No water, just weeds? Managing weeds in irrigation channels
August 16, 2019
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Are your channels a source of weeds on your farm?
Irrigation channels require regular monitoring as they are often a source for new weed incursions. Weed seeds can enter farms from irrigation water and any new weeds emerging need to be removed prior to seed set. Retaining irrigation water on farm can help to limit any potential spread.
Weeds in irrigation channels are an important source of weeds.
What do we know?
Channels are a source of seeds that can move into cotton fields adding to the weed seed bank.
Weeds in channels are a potential host for disease and insect pests.
They can interrupt water delivery reducing irrigation efficiency.
Large weed infestations can provide habitat for feral pests.
Larger weeds can also undermine the integrity and stability of the channel bank.
What are the options?
In addition to knockdown and residual herbicides always include mechanical removal as part of an integrated approach to weed management.
Valor (Group G) herbicide contains flumioxazin and is registered for knockdown and residual control of broadleaf and grass weeds on irrigation channel banks. It has low volatility and binds strongly to the soil so the risk of movement is very low. Valor binds tightly to the soil after 25mm of rain, if that doesn’t occur within 3 weeks after application the channel will require flushing and the waste water retained on farm. It is not degraded by solar radiation even if exposed for weeks prior to water or rainfall.
Pendimethalin (Group D) is a herbicide which provides residual control predominantly of grass weed species. Pendimethalin is very tightly bound to soil particles and has very low solubility; meaning it stays where it is applied. Pendimethalin should be applied to the bank after grading or reshaping and if rainfall does not occur for 14 days the channel should be filled with water and then waste retained for pre-irrigation of cotton fields.
Diuron (Group C) is a herbicide which provides residual control of a range of grass and broadleaf weeds. Diuron has limited mobility binding tightly to the soil, extremely low volatility and has a relatively low solubility rating. Application needs to be onto moist bare soil and prior to rainfall to fix the herbicide to the soil. If rainfall does not occur fill the channel, let stand for 72 hours and then drain the water into waste.
Glyphosate (Group M) is a knockdown herbicide with activity on a large number of weeds common to irrigation channels. The overuse of glyphosate for weed control in non-field areas is a contributing factor to the development of herbicide resistance; thus its use around channels, road sides and non-field areas should be restricted. Any survivors from glyphosate application need to be removed to prevent weed seeds topping up the soil seedbank with a potentially resistant population.
Amitrole-T (Group Q) is a knockdown herbicide that controls a range of seedling grasses and young broadleaf weeds. Using Amitrole-T as a substitute for glyphosate will help to reduce the likelihood of glyphosate resistance developing.
The importance of pre-emergent and residual herbicides
Getting the best out of any herbicide is important, none more so than glyphosate.
By incorporating pre-emergent herbicides into our planting program we reduce the emergence of weeds in crop allowing growers to target a small population of weeds reducing the risk of herbicide resistance developing. Running the weed seed bank down and targeting small weed numbers is a robust strategy for ensuring complete weed control. It’s a numbers game, low weed numbers means there is less likelihood of resistant individuals within the population, and large weed numbers increases the risk of resistant individuals present in the population.
Demonstration trials at Wee Waa and Whitton in 2018-19 highlighted this clearly. In addition to the risk of resistance from spraying large weed numbers the yield penalties from these early weedy treatments were significant. A 3.5b/ha difference between the weedy control and those treatments with pendithethalin or diuron reinforces the clear message of diverse weed control options.
Learn more in this short video from CottonInfo’s weed tech lead Eric Koetz: