- Weed numbers are dynamic across a paddock. Any intervention to decrease weed seed set is not going to deplete the seed bank within a season.
- Narrow windrow burning and harvest weed seed destructor’s are viable options when presented with the right seasonal conditions.
- Weed management should be approached as a whole farm system and control should not be relied on by any single practice.
Stubble, weeds, IWM, burning, harvest
The burden of herbicide resistant weeds is an increasing problem for growers in the Victorian western district. For a region that is relatively new to the cropping game, the rise of herbicide resistance has been rapid in comparison to other cropping areas. The long growing season in the high rainfall zone generates an array of challenges, including a larger window for weed germination, suboptimal spraying conditions, and stubble loads that restrict the use of Integrated Weed Management techniques that have been effective in drier cropping zones.
Common district practice embraces the integration of livestock on stubbles over summer, with the aims of reducing stubble load, grazing weeds and utilising lost grain from harvest. These attempts range in effectiveness, often resulting in a paddock being burnt in autumn because the producer is looking for greater ease of sowing and improved pre-emergent herbicide efficacy. There is also the broad consensus that harvest weed seed capture techniques do not work in the high rainfall zone due to the heavy stubble loads and a tendency for the grass weeds to lodge before harvest. Southern Farming Systems set out in the 2014 harvest to further investigate the value of these management tools in the fight against weeds and address the large research gap around the topic.
Aim The trial was undertaken to determine the effectiveness of harvest weed seed capture techniques in high yielding paddocks in the Victorian high rainfall zone.
Trial details The paddock scale trial was undertaken in a wheat crop at Mininera, Victoria, with a mean annual rainfall of 526 mm. The paddock would typically record cereal yields exceeding 6 t/ha, however with the driest spring on record a 3 t/ha grain yield was achieved and accompanied by a 5.36 t/ha stubble load. The trial was comprised of three treatments including a conventional burn, windrow burn, and a harvester integrated weed seed mill. The treatments were aimed at control of annual ryegrass (Lolium rigidum) and great brome (Bromus diandrus).The paddock was harvested on 12 December 2014 with a Case IH Axial Flow 9120 at 15 cm above the ground. Weed counts after sowing were completed on 16 June 2015.
Results and Interpretation There was an observable contrast amongst the treatments but no significant difference in weed populations was detected. The windrow burn treatment appeared to have greater effect on weed reduction however the variability of the trial area produced a high coefficient of variance which rendered the data insignificant.
The statistics in Table 1 do not convey the full story about getting weed seeds into the harvester. The effectiveness of any of the treatments is directly influenced by the capacity of the machinery to consume the reproductive body of the target weed. Pre harvest cuts done across the trial revealed that as much as 35% of annual ryegrass and 34% of great brome seeds are located between 0-15cm above ground level. This begs the question why don’t we harvest lower? For a farmer in the high rainfall zone the proposition of harvesting lower then 15cm in a wheat crop would mean a substantial compromise in harvest efficiency.
The benefits of capturing weed seed at harvest has been established as a valid substitute to conventional practice in the western district. The drawbacks of this research is that it doesn’t define the full potential of these harvest weed seed capture. Unless is it paired with cumulative years of the treatment with a focus on weed reduction we cannot conclusively know the advantages of these techniques in high yielding crops. The fundamental advantage of harvest weed seed capture is increasing the ability to retain stubble in these systems. Any benefit to weed reduction is a bonus when we recognise the long term numbers game that it is.
One method alone is not going to be a silver bullet when it comes to the battle against weeds. The use of harvest weed seed capture when paired with other integrated weed management tactics is profitable option in the high rainfall zone.
By Aaron Vague
Research and extension officer
Southern Farming Systems
This research is being conducted by SFS as part of the GRDC Maintaining profitable farming systems with retained stubble initiative (project BWD00024 ‘Maintaining profitable farming systems with retained stubble in Victoria and Tasmania’).