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Controlling weed problems

18 January 2012

Potato growers will have to put greater reliance on post-emergence herbicides to control problem weeds as residual chemistry comes under pressure from both regulators and the weather. 

Crops could soon be treated with a series of low-dose contact-only materials much like onions, carrots and brassicas, says John Keer of Richard Austin Agriculture.
“Any material that has a residual life in the soil is a candidate for withdrawal because of potential problems with the Water Framework Directive,” he explains. “We are slowly going to lose more residual herbicides and manufacturers are not developing new material to replace them.
“We have also had a run of dry springs in recent seasons. Pre-ems only work when there is sufficient moisture in the soil, so weeds have been growing through from depth.”
Work to establish suitable alternative regimes is required now. “The idea is to use small rates of herbicide for small weeds,” says John. “This approach has never really featured - our limited list of contact products has generally been used as fire-engine treatments against big weeds, necessitating high rates which can be pretty harsh on the crop.
“I’m proposing much lower rates that should be much better in terms of crop safety and capable of controlling a flush of small weeds.”
A demonstration at last year’s AHDB Potatoes East Midlands Potato Day tested three herbicide treatments across 30 varieties to test for crop safety.
One block was treated with 0.5kg/ha of metribuzin, a third of the recommended full rate, at a crop height of 15cm. Rather than a treatment itself, this was used mainly to update information on variety susceptibility.
“Metribuzin used post emergence can be very hard on some varieties. A lot of the varieties under test had not been screened for metribuzin tolerance before. We used this treatment as a confidence test - those varieties that come through this treatment unscathed should be suited to an even lower rate approach,” John explains.
A second block was treated with 0.2kg/ha of metribuzin. While it is too soon to glean any firm recommendations, this treatment appeared safe to most varieties, says John. 
Notable exceptions were Innovator and Cabaret, which suffered unacceptably high levels of scorch and vigour loss due to metribuzin damage.
“It’s worth noting that metribuzin damage can vary from season to season, so much more work is needed before we can definitely say which varieties will suit this approach,” says John.
The third treatment was bentazone at the full rate of 1.65kg/ha +2 litres/ha of adjuvant oil. “I would like to have gone lower but the chemical doesn’t control a broad range of weeds. We wanted to assess its performance this year at the full rate to assess its scope as a sequence or mixture partner in future years.” 
Bentazone also caused some crop damage due to the high light intensity this spring, with Markies and Russet Burbank proving particularly prone.
There is room for further work looking at sequences of low-rate metribuzin, with and without rimsulfuron, bentazone and adjuvants. 
Rimsulfuron is probably going to be the most useful product around which a low-dose programme could be built, thanks to its good crop safety and decent spectrum of weeds controlled, says John.
Other techniques to complement reduced rate contact sprays, including mechanical control or total weedkillers used pre-emergence or through directional or shielded nozzles, could find a place in spray programmes of tomorrow, he adds. 
Printable Version Grower Gateway - Issue 1, 2012
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