Beet crop potential for boosting black-grass control

THE opportunity in sugar beet herbicide programmes to use active ingredients that work against ACCase-resistant black-grass is the subject of a three-year trial funded by the British Beet Research Organisation.

ACCase resistance is causing black-grass control problems in sugar beet. That is according to authors of a report into the first year of a Rothamsted Research’s Broom’s Barn and British Sugar trial, with independent agronomy input.

Grass-weed actives commonly used in sugar beet herbicide programmes, such as fluazifop, propaquizafop, quizalofop, cycloxydim and tepraloxydim, are all subject to issues with ACCase (target site) resistance, which blocks the site of activity specific to them and results in poor weed control.

Independent agronomist Pat Turnbull, who has acted as consultant to the trial, says: “The aim is to optimise control of ACCase-resistant black-grass to prevent build-up throughout the rotation.”

The trial is being conducted on two sites in Suffolk and Lincolnshire, where ACCase resistance is suspected and is being tested.

Dr Turnbull says: “Some actives approved for use in sugar beet work against ACCase-resistant black-grass, and these were tested in the trials alongside conventional treatments.”

As pre-emergence treatments were used in the trial, the sites were selected according to past black-grass population history and resistance status.

The weather of 2012, however, made for a difficult first season of the trial. 

“Populations were low when compared with those typically found in winter wheat and oilseed rape crops, with an average 15 plants/sq.m in the untreated Suffolk plots, and eight plants/sq.m in the untreated Lincolnshire ones.

“Later on, waterlogging was an issue at the Lincolnshire site, which may have impacted negatively on the pre-ems.”

Initial findings from the two trials have been encouraging, with significant reductions in black-grass numbers being achieved by some treatments.

“These results should be treated with caution and advice will be updated as the project continues,” says Dr Turnbull.

“In total, 15 treatments were tested at each site, but performance of each treatment varied between sites.”

A programme of one pre- and two post-emergence treatments based on ethofumesate at all timings (see table) gave a significant reduction in black-grass levels at the Suffolk site, at a cost of about £170/ha (£69/acre).

The two post-emergence timings were spaced a fortnight apart, at early tillering and full tillering of the black-grass plants.

Pre-em control

Preliminary advice from the trial results suggests, after the resistance status of the black-grass present is confirmed, pre-emergence applications of ethofumesate at a minimum of 500g ai/ha, plus a minimum 1,400g ai/ha of metamitron, provides the best start for an ACCase-resistant black-grass control programme.

The suggestion is to then monitor emergence and continue with a metamitron and ethofumesate programme if new plants appear.

“The important thing, obviously, is to ensure label rates are not exceeded, as permitted maximum individual doses of different ethofumesate products do vary,” says Dr Turnbull.

“In this way, it is possible to hold some ethofumesate in ‘reserve’ for post-emergence applications.”

Results from treatments containing tri-allate were disappointing, she says. This has been attributed to the fact application was made manually, rather than using a plot applicator.

“We plan to continue work with tri-allate, though, as it is considered to be a potentially useful active for controlling resistant black-grass and has been used successfully by some growers.”

 

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