Harvest weed seed control – beyond windrow burning

Take home messages

• Harvest weed seed control (HWSC) comes in many forms – bale, burn, graze, mill or rot.
• Match the harvest weed seed control tactic to your farming system, crop types and location.
• Capturing weed seeds in the chaff fraction when chaff lining or using chaff decks requires attention to detail in harvester setup.
• Harvest weed seed control cannot be used effectively in isolation – adopt the ‘big six’ and top shelf agronomy to drive weed numbers to zero.

Background

Herbicide resistance remains an ongoing challenge for Australian grain growers but the industry is continually innovating to minimise the risks. Non-chemical tools are becoming mainstream practice so that growers and advisers can deal with herbicide resistance by reducing weed seed banks and protecting chemistry.

One of the most popular weed management tactics being adopted in recent years is harvest weed seed control (HWSC). This process takes advantage of seed retention at maturity by collecting weed seeds as they pass through the harvester. Problematic weeds such as annual ryegrass, brome grass and wild radish retain 77-95% of their seed above a harvest cut height of 15cm at maturity, creating an ideal opportunity for seed collection.

Seed retention will change over time with the proportion of retained weed seeds declining the longer harvest is delayed past crop maturity. Therefore, crop and weed maturity will have a significant impact on the success of harvest weed seed control. Harvest height is equally important for harvest weed seed control, with a 15cm cut height preferred to capture 80-90% of the ryegrass seed at maturity - this can be challenging in high yielding cereals or bulky hybrid canola crops.

In the southern cropping region, low harvest height has been a barrier to adoption with growers not wanting to slow harvest down, incurring higher fuel costs and reducing harvester efficiency. Growers and researchers have since been looking at tactics that will enhance the efficacy of harvest weed seed control without slowing harvest. One option being adopted is sowing crops at narrower row spacings or higher plant populations. Weeds are then forced to grow taller to compete for light, therefore producing seed higher in the crop canopy. Stripper fronts are also being investigated to gauge any differences with weed seed capture and harvest efficiency, reducing the need to cut low whilst minimising fuel consumption.

Harvest weed seed control (HWSC) practices

Originally pioneered 30 years ago with chaff carts in Western Australia, harvest weed seed control has now been adopted nationally as growers tailor their options to suit different farming systems and locations. The harvest weed seed control options are all slightly different with narrow windrow burning (NWB) and bale direct taking in both straw and chaff for burning or baling. Newer harvest weed seed control practices only take in the chaff fraction containing weed seeds for rotting, grazing or destruction through a mill. This includes chaff lining or chaff decks, chaff carts and emerging mill technology using the Integrated Harrington Seed Destructor (iHSD) or Seed Terminator.

Research by Walsh et. al., 2014 highlighted that harvest weed seed control tactics are equally effective in reducing weed seed production. The use of chaff carts, narrow windrow burning or the Integrated Harrington Seed Destructor, were compared at 24 sites across Australia with an average reduction in ryegrass of 60% germination the following autumn. This was achieved by removing 70-80% of the seed at harvest through either burning or destruction of weed seeds.

Research has recently commenced to gauge the impacts of chaff lining and chaff decks on the rotting of weed seeds under different crop types. Preliminary data suggests poor seed survival under canola or barley chaff because of an allelopathic effect; however in wheat there was high ryegrass seed survival underneath the chaff row which is unexplained. Michael Walsh from Sydney University and John Broster from Charles Sturt University are currently working to quantify the value of rotting under chaff line and chaff deck systems.

Each harvest weed seed control practice has its own benefits and challenges with growers leading the charge, working with a small group of researchers to develop harvester modifications that maximise weed seed control with harvest height and seed retention. For harvest weed seed control to be successful at the farm level the practice needs to be both cost effective and practical to fit in with existing operations.

Harvest weed seed control cannot be used in isolation for weed management; growers and advisers should implement a range of diverse weed management practices to drive weed numbers down. Defined as the ‘Big six’ (www.weedsmart.org.au/the-big-six), these management practices include diverse rotations, mix and rotating herbicides, crop competition, double knocks, crop topping/hay to stop seed set and harvest weed seed control. The ‘big six’ complements best practice agronomy such as calendar sowing combined with effective pre-emergent herbicide packages.

Harvest weed seed control adoption

An online twitter survey was conducted in November 2017 by WeedSmart with 269 growers responding. The results indicated that harvest weed seed control practices are changing, with narrow windrow burning declining at the expense of chaff lining and chaff decks. 32% of growers were planning to use narrow windrow burning in 2017 whilst 26% would be chaff lining and 9% using chaff decks. Chaff carts were stable at 13%, mill technology at 3% and 14% would be doing nothing.

