IMI-tolerant crops – use sparingly and to best effect

This new technology also increases the potential for over-use of the broad-spectrum imidazolinone (or imi) herbicides, along with an increased risk of escalating weed resistance. To manage this risk, it is essential that growers use a calculated approach to crop and herbicide rotation, and use multiple weed control tools in the year of imi herbicide application to prevent seed set – as recommended in the WeedSmart Big 6.

Since the Clearfield technology was commercialised in 1992, plant breeders in Australia have developed the range of imi-tolerant summer (maize, and soon sorghum and sunflower) and winter (wheat, barley and canola) crops, along with lentils with improved tolerance of imi herbicides.

Imi-tolerant sorghum hybrids (centre) currently in the development phase are showing promise for improved control of grass weeds in-crop. (Photo: Rob Crothers, DuPont Pioneer)

Understanding the risk

Plant breeders use conventional methods to breed the single gene imi-tolerance trait into a range of crops. Unfortunately, weeds can also naturally evolve resistance to Group B herbicides, all of which work by targeting the function of the ALS enzyme required to produce essential plant proteins.

Weeds that are resistant to one Group B herbicide will often be resistant to others in the group, even if there has been no exposure to the other sub-groups. This propensity for cross-resistance is a considerable risk and strategies must be in place to minimise exposure across the whole Group B MOA, not only the imidazolinone sub-group.

To protect the imi-tolerant crop technology it is essential to implement the stewardship guidelines for these crops. Richard Holzknecht, BASF technical services manager for Queensland and NSW, says growers will see optimal results in imi-tolerant summer crops when they use post-emergent imi herbicide products to control weeds.

“Pre-emergent imi products are generally less effective in controlling germinating weeds compared with well-timed post-emergent products,” he says. “Some pre-emergent imi herbicides, such as imazapic, have longer soil half-life and can restrict crop rotational choice or cause adverse plant-back effects.”

“The residual use of imazapic products also exposes new weed germinations to ever-reducing sub-lethal doses of product, which increases the risk of weed resistance.”

There are limited post-emergent grass weed options available in summer cropping due to increasing incidence of weed resistance to Group A and B (SU) and potential crop damage. One solution is the use of the post-emergent imi herbicide, imazamox (e.g. Intervix^®) to provide broadleaf and grass weed control in the one product.

Stewardship recommendations

“Post-emergent application targets a known weed population and takes advantage of crop canopy closure to suppress late germinations,” says Mr Holzknecht. “Growers also have the option to use an alternate MOA for their pre-emergent and fallow applications to layer control measures to achieve better season-long weed control.”

Imi herbicides can then be used to control a broad spectrum of grass and broadleaf weeds, including weeds that are closely related to the imi-tolerant crop. Crops with imi-tolerance are also a useful option if residues are present in the soil from previous applications that would otherwise prevent the production of conventional varieties or hybrids.

The over-arching recommendation for imidazolinone resistance management is to limit using Group B herbicides to twice in every four-year period per paddock and to thoroughly control any surviving weeds.

Mr Holzknecht says it is necessary to understand and utilise each herbicide’s strengths and to use each product to its best advantage. “Weed species biology, crop rotation and herbicide choice all need to be taken into account when planning an integrated weed management program,” he says. “Growers must avoid over-use of imidazolinone products in cropping systems, as the risk of resistance to these herbicides is high.”

“Across the rotation, look for ways to layer control measures by using alternate modes of action for fallow, pre-emergent and post-emergent applications in different crops,” he says. “And consider a tank mix with a non-ALS mode of action herbicide at full label rates for in-crop weed control.”

Imi-tolerant sorghum and sunflower hybrid releases

Nuseed’s program manager for summer crops, Chris Haire has been involved in the development of imidazole (imi) tolerant sunflower hybrids that will increase the opportunities to safely plant sunflower into situations that would be untenable with current hybrids.

“With an imi-tolerant hybrid, growers will be able to safely plant into paddocks where imidazole (Group B) chemistry has been recently applied, say after an IT-maize, IT-wheat or pulse crop, negating the current 12+ month plant-back requirement, which restricts the choices that growers have in summer,” he says.

“The second major benefit will be that an imi-tolerant sunflower hybrid could be planted into a paddock that had a broadleaf weed burden—a situation that would currently not be advisable.”

Nuseed expect to be conducting in-country trials required to gain chemical registrations for the new use pattern in 2017–18. Mr Haire is hopeful that a small amount of seed may be commercially available in 2018–19, opening up more cropping options for growers in Central Queensland and the western Darling Downs particularly.

Imi-tolerant sunflower trial – 28 days after application after planting into a weedy situation (0 days after application inset).

Similarly, Pioneer Seeds have sorghum hybrids with Group B tolerance coming through their plant breeding program, adding to their suite of IT canola and corn hybrids. Rob Crothers, Australian sorghum and northern corn product manager said that INZEN hybrids will be evaluated in field trials again this coming summer.

“When we are successful in bringing this new technology to the market place it will enable growers to spray grass weeds with a post-emergent herbicide in crop for the first time,” he says.