Putting in practice integrated weed management (IWM)
Author: Chris Davey (YP AG, Kadina) | Date: 12 Feb 2019
Take home messages
- Try to add as many ‘one percenters’ into your farming system as possible to reduce annual ryegrass setting seed into your soil weed seed bank.
- Implement both sowing and harvesting integrated weed management (IWM) strategies.
- There is a fit for east-west (E/W) sowing, but the yield response is greater for wheat compared to barley.
- At sowing, the choice of pre-emergent herbicide(s) should be made following the decisions regarding crop type, variety and sowing rate and orientation.
- Pre-emergent herbicide stacking provides the most effective means of chemical control for annual ryegrass.
Annual ryegrass (ARG) continues to present a major hurdle to agricultural production, as it continues to evolve and adapt to our farming systems and thereby evading our methods of weed control.
On the Yorke Peninsula (YP) of South Australia (SA), growers’ ARG weed control focus has switched from post-emergent control with herbicides to the use of pre-emergent herbicides. With the advent of resistance to common pre-emergent herbicides interest has grown in non-chemical weed management practices at both sowing and harvest.
A multi-pronged approach to reducing the number of ARG seeds entering the weed seed bank will consequently improve the management of ARG into the future.
Results and discussion
Chemical control of annual ryegrass
In the past couple of years, Group J and Group K resistant ARG has been found (Brunton et al. 2018). As these herbicides are commonly used in our farming systems, one of our main weapons of control is under threat.
A YP AG trial south of Paskeville has observed an ARG population that has been treated with triallate (according to label recommendations) evolve from low resistance to highly resistant within a single year. After resistance testing in 2017, the ARG population was measured to be 50% resistant and moved to a 100% resistant population when tested for resistance in 2018.
In this trial however, premium, stacked pre-emergent herbicide mixes gave better ARG control than conventional mixes and untreated paddocks. Stacking herbicides refers to the practise whereby more than one pre-emergent herbicide is mixed and applied to the paddock at seeding, (within label recommendations) rather than relying heavily on a sole pre-emergent herbicide.
Despite employing practices such as herbicide stacking, ARG resistance is still increasing (unpublished YP AG data). Therefore, in my opinion, the choice of pre-emergent herbicides should be made after pre-sowing decisions regarding non-chemical control practices such as crop type, variety and sowing rate and orientation. In this way, we are not relying as heavily on herbicides, and thereby, placing less pressure on the development of ARG resistance.
Non-chemical control practices for annual ryegrass
The most common non-chemical practices for control of ARG in use on the YP of SA involve crop competition at sowing and harvest weed seed control (HWSC) at harvest.
Compared to the use of pre-emergent herbicides, crop competition provides an easier, and cheaper way of reducing the seed set of ARG.
Crop competition tactics include the choice of crop type, variety, sowing rate and the orientation of sowing. Each of these have an impact on the effectiveness of the crop to compete with ARG, and therefore, how much seed ARG can potentially produce.
Harvest weed seed control
Farmers on the northern YP have adopted HWSC strategies rapidly over the past couple of years.
When weighing up what HWSC strategy to implement, the main considerations are cost, nutrient removal through burning, maintenance and convenience/logistics. This has been well documented by Planfarm, AHRI and WeedSmart.
On the YP there has been a swing from chaff carts and narrow windrow burning to chaff lining and chaff decks (chaff tramlining). Some Harrington Seed Destructors (HSD) and Seed Terminators are also present on the YP.
The key point to note with any HWSC strategy is that regardless of what approach you use, the strategy only deals with the weed seed that enters the front of the header and not those seeds that escape the header front.
Due to differing weed species attributes, some weeds are more suited to HWSC than others. A Northern Sustainable Soils grower group trial that ran in 2011 with a focus on brome grass showed that the number of weed seeds captured by the header may be as low as 52% of total weed seed produced for brome grass. This number is generally regarded to be higher for ARG and higher again (up to 80-85%) for wild radish, given its standability at harvest.
Annual ryegrass continues to evolve resistance to more mode of action groups.
As agronomists and growers, we must try to keep one step ahead of it to prevent resistance taking hold and reducing our viable control options.
With Group J and Group K resistance starting to surface, stacking pre-emergent herbicides together will ensure that the strongest possible herbicide option is applied prior to seeding. There are very few post-emergent herbicide options left that work on ARG, so currently we have a heavy reliance on the pre-emergent herbicides in our battle against ARG.
Growing a competitive barley variety instead of wheat, increasing the sowing rate, decreasing the row spacing and sowing E/W will all help contribute to crop competition with ARG.
Crop competition strategies will help ensure that there are very few ARG plants that make it through to harvest, and those that do, will be spindly and uncompetitive.
Adopting a form of HWSC will provide another option for controlling weed escapes.
Utilising all of these ‘one percenters’ will contribute to a sustainable and profitable future for our farming systems.
Brunton DJ, Boutsalis P, Gill G, Preston C (2018). Resistance to multiple PRE herbicides in a field-evolved rigid ryegrass (Lolium rigidum) population. Weed Sci. 66:581-585.
The author would like to thank the following people involved in this study: David Brunton, University of Adelaide; Northern Sustainable Soils (NSS) farmer group; Peter Newman, AHRI; WeedSmart and YP AG.
Was this page helpful?