WeedSmart Big 6 on Mid North, Yorke Peninsula and Lower Eyre Peninsula farms
March 27, 2023
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Advice from Chris Davey, WeedSmart southern extension agronomist (SA & Vic)
Annual rainfall: 350–500 mm
This mid-rainfall zone of SA is typified by two distinct soil types. On acidic soil types, found mainly in the Mid North and lower Eyre Peninsula, canola is the main break crop. On alkaline soils through the Yorke Peninsula, Lower North and Adelaide Plains, pulses are the main break crops. The main pulses grown are lentils and field peas, with smaller areas of chickpeas, faba beans and vetch.
Weeds and herbicide resistance status:
There is widespread herbicide resistance in annual ryegrass and wild radish across the region and in brome grass on alkaline soils. Resistance in milk thistle, Indian hedge mustard and prickly lettuce is increasingly problematic. Annual ryegrass and brome grass are known to have resistance to herbicides in Groups 1[A] (fop and dim), 2[B], 3[D] and 9[M], and wild radish is resistant to Groups 2[B], 12[F] and 4[I].
In 2011, the first confirmed case of glyphosate-resistant brome grass in Australia was from the Yorke Peninsula.
WeedSmart Big 6 for integrated weed control programs on farms in the Mid-North, YP and Lower EP region.
Crop and pasture rotation
The close wheat – break crop rotation that has dominated farming systems in the region in recent years is a weak link for weed control. The economic drivers for this short rotation are strong, but other profitable rotation options can assist in reducing weed pressure. The main problem with the short rotation is that weeds are exposed to the same herbicide modes of action every two years.
In canola rotations, this has led to ryegrass populations evolving resistance to multiple herbicides, often called ‘alphabet resistance’.
Although Roundup Ready canola has been very useful in reducing annual ryegrass numbers, many growers have been reluctant to switch from wheat to barley due to the lower gross margins. The wheat phase is the weak link in the rotation due to its inability to compete with the ryegrass.
Including barley would give growers more flexibility and options for weed control across their rotation.
In pulse rotations, the short rotation has increased broadleaf weeds such as milk thistle and prickly lettuce. Bifora, tares and medic can explode in the lentil phase in some years. High weed numbers result in a high weed seed bank for the following year and increase the risk of herbicide resistance evolution.
Weeds such as bifora, medic, snail medic and tares have long dormancy periods, so a blow-out in these weed species results in long-term problems in affected paddocks.
Although imi-tolerant (Clearfield) pulse varieties have been beneficial in allowing the use of Group 2 [B] herbicides in the crop or previous seasons, the alkaline soils on the Yorke Peninsula have expedited the more rapid evolution of Group 2 [B] resistance in wild radish, mustard, milk thistle and ryegrass.
Including more canola and barley into the rotations gives growers more flexibility and options for weed control across a rotation. Oaten hay was a former strong option on the Yorke Peninsula, but with the demise of the export hay market in the area, the percentage of paddocks sown to oaten hay has diminished. A hay company located on the Eyre Peninsula makes hay an option on the Lower Eyre Peninsula, while silage can be an option also in the Mid North.
Double knock to protect glyphosate
Glyphosate-resistant ryegrass is widespread in the region, primarily along fence lines. Removing fences to form larger paddocks increases the risk that harvesters will spread the resistance gene.
An annual double-knock application before seeding is considered very important to help protect the efficacy of glyphosate and is widely practised in the region when the season permits. Sowing earlier to achieve a yield advantage and dry sowing can impact the use of the double-knock. Under dry, dusty conditions most growers will choose two contact herbicides, such as paraquat or paraquat / diquat, rather than glyphosate / paraquat for the double-knock.
There is an increasing usage of Group 14 [G] knockdown herbicides, and other herbicides that may be synergistic with the paraquat component of the double-knock for weed control.
Observations on the eastern Eyre Peninsula after the considerable rain in January 2022 were that annual ryegrass had started to germinate earlier than usual. Early germination led to further exposure of ryegrass to glyphosate from the summer weed herbicide mixes, which added large quantities of glyphosate-resistant annual ryegrass seed to the seedbank.