The overall trend is positive and reflects the high value growers are increasingly putting on harvest weed seed control as a mainstream weed management tool, it does not come easy and looking at each practice in detail (Table 1) highlights what growers and advisers need to be aware of.

Table 1. Harvest weed seed control options

Narrow windrow burning (NWB)

Developed in the northern WA cropping zone narrow windrow burning has been highly effective at reducing annual ryegrass and wild radish seed banks across the nation. A chute is attached to the back of the harvester to concentrate straw and chaff into a 500-600mm narrow windrow; these rows are then burnt the following autumn. The practice is low cost and highly effective with rows burning hotter for longer than a standard stubble burn. Up to 99% of weeds seeds are controlled in a well-managed hot burn where temperatures reach 400^oC to 500^oC for at least 10 seconds.

Despite its simplicity and popularity, the practice is now in decline due to several factors. Burning is the major challenge, especially if fire escapes from the rows to burn the whole paddock or trees. Rows becoming wet after summer rains can create challenges waiting for the rows to dry out for the fire to burn hot enough and destroy weed seeds. Nutrient redistribution and ground cover loss are also key issues for growers using narrow windrow burning, particularly on lighter soil types.

Smoke in built up rural communities has been problematic for narrow windrow burning, where smoke lingers late into the evening when wind inversions occur. Some growers are actively looking at alternative options to narrow windrow burning, whilst for those where the process works it will remain a key tool in their harvest weed seed control tactic toolbox.

Glenvar Bale Direct

Chaff and straw are collected during harvest then baled directly using a baler attached to the harvester. There is a moderate level of groundcover removal with straw and chaff removed, whilst weed seed removal is high. A large capacity harvester is needed to operate the baler but does not slow the harvesting operation down. Growers would require access to markets to utilise the bales for bedding or as a feed source.

Chaff carts

The first harvest weed seed control tactic tool introduced from Canada for the collection of chaff material for feeding to sheep. A cart is towed by the header which collects chaff and weed seeds then dumps it in piles for grazing or burning. The original blower delivery system was improved with a conveyor belt elevator which allows some small straw into the chaff fraction. The increased oxygen levels in the chaff has resulted in a quicker, hotter burn. Burning of chaff piles has created similar issues to narrow windrow burning with chaff piles smouldering for long periods.

New research is proving the value of chaff dumps not only for weed seed reduction but also sheep feed. Chaff piles can be grazed by sheep directly or baled for sale into feedlots or other associated markets. Ed Riggall is a sheep consultant from WA who has found that sheep grazing chaff piles gained 3kg/head more over three weeks than those without chaff piles. This was despite the sheep taking one week to get used to the chaff piles. Chaff piles are reducing supplementary feeding costs and increasing scanning results while reducing weed seed numbers. Studies have shown that sheep do not spread weed seeds, with only 3-6% of seed remaining viable after passing through the rumen. Cattle are less effective at destroying ryegrass seed with 15-20% of the seed remaining viable.

Chaff lining

Developed by Esperance grain growers, chaff lining involves separation of the chaff and weed seed fraction from the straw residue, with chaff dropped into a narrow line behind the harvester via a chute attached to the main sieve. The chaff line remains on the soil surface where weed seeds are left to rot, while the straw travels through the rotor to be chopped and spread.

Chaff lining is repeated on the same runs year after year to allow weeds to continually rot in a defined area. There is limited research data to quantify the full impacts of seed rotting but observations to date indicate the undisturbed chaff row is a hostile environment for weed seeds. Growers don’t need to be on a full controlled traffic farming system but ideally the header needs to run on the same lines each year.

Chaff lining is low cost, involves no burning and growers have the option to graze chaff lines with similar feed values as that found with chaff carts. Chaff lines have been successfully grazed in stubble over summer but also in winter when sown to a dual-purpose grazing crop.

Harvester setup is critical to maximise weed seed capture with growers adding a separating baffle above the sieves to ensure chaff stays out of the straw and exits via the chute. Grain needs to be threshed hard to get weed seeds out of the head, with the grates of the harvester opened up to get as much material out of the rotor and onto the sieve for collection.

Growers have built their own chutes and baffles to suit a wide range of harvesters with 2017 being the first season many growers adopted the practice. There were several situations where chaff lining setups caused issues at harvest including a build-up of excess fine chaff on the air cleaner or blockages at the rear of the baffle in canola. Refinements to chaff lining are ongoing as growers work with each other and industry to achieve continuous improvement with the practice.

