Clearfield tolerant oilseed rape varieties were announced at Cereals 2011. Mike Abram takes an in-depth look at the technology.

Mention herbicide-tolerant crops and most growers immediately think of genetic modification, such is the hold of Roundup Ready and, to a lesser extent, Liberty Link crops on the collective imagination.

But herbicide tolerance can be bred into varieties using conventional breeding techniques, and that is exactly how Clearfield varieties have been created.

The technique used is called mutagenesis, and has been used in breeding since the 1930s, explains Matthias Pfenning, senior herbicide technical manager for BASF in Germany. “It is not a targeted approach, but one that does speed up the breeding process. It is a method breeders have used successfully to create brewing barley, seedless grapes, semi-dwarf and HO,LL oilseed rape varieties.”

Clearfield itself is not new either. “The starting point was spring oilseed rape, Canola, which was launched in Canada in 1995.”

Researchers took pollen from conventional oilseed rape, used a chemical to induce variation in pollen cell cultures in the laboratory, some of which were found to be tolerant to imidazoline herbicides.

Imidazoline herbicides are from the same family as the sulfonylurea ALS-inhibiting weed killers commonly used in other crops, Mr Pfenning says. “Oilseed rape has an ALS enzyme, so it is sensitive to ALS-inhibiting herbicides, which bind to the enzyme so it can no longer produce essential amino acids and the plant starves to death.

“The chemical used in the pollen cell cultures induces a change so the herbicide can no longer bind to the enzyme, so it is ineffective. The herbicide is metabolised quickly by the plant so there is no harm to the plant.”

The tolerance is specific to imidazoline herbicides, Mr Pfenning says. “In theory, it is possible for the plant to be tolerant to other ALS-inhibitors, and there is some cross-tolerance, but there is not sufficient tolerance for it to be crop safe.”

That is because there are a number of binding sites within the ALS-enzyme that herbicides can bind to, and other ALS-inhibitors will still bind to other binding sites and inhibit the production of those essential amino acids.

Indeed to make sure oilseed rape is tolerant to the imidazoline herbicides, both parent lines need to be tolerant, otherwise crop safety in the resulting hybrid variety is not good enough, Mr Pfenning says.

A rigorous testing procedure for all Clearfield hybrid varieties has also been put in place to check tolerance is strong enough in field situations before commercialisation.

“The breeders send us seeds, which we plant in qualification trials in the UK, France and Germany, where the varieties are treated with double and quadruple doses. With molecular analysis you can prove the trait is there, but there are a lot of environmental impacts that could affect whole- plant tolerance that we need to check under field conditions too.”

That is the same as normal herbicide tolerance work, says Will Reyer, BASF UK oilseed rape manager. “With any new herbicide you check to make sure it is safe under a range of conditions, such as cold or dry.”

Before that, a rigorous breeding process has to take place to first backcross the trait into the preferred in-bred lines up to seven or eight times to remove undesired genetic material that the initial chemically induced genetic variation also causes, Mr Pfenning says. “That is not different to a normal breeding process.”

There will also be clear stewardship rules for growers of Clearfield oilseed rape to help optimise the system and to ensure grower satisfaction. All Clearfield varieties will be clearly labelled with clear branding and a CL-suffix to help growers avoid treating non-tolerant varieties with the Clearfield herbicide, Cleranda (imazamox + metazachlor).

Equally importantly, growers will also be provided with advice on how to control Clearfield oilseed rape volunteers in the rotation. Currently, many growers rely on a sulfonylurea herbicide, such as Atlantis (mesosulfuron + iodosulfuron), for example in wheat, to remove oilseed rape volunteers, Mr Reyer explains.

“It won’t be the same if a Clearfield variety has been grown. While you can’t use other sulfonylurea herbicides on the Clearfield oilseed rape as you will get high injury to the crop, you also cannot rely on them to control Clearfield volunteers.

“But there are a range of other weedkillers you will be able to use, such as phenoxy herbicides such as 2,4-D and MCPA. We will provide a full list of options.”

Cleranda

Easier timing could be one of the key benefits from using the Clearfield herbicide, Cleranda, says BASF UK oilseed rape manager Will Reyer.

Virtually all of the widely used residual herbicides need to be applied either pre-emergence, usually within 48 hours of drilling, or from the expanded cotyledon stage early post-emergence, he explains.

That gives limited flexibility at what is often a busy period on farm. Cleranda will give growers the opportunity to go to a true post-emergence timing, three to six weeks after emergence.

It takes metazachlor beyond its usual application window, but it still will play an important role, Mr Reyer says. “There are a number of weeds that will respond to contact and residual activity. If you were using metazachlor alone, it wouldn’t be enough, but imazamix turbo charges the system.”

On a number of weeds the mixture is synergistic, adds Ron Kehler, BASF oilseed rape crop manager in Germany. “It means you get more activity than you would see from one active alone. The combination will give the broadest spectrum of activity in a single product, including volunteer cereals activity.

“It will also give more consistent performance in a range of conditions because it will have two routes of uptake. That will also mean it is less dependent on seed-bed conditions.”

Growers who establish oilseed rape through min-till methods could also benefit because it is more forgiving of poorer seed-bed conditions, he adds.

Nine challenges BASF says Clearfield oilseed rape could help address

1) Few alternatives to pre- and early post-emergence residual herbicides

2) Limited timing flexibility with current residual herbicides

3) Wide germination window of broad-leaved weeds

4) Increasing problems with cruciferous weeds in oilseed rape

5) Problem weed seed impact on oilseed rape seed quality

6) High workloads around planting time

7) High dependence on good seed-bed conditions

8) Need for rapid establishment

9) Lack of in-crop volunteer oilseed rape control options

Clearfield adoption

Clearfield technology is available in eight crops across the world, including oilseed rape, wheat, rice and sunflowers, says Ron Kehler, BASF oilseed rape crop manager in Germany. “There will be around 7.5m ha of Clearfield crops this year, with over 1m ha of oilseed rape.”

Adoption of the system is strongest where the varieties are most competitive, he admits. “We’ve had years where the market share in oilseed rape in Canada was 20%. Now it is 10% because our competitors have had varieties with very good yield performance.”

UK oilseed rape varieties will also be behind on yield initially. The first two Pioneer varieties approved to the European Common catalogue are said to be 3-4% behind reference varieties in trials.

“The first varieties are not best in class – that’s usual with a new technology, but when they are we will see quick adoption,” Mr Kehler reckons.

That could take three to four years, but there will be benefits from using the system immediately, particularly where growers have significant problems with key target weeds, such as cruciferous weeds, BASF UK oilseed rape manager Will Reyer says.

First commercial sales in the UK will be in autumn 2012. “This year we are working with our customers to make sure they understand the technology, and have the right information to make it a success. They need a chance to try it, so they can the best from it.”

In addition, to the two Pioneer varieties, at least two others are expected to be available in 2012.

Clearfield in wheat

Clearfield varieties have been developed for wheat in North America, South America and Australia, but it has been a slow process and one not likely to be undertaken in Europe, says Ron Kehler.

“It has taken a long time to get the right varieties to market in the countries where it has been developed,” he points out.

And with a wheat variety market so clearly differentiated between countries and regions, the economics of developing Clearfield wheat in Europe doesn’t stack up, he says.

“Plus we will lose the benefits of volunteer cereals control in oilseed rape and sunflowers if we have Clearfield wheat varieties too.”