Difficulties controlling charlock and runch were behind Norfolk grower Tim Mack’s decision to see if an oilseed rape variety with conventionally-bred herbicide tolerance could be used to help overcome the brassica weeds problem.

With root crops, cereals and oilseed rape being grown at The Grange, Surlingham, near Norwich, Mr Mack points out that avoiding any contaminants in his oilseed rape is essential to the success of his oil business, Yare Valley Oils.

“Just over a year ago, we started cold pressing and bottling the oil from the rape grown on this farm, selling it locally and through retailers. The business has gone very well, but it is important that we keep the oil in its purest form, as that is one of its great benefits.

“As a result, any weed seeds in the sample would be a problem. The taste of the oil is distinctive.”

Last autumn, on the advice of his agronomist, Brian Ross of Frontier Agriculture, Mr Mack drilled one 15ha field with PX100CL. A semi-dwarf hybrid type, it has in-bred tolerance to the imadazoline herbicides, and is one of two Clearfield varieties being assessed by Frontier.

The tolerance trait in Clearfield varieties allows the use of post-emergence BASF herbicide Cleranda (metazachlor + imazamox) on the crop, which controls a wide range of broad-leaved weeds, as well as volunteers and some grass weeds, explains Mr Ross.

“In this particular situation, we were really targeting charlock and runch,” he says. “The pre-emergence options don’t touch them and both weeds have become a real issue on this farm. As a result, there are certain fields, which can’t be used to grow oilseed rape.”

The Clearfield technology is a possible solution but its use requires planning and management, he stresses.

“It’s important to remember that imazamox is an ALS inhibitor, so it has the same mode of action as sulfonylurea herbicides.

“What this means in practice is that growers can’t rely on other sulfonylurea herbicides to control volunteer oilseed rape in following broad-leaved crops. That’s why its management will be so important.”

Not surprisingly, Mr Mack admits to some anxiety about his sugar beet, as he currently uses Debut (trisulfuron-methyl) to control volunteer oilseed rape in the crop.

“I’ve been told that there are other herbicide options available, which won’t have reduced efficacy, so only time will tell if it becomes a problem. We’ll certainly have to pay more attention to stubble management and cleaning, to limit volunteers.”

The fact that he also has other non-Clearfield oilseed rape in the ground means that there’s no room for complacency at spraying time. “If Cleranda was applied by mistake to the Sesame we are growing elsewhere on the farm, it would be killed.”

Since it was drilled, the field of PX100CL received a graminicide when the crop was at the two leaves stage, followed by Cleranda once it had reached 4-6 leaves.

“The recommendation for Cleranda is to apply it when the target weeds are at 2-4 leaves,” advises Mr Ross. “However, where brassica weeds predominate, there seems to be an advantage in going a bit later. So it was used later than the label recommendation.”

That advantage could be because runch, in particular, will keep germinating over a period of time, he adds. “There is some residuality in Cleranda, so it will control brassica weeds even if they’re very big.”

For other broad-leaved weeds, applications should be made when they are smaller. “It’s effective on poppies, cranesbill, speedwell, cleavers, mayweed, chickweed, shepherd’s purse and hedge mustard at the 2-4 leaves stage.”

Cereal volunteers and annual meadow-grass are also controlled, but not blackgrass, he stresses. “So the system is not the answer for everyone and every situation. Without good stubble hygiene, stale seed-beds and timely spray applications, it’s unlikely to be successful.”

To date, the results are impressive, reports Mr Mack. “I deliberately left a small patch unsprayed, so I could see any differences. Charlock, runch and volunteer barley are present in that patch, while the rest of the field is clean. So it’s been a good start.”

However, until prices for the seed and herbicide are released, he has no way of knowing whether Clearfield technology has a place on the farm in the future.

“There’s certainly a management advantage and it does mean that seed-bed condition isn’t critical, because we are less reliant on the pre-emergence herbicides. A HOLL Clearfield variety would be of interest, as that would meet our edible oil end market.”

The current 3-4% yield penalty in Clearfield varieties doesn’t worry him too much, nor does the fact that they’re not on the Recommended List. “I expect that yield gap will close as new varieties come along.”