventional harvester fronts was compared in wheat paddocks on two different farms near Wagga Wagga (Figure 1). At Site 1 the proportion of annual ryegrass seeds collected by the stripper front was identical to that of a conventional front. In contrast, at Site 2 a lower proportion of ryegrass seed was collected by the stripper front. Row spacing was greater at this site and the harvester was running faster and higher than at Site 1, which could have reduced the seed collection and therefore HWSC efficacy.

Figure 1. Percentage of ryegrass seed collected by stripper front and conventional front harvesters at two locations (means with same letter are not significantly different) (Broster et al. 2018).

These results suggest, with due diligence, a stripper front can collect a similar proportion of annual ryegrass seeds as a conventional front and can be used in conjunction with HWSC systems.

Fate of weed seeds

Weed suppression

There have been anecdotal reports that weed seeds die or fail to emerge from chaff lines and chaff tramlines. Pot trials were conducted at three locations (Toowoomba, Wagga Wagga and Narrabri) to investigate the influence of wheat, barley, canola and lupin chaff on seedling emergence of annual ryegrass (Broster et al. 2018; Ruttledge et al. 2018). Although the amount of chaff required to significantly reduce germination differed between studies, increasing amounts of wheat chaff reduced annual ryegrass germination and emergence at all three locations. Wheat chaff at 24t/ha reduced annual ryegrass emergence by approximately 15% to 35% across the three pot trials.

The type of chaff can also influence the emergence of weed seedlings. A pot study conducted at Wagga Wagga explored the emergence of annual ryegrass under four chaff types (barley, canola, lupins and wheat). While there was an interaction between crop type and chaff amount (Figure 2), overall barley inhibited emergence more than wheat, and both were better at suppressing emergence than canola and lupins. For each chaff type, the effect of rate was significant (i.e. for all four chaff types, weed emergence decreased as the amount of chaff increased).

Figure 2. Emergence of annual ryegrass through wheat, lupins, barley and canola chaff at eight different rates (t/ha) in a pot trial conducted at Wagga Wagga (Broster et al. 2018). NB: Means with the same alphabetical characters are not significantly different at the P = 0.050 level based on LSD values (average LSD is shown on bar).

Pot experiments conducted in Toowoomba compared the emergence of common sowthistle under wheat and barley. In the first study, wheat chaff suppressed emergence more than barley chaff, although there was no significant difference between the two chaff types (data not shown). In a subsequent repeat of this pot study, wheat did have significantly greater suppression of common sowthistle compared with barley chaff (Figure 3). In both studies, 12t/ha was sufficient to significantly reduce sowthistle emergence in the wheat chaff treatment.

Figure 3. Emergence of common sowthistle through barley and wheat chaff at nine different rates (t/ha) in a pot experiment conducted at Toowoomba  (Ruttledge et al. 2018). NB: Means with the same alphabetical characters are not significantly different at the P = 0.050 level based on LSD values (average LSD is shown on bar).

Weed seed decay

There are anecdotal reports of weed seed rot in chaff lines and chaff tramlines. On-farm research conducted on the Queensland Darling Downs has compared the viability of weed seeds inside and outside of chaff tramlines, with tramlines produced by both Draper (conventional) and Shelbourne (stripper) harvest fronts. Seeds of annual ryegrass, sowthistle, turnip weed and wild oats were placed in fibreglass mesh bags and left for six months in two barley paddocks (one harvested using a conventional draper front and one harvested using a stripper front) and in a wheat paddock (conventional front only). At the end of this period, the viability of the remaining seeds was determined. There were no significant differences between the seeds placed inside tramlines and those placed outside of tramlines, for any of the three paddocks (Figure 4). In other words, weed seeds placed in tramlines did not appear to decay any faster than those left on the soil surface outside of tramlines.

Figure 4. Viability of weed seeds collected after six months in barley and wheat fields, both under chaff tramlines and outside tramlines (control treatment). In barley, tramlines were produced using both Draper (conventional) and Shelbourne (stripper) harvest fronts (Ruttledge et al. 2018).

However, multiple factors are involved in weed seed decay, including temperature, moisture, chaff type and amount, and characteristics of the weed seeds themselves. In other words, what happens to weed seeds in chaff lines or chaff tramlines is likely to vary according to site, season, crop type and weed species.

Chaff production

As outlined above, research indicates that weed suppression increases with the amount of chaff cover. The amount of chaff that accumulates in chaff tramlines or chaff lines could also have implications for other farming operations (e.g. planting).

It is unlikely that all crop types will have the same chaff load percentage. A barley crop is likely to have a lower chaff proportion than a wheat crop. With a 12m wide conventional harvester forming a chaff line 30cm wide and a chaff proportion of 0.3, then 42t/ha of chaff equates to a 3.5t/ha crop (Figure 5). However, if the chaff proportion is 0.2 then 42t/ha equates to a 5.25t/ha crop. Conversely, with a chaff proportion of 0.5, only 2.1t/ha crop is required to produce a chaff line of 42t/ha (Broster et al. 2018). It should be noted that in chaff tramlining, two chaff lines are formed, which means that only half the amount of chaff residue is deposited in each line.