Mix and rotate herbicide groups
There is a heavy reliance on pre-emergent herbicides in the region, and in weedy paddocks growers need to use additional shots to drive down weed numbers to preserve yield. In cereals, applications of pre-emergent herbicides were often spiked with triallate to strengthen the pre-emergent efficacy because there were no in-crop herbicide options in wheat and barley crops. The registration of Mateno Complete in 2022 provides growers with an early post-emergence herbicide option in wheat, with further registration in barley occurring in 2023.
The new release pre-emergent herbicides, Luximax, Overwatch and Mateno Complete, makes it easier to rotate chemical mode of action groups.
Pre-emergent herbicides are very important in lentils as the main break crop to reduce ryegrass numbers and reduce pressure on the clethodim / Factor mix in-crop. In canola, propyzamide reduces the pressure on the clethodim and/or glyphosate mix in-crop to control annual ryegrass. Overwatch can also be used in canola, opening up another avenue to rotate herbicide groups for grass and broadleaf weed control.
The recent release of Ultro (Group 23 [E]) and Reflex (Group 14 [G]) adds another chemical group to a rotation that includes pulses. Ultro can be rotated with propyzamide for ryegrass control in the pulse phase and provides effective control of barley grass and brome grass. Ultro is a particularly valuable tool for controlling brome grass resistant to other herbicide modes of action.
Trifluralin susceptibility in ryegrass has been very low since the late 1990s. It is no longer a tank mix option unless targeting broadleaf weeds like wireweed or three corner jack, or suppression of brome and barley grasses.
Stop weed seed set
Growers in the region generally use their late fungicide application in August or September to scout for weed escapes in-crop. Taking a nil tolerance approach, growers might hand-pull small areas or spot-spray individual plants or small weedy patches.
Using paraquat or paraquat / diquat, growers can avoid using glyphosate on potentially resistant individuals when chemically fallowing areas of their crop. The permit for Weedmaster DST use to crop-top barley, wheat and canola provides a useful control tactic for radish and ryegrass at the end of the season but is often too late for brome grass, which has usually already set seed by this stage of the crop.
Windrowing canola allows weed seed heads to be cut earlier than usual and places them into a row for harvest weed seed management. Windrowing is also very useful in RR canola, where using a registered glyphosate does not result in uniform crop maturity.
In blow-out situations, consider taking the ‘short-term pain for long-term gain’ of a chemical fallow using paraquat or Spray.Seed®. Cutting cereal crops for hay has also proven useful to stop seed set in resistant annual ryegrass.
Chemical fallow is the best option if weed numbers are building up in a paddock. The performance of the following crop usually makes up for the one-year sacrifice due to increased nutrients and moisture availability. Some growers plan to include a small portion of the rotation to be sown as a chemical fallow, while others use chemical fallow only in a failed crop or for a weedy section of a paddock.
Where the soil type allows, growers in the region have readily adopted east-west row orientation. This strategy may be difficult or impossible on lighter, sandier soil types, dune swale landscapes and farms with controlled traffic farming (CTF) already in place.
Barley is the most competitive crop option for the region. Growers usually consider choosing the most competitive cultivars available coupled with high sowing rates and narrower row spacing (17–17.5 cm), especially using disc systems.
When choosing a canola variety, hybrids are preferred for their better seedling vigour, providing early competition for weeds. Herbicide tolerance traits give growers additional options for weed control in canola crops.
Soil amelioration and improved management of non-wetting sands, acidity and compaction can lead to healthier crops that compete against weeds and generate more profit for growers.
Harvest Weed Seed Control (HWSC)
In response to increasing herbicide resistance, many growers in the region have adopted some form of harvest weed seed control within their weed management program. Chaff-lining or chaff-tramlining are the most common methods used. There is also rapid adoption of seed impact mills, which are particularly effective for weed management in lentil crops.
Brome grass can be difficult to control using HWSC as its flexible stem doesn’t always get cut and can flick back up once the harvester has passed, effectively evading the first stage of the process. However, even with a 50 to 60 per cent capture of brome grass, depending on the season and when harvest occurs, HSWC is still an important part of any weed management program.
In some years ryegrass will lodge and the harvester will not picked up the seedheads, while in other years, ryegrass will stand up well, and the harvester will capture 80 per cent of the seedheads.
Wild radish generally stands erect in the crop canopy, allowing growers to capture a high proportion of the viable seed at harvest with their header.
Once weed seed is in the header, it doesn’t matter what HWSC method is used – all are effective. The main consideration is for growers to adopt practices that aid in optimising the amount of weed seed coming into the front of the header.