Pre-emergent herbicide, metolacholor and S-metolachlor, have been used in Australia primarily to control grass weeds for over 40 years. To date, no instances of resistance to this active ingredient have been documented in Australia and even internationally the few cases of resistance are in broadleaf weeds only. Resistance in northern summer grasses is probably low at present.
Syngenta’s field biology manager, Rob Battaglia, says some recent changes to the label use patterns and the launch of a new seed safener product will further expand the usefulness of S-metolachlor products, such as Dual Gold® and Primextra Gold®, in summer crops while protecting sorghum seedlings from herbicide damage.
“S-metolachlor is registered for use in a wide range of crops, including sorghum, maize, sweetcorn, soybean, sunflower and cotton, as well as in fallow situations,” he says. “The new use patterns in sorghum, cotton and fallow centre on extending application flexibility and residual activity of the herbicide on target weeds.”
In sorghum, a rate of 1.0 to 2.0 L/ha can be applied either as a single application before the crop or weeds emerge, or as a split application pre and post crop emergence (up to 6-leaf stage). Similarly, the full rate can be applied after harvest to establish the fallow or 1.0 to 1.5 L/ha can be applied pre-emergent to weeds at fallow establishment followed by 0.5 to 1.0 L/ha within 4 weeks. Be sure to observe replant intervals when planning the next crop.
In cotton, a single application of 1 L/ha either pre-emergent (before, at, or immediately after, planting), over-the-top or directed stand-alone spray or over-the-top mixed with Roundup Ready herbicide on Roundup Ready FLEX® cotton.
“There are several factors that affect the length of effective residual activity of S-metolachlor, some of which are outside the grower’s control,” says Rob. “The product needs to be incorporated and has interactions with rainfall, temperature, soil type, soil organic matter and stubble. Having more flexibility in the application timing gives growers the ability to compensate for some of these factors and control new waves of weed germinations for longer.”
S-metolachlor is only active on weeds that have not yet emerged and has no effect on weeds that have already emerged from the soil or that do not come in contact with the herbicide as they emerge from the soil. It is essential to control already-emerged weeds first and then apply the pre-emergent to control subsequent flushes of weed germinations.
“There are some situations where the residual herbicides will appear to be less effective than they should, but this can often be attributed to compromised application timing, limited rainfall and soil conditions, rather than herbicide performance,” says Rob. “The difficultly in achieving perfect application timing and incorporation with rainfall is one of the main reasons why the new use patterns for Dual Gold® have been registered. Pre-emergent herbicides are best used within the WeedSmart Big 6 integrated weed management program and not as a stand-alone control measure.”
While other crops listed on the S-metolachlor label can metabolise the herbicide, sorghum requires a seed safener to protect the germinating plants from the herbicidal effects of S-metolachlor. The new Epivio C® seed safener replaces Concept II® and provides better overall protection for sorghum seed.
In Syngenta’s product development trials in Queensland and NSW, they measured an 11 per cent increase in plant stand and a 7 per cent increase in sorghum yield in crops where the seed was treated with Epivio C® compared to Concept II®. Epivio C® has also demonstrated improvements in seed safety and shelf-life for carry-over seed.
“Epivio C® is applied as seed treatment and when the seed is planted into moist soil the product is taken up into the plant and improves the metabolism of the herbicide in the seedling, resulting in no crop symptoms,” says Rob. “The result is better crop establishment and healthier plants that can better compete against weeds that germinate later in the season.”
There is evidence that growers can maintain crop yield and reduce summer grass seed production by planting sorghum crops at a density of 10 plants/m2 and a row spacing of 50 cm.
The WeedSmart Big 6 includes diversity in crops and pastures, crop competition and mixing and rotating herbicide mode of action groups.
For more information about pre-emergent herbicide use in sorghum crops, please visit the website: www.weedsmart.org.au
Attribution: Cindy Benjamin, WeedSmart
WeedSmart’ is the industry voice delivering science-backed weed control solutions to enhance on-farm practices and promote the long term, sustainable use of herbicides in Australian agriculture.
WeedSmart has support from the Grains Research and Development Corporation (GRDC), major herbicide, machinery and seed companies, and university and government research partners, all of whom have a stake in sustainable farming systems.
The GRDC is a Platinum investor in WeedSmart to ensure Australian grain growers have access to world class research in strategies to mitigate weeds and control herbicide resistance.