Figure 5. Estimated crop yield for various chaff rates at different chaff proportions for a 12m wide harvester and 0.3m chaff line (Broster et al. 2018).

Note: Calculations are regardless of header type, but different chaff proportions could relate to the different fronts, e.g. 20% = stripper front, 40% = draper front.

The type of header front is also important in determining the amount of chaff exiting the header. Research has established that draper fronts produce more than twice the amount of chaff compared with a stripper front (Figure 6). This is likely due to the larger amount of crop material collected by a draper front compared to a stripper front, resulting in a significant amount of straw material exiting in the chaff fraction when using a draper front (Broster et al. 2018).

Figure 6. Amount of chaff fraction (kg) produced when using two different harvester fronts (means with same letter are not significantly different) (Broster et al. 2018).

Conclusions and recommendations

Chaff lining and chaff tramlining are forms of HWSC with the potential to capture weed seeds at harvest time and concentrate them in the chaff residues. Weed seeds buried in chaff can have reduced emergence, but the extent of suppression depends on the characteristics of the chaff (crop type, chaff thickness) and also on the weed species. Depending on the seed ecology of weed species, some weeds will be more susceptible to burial in chaff than other weeds.

Research conducted to date does not indicate that weed seeds rot more quickly when concentrated in chaff tramlines. However, the fate of weed seeds is strongly determined by environmental conditions (moisture, temperature, predation, etc.), as well as the characteristics of the weed seeds themselves. This means the extent and rapidity of seed decay is likely to vary from site to site, season to season, and between chaff type and weed type.

The amount of chaff deposited in chaff tramlines is likely to influence the fate of weed seeds, and this is in turn dependent on crop type, planting density, row spacing, header set-up, harvest speed, and the type of header front used (with less chaff produced by a stripper front).

The efficacy of HWSC techniques, including chaff lining and chaff tramlining, will depend on maximum capture of weed seeds at harvest time. The use of stripper fronts is compatible with HWSC, but care is needed in harvester operation to maximise seed collection. Header settings (e.g. height) and harvest speed can all influence the efficacy of weed seed collection by both conventional and stripper harvester fronts. Further research is needed to determine how agronomic considerations (e.g. crop architecture and planting design) can optimise weed seed collection during harvest. In addition, attributes of the weed species, the weather and the amount of seed shed which has occurred before harvest will have an impact on the accuracy of weed seed collection using HWSC practices (Broster et al. 2018).

In summary, weed seeds captured during harvest and concentrated into chaff residues can have reduced emergence due to the presence of the crop chaff, especially at high chaff loads. From current evidence, it is apparent weed seeds of key species can remain viable in a tramline environment for six months, and potentially longer (work being conducted at present will establish the viability of seeds after 11 months in the field). However, by using chaff tramlining or chaff lining, weed seeds captured during harvest are concentrated into one or two lines per header pass, where they can be monitored and treated in targeted weed control strategies (e.g. using high rates and a shielded sprayer).

References

Broster, J.C., Rayner, A., Ruttledge, A., Walsh, M.J. (2018). Impact of stripper fronts and chaff lining on harvest weed seed control. Available at https://grdc.com.au/resources-and-publications/grdc-update-papers/tab-content/grdc-update-papers/2018/07/impact-of-stripper-fronts-and-chaff-lining

Heap, I. The International Survey of Herbicide Resistant Weeds. Available at www.weedscience.com

Ruttledge, A., Widderick, M., Walsh, M., Broster, J., Bell, K., Rayner, A., Jalaludin, A., Cooray, O., Heuke, L., Robilliard, S. and Chambers, A. The efficacy of chaff lining and chaff tramlining in controlling problem weeds (2018). Available at https://grdc.com.au/resources-and-publications/grdc-update-papers/tab-content/grdc-update-papers/2018/07/the-efficacy-of-chaff-lining-and-chaff-tramlining-in-controlling-weeds

Walsh M.J., Newman, P., and Powles SB (2013). Targeting weed seeds in-crop: A new weed control paradigm for global agriculture. Weed Technol. 27:431-436.

Acknowledgements

The research presented in this paper was made possible by the significant contributions of growers through both trial cooperation and the support of the GRDC. The authors would like to thank them for their continued support. The authors also gratefully acknowledge the following people for their assistance with the establishment and assessment of experiments – Allison Chambers (CSU), Linda Heuke (USyd), Shona Robilliard (USyd), a number of CSU students, Onella Cooray (QDAF), Jeff Werth (QDAF) and Adam Jalaludin(QDAF). The authors would also like to thank Kylie and Peter Bach for hosting the QLD trial site.

Contact details

Annie Ruttledge
DAF QLD
PO Box 2282, Toowoomba QLD 4350
07 4529 1247
Annemieke.Ruttledge@daf.qld.gov.au