Residuals which don’t lock up your rotation

Logistics, labour and suitable spray conditions don’t always align.

Fast-growing fallow weeds which recover from, or emerge after, a knockdown treatment can often set seed before the next knockdown treatment is applied.

Tony Lockrey, senior agronomist with AMPS Moree, says the use of residual (pre-emergent) herbicides in the weed control program can provide broad-spectrum weed control and reduce weed seedbank replenishment.

“Many growers introduced imazapic to their herbicide programs when glyphosate-resistant grasses, such as feathertop rhodes grass and barnyard grass, began to proliferate around 2010,” he says.

“This Group 2 herbicide provided effective control of glyphosate-resistant grasses as well as broadleaf weeds, but it also locked growers into fixed rotations to avoid the risk of crop damage in situations where the herbicide did not degrade sufficiently before the next crop was planted.”

At the time, the Northern Grower Alliance (NGA) conducted extensive trials using a range of residual products to identify which products best controlled and suppressed each weed, and when it was safe to plant various crops back into the soils after residual herbicides were applied.

Using this research and information available on the solubility and binding characteristics of the various products, AMPS and other northern region agronomists began to tailor residual herbicide recommendations to manage a particular set of weeds within a desired crop rotation, which were also flexible enough to adapt to variable seasons by having a second or third crop option pencilled in for each paddock.

“What is really important is growers and agronomists gain a deep understanding of how residual herbicides work, their limitations and their benefits,” Tony says.

“There are so many scenarios, and these products fit very well in the WeedSmart Big 6 ‘mix and rotate’ and ‘spray effectively’ suite of tactics,” he adds.

“Done right, they have a lot to offer growers contending with glyphosate-resistant weeds in zero-till systems, particularly.”

How to integrate residuals into the spray program

Immediately post-harvest is the best time to apply residuals, so the benefits of reduced weed emergence are capitalised on over summer.

However, sometimes the rotation plan isn’t clear until some rain has fallen, beginning to refill the profile for the next crop.

Fortunately, residuals can be applied at varying times, and sometimes to great effect, leading into a winter crop or winter fallow, provided the plant back restrictions are observed.

Residuals can be applied on bare stubble post-harvest, as a blanket spray in combination with either a knockdown or double-knock spray across the whole paddock or to areas within a field where the soil type or weed pressure calls for additional attention.

Another option is to apply residual products through an optical spot sprayer to suppress the emergence of seedlings around individual plants or known weedy patches.

When formulating a residual herbicide plan, study the labels and factor in your experience of local soils and climate risks, then generate a replant guide of the available options, and how the rotation could work for summer and winter crops following the residual application. Factor in the crop systems, soil, organic matter, pH, leaching fraction, rainfall, planned crops, sowing depth and coleoptile length.

Each grower also has a unique attitude to risk and varying operational and logistical constraints to consider.

To see the best results, it is critical to understand the weed spectrum, the product’s characteristics and how it interacts with the soil type and crop residue in the paddock.

Key resources to use when planning a herbicide program include the product labels, the mode of action resistance risk chart from CropLife and GRDC’s ‘Soil behaviour of pre-emergent herbicides in Australian farming systems’ publication. No single residual product is likely to control all the weeds present, so a combination of two or even three products may be necessary.

Achieving the best return on investment from late fallow residual herbicides

The main advantages of residual herbicides in the fallow are their ability to complement knockdown herbicides and suppress future germination flushes.

If fewer weeds are present at the end of the fallow, sowing a competitive crop and providing effective early weed control reduces pressure on in-crop weed control tactics.

Moving to a double-knock with a systemic product first, followed by a systemic plus residual, provides high-level control, giving crops early protection from weed pressure.

A Group 14 spike (e.g. Sharpen or Voraxor, at the higher rate) improves paraquat knockdown control.

Another strategy is adding isoxaflutole (Group 27, e.g. Balance) or a triazine (Group 5, e.g. Terbyne) to improve paraquat knockdown and provide ongoing soil-residual activity, particularly for broadleaf weeds.

A number of pre-emergent herbicides (e.g. Sakura) have been shown to provide excellent control of feathertop Rhodes grass for at least 14 weeks, allowing crops to establish and suppress in-crop weed germinations. Suppression of feathertop Rhodes grass germination can continue into the summer fallow after the wheat harvest.

For glyphosate-resistant grasses, a split application of S-metolachlor (Group 15, e.g. Dual Gold) is a good residual option in fallow, or in-crop where applicable, with no known resistance worldwide.

As with all weed control tactics, there is no one-size-fits-all pre-emergent herbicide product or use pattern.

Working out where a pre-emergent product will fit in the crop rotation is essential to avoid plant-back issues that might restrict future crop choices.

Microbial breakdown – be aware

The breakdown of pre-emergent herbicide molecules in the environment is driven by time, exposure to the elements (UV and moisture), and how the product binds to the soil (influenced by soil pH, cation exchange capacity and organic matter content).

These give rise to the plant-back caution on the herbicide labels that indicate the amount of time and rainfall that must occur before it is safe to plant specified crops.

A fourth factor – soil microbial activity – is much more site-specific and harder to determine.

Although microbial breakdown is well understood in control situations, it is unpredictable in field settings.

It is thought repeated use of a product could favour the build-up of microbial populations which break down that molecule, resulting in a much shorter-than-expected residual efficacy. This is another reason to focus on herbicide product diversity.

WeedSmart provides a practical planning framework and farming systems approach to integrated weed management to keep weed numbers low, maximise crop yield and improve long-term farm profitability.

How to ask a WeedSmart question

Ask your questions about using the WeedSmart Big 6 to develop a weed management plan on social media using @WeedSmartAU (Twitter, Instagram and Facebook).