Resistance is certainly partly to blame, but the weather has almost certainly played a bigger part in the much more evident uncontrolled blackgrass and wild oats that appeared in fields this summer.

Slow autumn growth in 2009 meant many post-emergence herbicides were not applied and the cold winter encouraged greater wild oat germination. In the following spring, cool, dry conditions were not conducive to good herbicide activity.

Herbicides such as Atlantis (mesosulfuron + iodosulfuron) and Axial (pinoxaden) do not work as well in cool dry conditions – they work better on actively growing plants.

Resistance is a bigger factor with blackgrass than with wild-oats. While complete failures are rare, resistance more commonly reduces the efficacy of herbicides. When other factors are working against you, such as trying to control large weeds in adverse weather, resistance may well tip the balance towards poor control. This, I believe, was the situation in spring 2010.

So what do you do? The key message must be to consider all control options – not just herbicides.

With the loss of several older herbicides, such as trifluralin and isoproturon (IPU), the lack of any effective new herbicides and increasing resistance means that farmers will have to rely on an aging armoury, many of which are becoming progressively less effective. Like it or not, cultural and non-chemical methods will become increasingly important (see table).

As blackgrass seed will be much more abundant in many fields, it will be important to destroy as much as possible pre-drilling this autumn. The hot dry conditions during seed maturation should mean seed has low dormancy and much should germinate on the stubble – provided there is sufficient moisture. If there is not enough soil moisture for germination, the dormancy status of seeds becomes largely irrelevant.

Although many farmers undertake shallow cultivations post-harvest, there is little evidence that this helps encourage blackgrass to germinate, or increases seed losses. Indeed, with wild-oats, seeds are best left on the surface for as long as possible to increase natural losses as early burial helps preserve them.

Shallow stubble cultivation does increase germination of sterile brome and volunteer cereals, spread straw and chaff and remove surface compaction, and may improve the chances of achieving a good seed-bed which might favour subsequent pre-emergence herbicide activity. So shallow post-harvest cultivation has real benefits, but it is best to consider it as neutral from a blackgrass control aspect, and negative in terms of wild-oat control.

Delayed drilling will give more opportunity to destroy blackgrass prior to sowing, and could be particularly relevant this autumn with such a heavy seed burden in many fields, provided there is sufficient soil moisture.

While many farmers dismiss the idea as being too risky, an increasing number are delaying the sowing of winter wheat until mid October in particularly bad fields. In really badly infested fields, it may even be worth fallowing for a year or two.

Ploughing too can substantially reduce subsequent blackgrass populations. The greater the seed burden on the soil surface, the greater the benefit of ploughing, as a larger proportion of seed is buried compared with that brought back to the surface. Good inversion is critical, and this will be dependent on favourable soil conditions.

In conclusion, the key message is to think beyond what herbicides to use in your next crop. For many farmers, especially those on heavy land with few cropping alternatives to winter wheat and oil-seed rape, integrated weed control will become essential.

Spring cropping

Spring cropping can prove useful in reducing blackgrass populations as shown by a recent Rothamsted trial.

While delaying drilling from 19 Sept to 19 Oct had only a modest effect, due to dry autumn conditions, a more substantial reduction (80-90%) occurred in spring barley and spring wheat (see figure). Spring barley was more competitive than spring wheat, and consequently bigger reductions in blackgrass head numbers were obtained.

Dry spring conditions may have reduced blackgrass growth, and exaggerated the benefits, but other trials have shown an 80% reduction in blackgrass in spring cereals.

However farmers must be aware that fewer herbicides are available for use in spring cereals, although Stomp (pendimethalin), Avadex (triallate), Defy (prosulfocarb) and chlorotoluron are all possibilities pre-emergence, with Axial (pinoxaden) the best post-emergence option.

If growing spring cereals to help get on top of grassweed problems, farmers will have to put greater reliance on pre-emergence herbicides – just as they are doing with winter cereals.

Herbicide resistance

With widespread resistance and consequential declining performance of many post-emergence herbicides, pre-emergence herbicides will become increasingly important.

These are all affected by resistance, but their efficacy is usually only partially reduced. Liberator and Crystal (flufenacet), Defy (prosulfocarb), Stomp (pendimethalin) and Avadex (tri-allate) can all give a good level of control of blackgrass in cereals, and Hurricane (diflufenican) is useful in mixtures.

Studies at Rothamsted Research show that flufenacet is the herbicide least affected by resistance, so this should feature in any pre-emergence programme.

Using additional active ingredients “stacking” either in mixture pre-emergence or in sequence very early post-emergence, is a useful way of increasing control. Most studies show that the more active ingredients applied pre or early post-emergence, the better the weed control, but with higher cost and an increased risk of crop damage.

Atlantis is by far the most widely used post-emergence herbicide for grassweed control, with over 1 million ha treated in 2008. Resistance has now been confirmed on over 300 farms in 23 counties.

Our research shows that resistance is unlikely to decline, even if farmers stop using affected herbicides. In the absence of new modes of action farmers must make the best of what they still have available and become less reliant on an “answer in a can.”