As the potato season edges closer, growers will be pondering herbicide programmes and asking which strategy is likely to deliver the greatest bang for their buck.

The first decision confronting growers will be whether to use metribuzin, or not? With it, weed control is almost always easier and less expensive, but the long list of varieties sensitive to this substance means many growers have to consider the alternatives.

“Metribuzin tolerance, or lack of, is often the first consideration when formulating a weed control programme. Without it, control is still achievable, but is often more complex and, potentially, costly, so care needs to be given to what products are used if unnecessary expense or false economies are to be avoided,” said Eric Anderson, senior agronomist at Scottish Agronomy.

The pressure on metribuzin is amplified by a reliance on rented land where knowledge of the weed spectrum present is often less than complete.

This was accentuated by the loss of diquat ahead of the 2020 season which forced a change in pre-emergence tactics because the alternative contact herbicides are less effective against established broad-leaved weeds and annual meadow-grass.

“Specifically, the issue was what contact herbicide to use at the pre-emergence stage? Carfentrazone-ethyl or pyraflufen-ethyl, both have limitations,” said Mr Anderson.

The change in tactics, however, had implications for timing. “Some growers were unprepared for the need to bring herbicide applications forward.

"The first residual herbicide needs to be made seven to 10 days after planting; typically once ridges are settled. This can be sequenced with contact sprays, such as Gozai or Shark (carfentrazone-ethyl) which are restricted to no more than 10% crop emerged, any later and growth will be seriously checked,” pointed out Mr Anderson.

One consequence of this was to increase the pressure on the residual component of the programme. Concerns over the future of metribuzin in a programme are not too far removed from the questions posed at the start of last season, ie which of the herbicide choices and mixing partners can be expected to give the greatest control?

“The central issue is one of weed spectrum to be controlled. Failure to understand the weeds present in the field will result in higher costs in the long-term,” said Mr Anderson.

“On soils where the variety is approved for metribuzin then it’s a no-brainer. Metribuzin has a wide spectrum of activity including annual meadow-grass and many broad-leaved weeds so should be incorporated into the top 10-15 cm of soil during the final cultivation with the rotary tiller.

"Applying Emerger (aclonifen) pre-emergence will bolster control of small nettle, brassica weeds, fat-hen, orache, polygonum species, chickweed and black-bindweed,” he said.

Soil moisture is another consideration. Emerger is a diphenyl-ether, which is taken up by the hypocotyls, cotyledons and coleoptiles of weeds. It’s then translocated to the meristems causing bleaching and chlorosis of young shoot tissue as it develops.

As such, it is less affected by dry soils than Defy (prosulfocarb), so this may influence application timing.

“Defy needs to go on later, typically as late as you dare and often as a standalone application. This has obvious cost implications and weed control in potatoes is already twice as expensive as it was two years ago,” said Mr Anderson.

For some, there is still the suggestion that Praxim (metobromuron) has a place in the programme. Compared with Emerger, it has a smaller weed spectrum – just eight susceptible and one moderately susceptible weeds at the lower rate of 3 l/ha – and is more than twice the per ha cost in 2020 (see table).

“At the end of the day, Emerger has the wider weed spectrum and is more cost effective with better activity against fat-hen, black-bindweed and common field-speedwell, but both will require the addition of Defy (prosulfocarb) to bolster control of cleavers,” added Mr Anderson.


Typical on-farm herbicide costs (£/ha): 2017 v 2020

Weed control costs in potatoes

Weed control costs in potatoes