After over 30 years of PDQ + linuron as the standard weed control in potatoes, growers will need to totally rethink weed control programmes for 2009.

Last season saw the final use of PDQ, and next season a maximum of 600 g ai/ha linuron will be permitted. PDQ + linuron, applied at crop emergence, was capable of controlling all annual weeds that emerged and also provided sufficient residual control until canopy closure.

Such a radical change in our herbicide armoury prompted me to propose a weed control project to the Potato Council, designed to assess PDQ alternatives and look at how to bolster the performance of reduced rates of linuron.

Retro and Basta were brought to the market in an effort to replace PDQ. These two products joined Shark, already on the market as a contact herbicide in potatoes.

In the trials it became clear none of these PDQ alternatives possessed the same broad spectrum weed control. Shark (0.33 litres/ha) was the best against broadleaved weeds, but had virtually no grassweed activity. Basta (2.0 litres/ha) was probably best overall, but grassweed control was inferior to PDQ and small nettle was not completely controlled.

Retro (2.0 litres/ha) efficacy was greatly improved by the addition of a wetter (Activator 90), but grassweed control was not as good as PDQ, and knotgrass and small nettle were also not best controlled.

Crop safety was also compared. Retro caused the expected chlorosis and necrosis but was no more damaging than PDQ. Basta, although not permitted on the emerged crop, was also no more damaging than PDQ.

However, Shark caused a much higher level of crop necrosis compared with PDQ. Despite the label allowing early post-emergence applications of Shark, I think the product should only be used as a pre-emergence option.

Another advantage of PDQ was the flexible application timing. Linuron + PDQ was usually aimed at just prior to emergence but weather delays often meant the mixture was applied post-emergence. Growers wanting similar flexibility will be restricted to Retro.

Mixtures involving Basta or Shark should be applied probably one to two weeks earlier than traditional “at emergence” timings. These earlier timings will ensure crop safety without compromising weed control – so long as sufficient residual control is built into the tank mix.

Growers and agronomists will need to ensure only linuron formulations with the new MAPP numbers are applied this season. These labels allow a maximum of 600g ai/ha to be applied in 300 litres/ha water.

This low rate still contributes cost-effectively to the control of many weeds, especially the difficult polygonums weeds that are not well controlled by other residual herbicides.

However, 600g ai/ha provides insufficient residual activity in all situations and needed to be bolstered by mixing Sencorex, Defy, Artist or Gamit, depending on the expected weed spectrum. More work is required to refine mixtures of residuals – three way combinations may be required for difficult weeds.

Another way around the loss of PDQ is to apply higher rates of residual herbicides, before weed emergence, negating the need for contact type products. This is a viable option on sandier soils with a fine tilth, where herbicide was applied to suitably moist soil.

However, more bodied soil types did not suit the earlier, residual-only approach. The subsequent weathering of soil aggregates after herbicide application disrupted the herbicide “seal”, allowing weeds to escape.

PDQ (2.0l) controlled virtually all emerged annual weeds in potatoes, often with something to spare, with the partner herbicide merely providing some residual activity.

More thought is required now on how herbicide mixtures are put together. The residual component must also have contact activity on weeds not controlled by the contact partner, when dealing with emerged weeds.

Sencorex, Artist and Defy all offer useful contact grassweed control which will complement contacts with limited grassweed activity, for example Retro and Shark.

Sencorex, Linuron and Artist can similarly be used to plug the broadleaved weed gaps. Future work will seek to refine herbicide combinations and look at early applications of pendimethalin.