Potato growers and agronomist have a new challenge for pre-emergence herbicide timings this season – they won’t have the use fast-acting diquat.

The over-riding message is that pre-emergence residual treatments will need to be applied earlier, according to Syngenta’s potato technical manager, Michael Tait. “The focus is on mixtures, stacks and rates to deliver longer lasting control, and new demands on contact herbicide partners for a clean start,” he advised.

After the winter weather rains, growers’ key issue is expected to be waiting for heavier soils to dry out for cultivations and planting, with ample moisture for pre-emergence herbicides. On quicker drying, light soils, however, herbicide timing decisions will still likely to be dictated by prevailing weather and soil moisture conditions.

“Many agronomists are already looking at a split, or combination, of contact activity herbicides with their weed control recommendations, if conditions will allow,” reported Mr Tait. “That is likely to potentially include pyraflufen-ethyl in tank mix with the pre-em treatment where any weeds are already present, with a follow-up of a second pyraflufen-ethyl, or possibly a glyphosate treatment where label approval permits, immediately prior to crop emergence.”

Independent herbicide research last season showed a simple pre-em mix of Defy with metribuzin + pyraflufen-ethyl achieved complete control of key potato weeds - black bindweed, fat hen, annual nettle and annual meadow grass. Defy and a reduced rate of metribuzin was also equally effective when used in combination with metobromuron.

However, whilst the combination of aclonifen (1.75 l/ha) + pyraflufen-ethyl (0.3 l/ha) with Defy at 3 l/ha did show a weakness on some key weeds, adding metribuzin, at 0.3 kg/ha, to the stack bolstered the mix back to full control. Furthermore, the trials showed a Defy + pyraflufen-ethyl combination had the least impact on the emerging crop, with any transient effects quickly out grown.

“Where pre-emergence herbicides are applied at, or soon after, planting, trials suggest higher rates can achieve longer persistence of residual activity for weed control,” advocated Mr Tait. “Previous Syngenta herbicide research has highlighted Defy at 4.0 l/ha, for example, has shown a significant dose response increase in both efficacy and residual activity, compared to lower rates.”

Application covered for potato herbicides

Bare soil, and potato seedbeds in particular, pose a serious challenge for herbicide application, warned Syngenta application specialist, Harry Fordham.

Whilst contact herbicides need to hit emerged weeds, the residual element requires a consistent coverage across the soil surface, he advocated. “Spray drift and boom height are critical components of achieving an even result. Any shift in the spray pattern, frequently caused by gusts of wind, can affect the coverage of residual herbicides.”

Defy trials, for pre-emergence grassweed control, has shown that even when spray conditions were deemed ‘ideal’ at an average 1.6 m/sec, gusts of between 0.5 to 8.0 m/sec during application resulted in up to 20% differences in levels of control with standard flat fan nozzles. Application using 90% drift reduction technology (DRT) nozzles achieved far more consistent results, with higher levels of weed control.

Furthermore, with potatoes grown on deep ridges and beds, there is added effects of boom height and potential for shading on the windward side of the ridge. “Syngenta application trials have demonstrated that the 90% drift reduction nozzle technology is far more adept at maintaining spray pattern, and resulting weed control, at a range of heights, compared to flat fan or standard variable pressure nozzles,” reported Mr Fordham.

The research revealed that whilst blackgrass control performance from flat fan nozzles dropped by 11% with a boom height of 100 cm (64% control), compared to the ideal 50 cm (76%). The 90% DRT nozzles gave consistently better results, with 91% and 84% of control at 50 cm and 100 cm respectively.

“New 90% DRT nozzle technology to limit the effects of drift would also help to achieve improved coverage of both sides of the ridge and faces of potato beds,” he added. “Trials have also shown using a water volume of 200 l/ha has delivered consistently better results, compared to lower rates of 50 to 100 l/ha.”

Also, trials for the pre-em herbicide application campaign ‘Go lo; Go slow get covered’, have highlighted the improvement in results from operation at 10 to 12 km/hr, compared to 16 km/hr or faster modern sprayers are cable of achieving.

“Slower speed significantly reduces turbulence that can increase risk of drift, along with improvements in boom stability to maintain a constant 50 cm nozzle height above the seedbed,” advised Mr Fordham. “Additionally, maintaining slower operation, along with nozzle size selection, typically enables delivery of the required water volume at lower pressure that minimises production of drifty fine droplets and increases the proportion of large droplets that better hit and evenly cover the soil surface.”