Cart crush or cremate weed seeds to manage resistance
GroundCover™ Supplement Issue: 104 | 06 May 2013 | Author: Dr Michael Walsh and Peter Newman
If herbicides were the complete answer to weed control, we would have eradicated annual ryegrass and wild radish from Australian cropping systems long ago.
Instead, herbicide-resistant populations of ryegrass and wild radish are on the rise across Australia, with Western Australia leading the other states with the most resistant populations.
As herbicides are central to the success of Australian cropping systems, herbicide-resistant weeds represent an enormous threat to farm viability.
Despite this grim outlook, the good news is that Australia’s growing herbicide-resistance problem has stimulated an innovative and integrated approach to weed management, which is now paying dividends in growers’ paddocks across southern Australia. After adopting the new approaches many continuous croppers say they now have fewer weeds than ever before.
Exploit the weakness
Central to the integrated weed management approach has been a better understanding of weed biology and how to target vulnerable aspects of this to drive down weed seedbanks.
A major biological weakness of most southern Australian cropping weeds is that their seed does not shatter before harvest, providing the potential to remove the weed seed from cropping systems at harvest.
Research by the Australian Herbicide Resistance Initiative (AHRI) found that between 70 and 98 per cent of weed seeds remained on various weed species at harvest.
In the past, these retained weed seeds were harvested and then spread back across the paddock with the chaff fraction. However, new approaches now focus on removing the weed seeds from the cropping paddock. This is achieved by either removing the weed-laden chaff via baling, concentrating the chaff fraction into narrow windrows for burning, or pulverising the chaff fraction to crush and destroy the weed seeds before they exit the harvester.
The benefits of these harvest weed-seed control systems is that they remove both resistant and susceptible weeds that have survived earlier herbicide applications, slowing herbicide-resistance evolution and reducing the pressure on subsequent herbicide use.
Another weakness of southern Australian weeds is that their seed does not remain viable in the soil for very long and seedbanks decline rapidly if not replenished with annual seed production. Consequently, harvest weed-seed control methods can lower a very large seedbank of more than 1000 seeds per square metre to 100 seeds/m2 in only four years.
The harvest weed seed control systems have brought fresh optimism to growers who have long struggled with herbicide-resistant weeds. Many continuous croppers are now able to sow early with the confidence they have a low weed seedbank burden, and others have not had to use in-crop herbicides in some seasons.
AHRI research compared the effectiveness of the harvest weed-seed control methods – chaff carts, baling, narrow windrow burning and the Harrington Seed Destructor.
Carried out over 25 sites across four states and over two harvests, the researchers found that each of the harvest weed seed systems was equally as effective at removing ryegrass seed from cropping systems. Averaged across the 25 sites, each of the weed-seed harvest methods removed about 55 per cent of annual ryegrass seed.
The system chosen will therefore come down to personal preference and how best it fits within a particular farming system.
There has been a significant resurgence in the use of chaff carts in WA, with the addition of a conveyor belt to deliver the chaff fraction from the harvester to the cart making the system more user-friendly. The conveyor belt adaptation creates chaff dumps that burn out in a shorter period of time, thus reducing the risk of fire escapes.
An alternative to the in situ burning or grazing of chaff is to bale all chaff and straw material as it exits the harvester. Initially developed as a way to improve straw hay production, the Glenvar Bale Direct System developed by Graham Shields, a grower from Wongan Hills, WA, consists of a large square baler directly attached to the harvester, which collects and bales all harvest residues.
AHRI research shows that 95 per cent of annual ryegrass seed entering the harvester was collected in the bales. In addition to removing weeds, the bales can be sold or used as a feed source. However, as with all baling systems, the cost of removing nutrients from the cropping system needs to be considered.
Narrow windrow burning
A widely adopted alternative to chaff carts has been to funnel the harvested straw and chaff fraction into narrow windrows for subsequent burning in autumn. This system requires only a simple modification to the harvester and is cheap and easy to use. An estimated 50 per cent of WA growers have adopted the windrow system to control herbicide-resistant weeds.
According to AHRI and the Department of Agriculture and Food, WA (DAFWA), burning narrow windrows consistently destroys more weed seed than burning standing stubble. This is due to the higher-temperature burn achieved in the narrow rows. Concentrating the stubble into a narrow windrow and burning in a light wind increases the intensity of the burn and allows higher temperatures to be reached for longer, which is crucial for destroying weed seeds, especially wild radish.
The researchers also calculated that less than 10 per cent of a paddock is exposed to erosion when burning narrow windrows rather than the entire paddock.
Harrington Seed Destructor
Developed by WA grower and inventor Ray Harrington, the Harrington Seed Destructor (HSD) is a unique weed-seed control system that smashes the chaff and weed-seed fraction as it exits the harvester, destroying seed viability and returning the crushed fraction to the paddock. Unlike other weed-seed harvest systems, there is no need for autumn burning and chaff and weed nutrients remain in the paddock.
AHRI research has shown that the HSD consistently destroys 95 per cent of annual ryegrass, wild radish, wild oats and brome grass seed present in the chaff fraction.
Combined with a pre-emergent herbicide, DAFWA research has shown harvest weed-seed systems to be highly effective at removing the final few weeds in cropping paddocks (Figure 1).
The research has monitored the impact of pre-emergent herbicides with and without narrow windrow burning and chaff carts on the ryegrass populations of 31 northern WA cropping paddocks over the past 13 years.
Despite starting with a larger seedbank, growers who implemented regular harvest weed-seed control in the form of narrow windrow burning or chaff carts eroded their ryegrass population to very low levels in four years. By year eight, these growers had fully depleted ryegrass from their focus paddocks and since then have averaged fewer than 1.5 ryegrass plants/m2.
While the herbicide-only growers have also been very successful at eroding the ryegrass seedbank, such a heavy reliance on herbicides is likely to result in higher levels of herbicide resistance in these paddocks, making the few remaining plants expensive to contain.
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