Crop rotation will play an increasingly important role in weed control as growers grapple with the rising cost of inputs such as fertiliser and fuel in the 2022 season, according to New South Wales agronomist Greg Condon.

Speaking at a recent GRDC Grains Research Update on ‘new pre-emergent options and their fit in the farming system’, Mr Condon said growers had more choice than ever with herbicides, but “through the roof” input costs this year meant difficult decisions might need to be made about controlling weed seedbanks.

“In our part of the world (central and southern NSW), crop rotation is playing a bigger role than ever. For example, we can’t risk putting canola into high-pressure ryegrass blocks, so there’s a strategic perspective needed in terms of looking at paddocks in isolation,” he said.

“We’ve got some pretty exciting chemistry to choose from. The important thing we have been talking about is choosing a product that is going to be a winner financially and selecting where you are going to use it. The key is knowing which resistant weed population you are dealing with and picking the winner (product) for the paddock.”

Mr Condon, from Grassroots Agronomy and WeedSmart, was updating growers on the performance of seven new pre-emergent herbicide products in NSW in 2021.

The products were:

• Callisto, Syngenta
• Mateno Complete, Bayer CropScience
• Overwatch, FMC
• Reflex, Syngenta
• Terrad’or, Nufarm
• Ultro, Adama
• Voraxor, BASF

Conditions in the 2021 season in southern New South Wales included: good subsoil moisture but a dry start around planting time; this was followed by 80 to 100 millimetres of rainfall post-planting in June, which had a big impact on the influence of pre-emergent products; and a very cool winter and spring, which meant slow-growing crops.

Overall, Mr Condon said, the new herbicide pre-emergent products were successful in what was a profitable season.

Some of the key observations from the 2021 season were:

• low-organic-matter soils and sandy soils, as well as the level of stubble loads, created specific issues that need to be considered for each product;
• growers should be mindful of new formulations of mixing partners and their uses;
• accurate seed depth is critical, as is ensuring the furrow is closed;
• large seedbanks of weeds such as ryegrass cannot be controlled using herbicides alone; therefore “stacking” of multiple WeedSmart ‘Big 6’ tactics – including mixing and rotating herbicides, harvest weed seed control, crop competition and croptopping – are complementary strategies; and
• growers must always be mindful of sticking to labels.

Mix and rotate in diverse farming systems

Speaking at a previous GRDC Update, Mr Condon said an expanded range of herbicides creates opportunities for the rotation of herbicide modes of action and the ability to mix with existing chemistry.

“Research by Pat Tranel from the University of Illinois in the US found that resistance can be mitigated by mixing herbicides at full rates. Pat is quoted saying ‘rotating buys you time, mixing buys you shots’. Peter Newman from WeedSmart expanded on the concept to recommend that we mix herbicides and rotate modes of action so that we can ‘buy time and shots’,” Mr Condon said.

“Research by Roberto Busi from the Australian Herbicide Resistance Initiative found that rotating groups alone may not substantially delay resistance occurring. However mixing herbicide groups can be a highly effective tactic, even on resistant populations. Ryegrass from 140 fields across 58 farms in WA were tested for susceptibility to a range of pre- and post-emergent herbicides.

“The testing showed that a number of ryegrass populations were resistant to individual herbicides, for example 34 per cent of the ryegrass populations were developing resistance to trifluralin and 11 per cent developing resistance to triallate. Yet, when these two herbicides were combined in a mix, full control was achieved.”

Resistance management

The mix-and-rotate strategy would not only provide improved weed control but – more importantly – aids in resistance management where unpredictable patterns of cross-resistance are evolving, he said. “Even the best pre-emergent herbicides can be broken by resistance if not managed wisely.

“Populations of ryegrass from the Eyre Peninsula in South Australia have recently been confirmed as resistant to all of the pre-emergent herbicides – triallate (Avadex®), prosulfocarb (Arcade®), trifluralin, propyzamide and pyroxasulfone (Sakura). These findings by the University of Adelaide have huge implications for an industry now heavily dependent on pre-emergent herbicides in no-till systems, showing they can quickly break down in the face of metabolic cross-resistance.

“Repeated applications of the same herbicides in simple canola/wheat rotations has allowed ryegrass to develop metabolic cross resistance. This is in the absence of alternative tactics such as crop topping, hay, harvest weed seed control or diverse rotations which create opportunities to run down the weed seedbank.

“A heavy reliance on Group 15 (J and K) (eg Avadex®, Boxer Gold, Sakura) in no-till systems can be alleviated with the introduction of herbicides from Groups 23 (E), 30 (T) and 13 (Q) (eg Ultro, Luximax and Overwatch). The new chemistry used alone or in mixtures creates opportunities for targeting resistant weeds or managing resistance through alternative use patterns.

“The new Group 23 (E)product Ultro (carbetamide) provides an alternative to trifluralin or propyzamide for pre-emergent grass weed control in pulses. Ultro will also reduce the heavy selection pressure on post emergent grass herbicides such as clethodim or clethodim + butroxydim (Factor) mixtures.”

Crop rotation allows greater diversity with some of the new herbicide choices available, he said.

“For example, the new Group 14 (G) product Reflex (fomesafen) has shown good control of broadleaf weeds such as sowthistle and prickly lettuce, which are problematic in pulses. A heavy reliance on the use of Intercept, Group 2 (B) in Imi tolerant pulse crops has seen the development of resistance in brassica and thistle species. This new Group 14 (G) product allows growers to relieve pressure on Imi chemistry and strengthen the value of older herbicides such as simazine when used in a mixture.”

More information

Paddock Practices: Dry sowing and pre-emergent herbicides

GRDC Update presentation, Update presentation by Greg Condon

Summer weed management paddock practices guide,

Weed management is available from the GRDC-supported WeedSmart resource centre. WeedSmart has investment from the GRDC and commercial companies and delivers science-backed weed control solutions. GRDC is a Platinum Partner in WeedSmart.