Chaff decks

The chaff deck system operates on a similar principle to chaff lining but the chaff material is directed onto dedicated wheel tracks in a control traffic farming (CTF) system. Known also as chaff tramlining and developed in the Esperance region of WA, weed seeds exit the harvester off the sieves in the chaff fraction whilst straw is chopped and spread with no loss of harvest efficiency. Weed seeds are exposed to the same rotting effects as in chaff lining but there half the material given the split across the two wheel tracks.

Dust generated when summer spraying is minimised due to the presence of the chaff on the tramlines. Conversely the weed seeds are exposed to a level of disturbance on tramlines which increases their potential to germinate as opposed to continually rotting. This contrasts with chaff lining where the single chaff row is not exposed to any wheel traffic and potentially optimises its rotting potential.

Chaff decks systems have opened new opportunities for alternative forms of weed control not previously thought possible. Weed seed collection has been so effective that very dense populations have emerged in defined rows on the tramlines in crop. Due to the nature of permanent control traffic farming tramlines, growers can use a range of alternative chemistry or cultural practices throughout the season and not affect the main crop. For example, in a 12m control traffic farming system only 8% of the paddock is dedicated to wheel traffic therefore weeds in the chaff lines can be targeted using non-chemical options such as microwave, baling or crimping as potential forms of site specific weed control.

Agronomy for chaff rows created by chaff decks and chaff lining is a key issue and growers need to be aware of some issues that need to be managed. These include:

• Sow through the chaff rows with either a disc or tyne, unsown rows become too weedy without any competition, increase sowing rate on these rows if practical
• Increase herbicide rates on the chaff rows using higher output nozzles for all passes including knockdown, pre-emergent, post emergent and crop topping
• Graze with sheep where available to help to reduce the bulk of chaff rows
• Monitor for pests such as mice, earwigs, millipedes and slaters which can breed up in chaff rows, especially when sowing canola and consider on-row baiting or insecticide.

Integrated Harrington Seed Destructor (iHSD)

Recognised as the ultimate form of harvest weed seed control tactic, the mill technology conceived by Ray Harrington is now reaching commercial reality for growers. The Integrated Harrington Seed Destructor (iHSD) comprises of two hydraulically driven cage mills that are mounted within the back of the harvester (just below the sieves). The mills can destroy 93-99% of the weed seeds and then spread the material back out on the paddock without any loss of stubble or nutrients. Suitable for fitting onto all class eight, nine and ten harvesters the mill has been tested to destroy 96% of annual ryegrass seeds, 99% of wild oat seeds, 99% of wild radish seeds and 98% of brome grass seeds in the chaff.

Seed Terminator

Developed by Nick Berry and his group in South Australia the Seed Terminator uses a multi stage hammer mill on weed seeds in the chaff fraction. The mill uses a combination of processes to shear, crush, grind and high impact to destroy over 90% of weed seeds. More research is under way to further quantify this weed seed kill. The mill is mechanically driven with three stages of screen to sort material for size and can be operated at dual speeds of 2800 and 2950 RPM.

Conclusion

Growers now have available a diverse range of harvest weed seed control tactic tactics at their disposal depending on their farming system, location and scale. The options are becoming less labour intensive with a shift away from burning of windrows towards chaff lining or mill technology which leave crop residues and nutrients in place. Although intensive croppers have previously been the major adopters of harvest weed seed control tactic, mixed farmers can also benefit through grazing chaff dumps or chaff lines while reducing weed seed banks.

HWSC is part of a broader weed management package that includes improved herbicide management as well as crop competition, diverse rotations, double knocking and crop topping or hay to stop seed set. The implementation of some or all these tactics will ensure growers keep weed seed banks low but more importantly, remain profitable.

Useful resources

Broster J, et al (2015) Harvest weed seed control: ryegrass levels in south-eastern Australia wheat crops. 17^th Australian Agronomy Conference.

Walsh M, et al (2017) High levels of adoption indicate that harvest weed seed control is now an established weed control practice in Australian cropping. Weed Science Journal of America 31, 341-347.

Harvest weed seed control tools – they all work

Chaff carts - good for the crop and the sheep

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Acknowledgements

Funding for AHRI is provided through GRDC. 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.

Contact details

Greg and Kirrily Condon
PO Box 73 Junee NSW 2663
Mb: 0428 477 348 (Greg)
Email: greg@grassrootsag.com.au
Email: kirrily@grassrootsag.com.au
Twitter: @grassrootsag