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.
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.
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.
Harvest weed seed control (HWSC) practices
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.
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.
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.
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.
The key to HWSC is that the weeds must retain their seeds long enough, without shedding, to be captured by the combine. Seed heads must also be high enough on the plant to be harvested.
The objectives of the AAFC research were to evaluate weed seed control with the Redekop SCU, optimize blade configurations between all fan blades and a cutting blade/fan blade combination, and determine if weed seed control levels remain high at higher chaff feeding rates.
Redekop’s SCU is currently available for John Deere S Series combines. For 2021, the SCU can be added on to factory orders of John Deere combines.
Integrated impact mills were pioneered in Australia, where herbicide-resistant weeds, especially resistant annual ryegrass, have become very difficult to control. The idea is to prevent weed seeds from entering the seedbank at harvest.
In 2001 in Australia, a long-term study of HWSC on annual ryegrass populations in 25 commercial fields was initiated. The fields in the northern region of the Western Australian wheat belt were in continuous cropping rotations. HWSC was used every third year in 12 fields and compared to 13 fields with HWSC once every 10 years for 17 years.
Recent research by Breanne Tidemann, weed scientist at Agriculture and Agri-Food Canada in Lacombe, Alta., investigated a new Harvest Weed Seed Control (HWSC) tool from Redekop Manufacturing in Saskatoon. The Redekop Seed Control Unit (SCU) is an impact mill that incorporates a blade system in the centre of the mill with the goal to increase suction into the mill and airflow through it.
Volunteer canola was chosen for its high viability, limited primary dormancy and rapid germination, making it an ideal study species for HWSC. It is also the fourth most abundant weed in annual field crops on the Canadian Prairies. Canola also had similar destruction rates as other weed species when previously tested by other impact mills.