With increasing reports of resistance in black-grass it’s important to be aware of the background to resistance and the best advice on how to tackle it.
|Agronomic factor||Lowest Resitance Risk||Highest Resistance Risk|
|Cropping system||Good rotation of spring and autumn crops||Continuous winter cereals|
|Cultivation system||Annual ploughing||Continuous non-ploughing|
|Control method||Cultural only||Herbicides only|
Herbicide used throughout the rotation
|Different modes of action||Single mode of action|
Weed infestation level
|Resistance incidence||None in vicinity||Identified locally in similar cropping systems|
Herbicide resistance is inherited and occurs through selection of plants that survive herbicide treatment, to put it simply: it is ‘the inherited ability of a weed to survive a rate of herbicide that would normally kill it’. With repeated selection, resistant plants multiply until they dominate the population. There are three main types of resistance present in black-grass in the UK, which can occur independently in different plants in the same field, or altogether in one plant: Enhanced Metabolic Resistance (EMR), ACCase target site resistance (ACCase TSR), and ALS target site resistance (ALS TSR)
More than 1,000 black-grass plants/m2 containing many with a RRR resistance rating is most growers’ worst nightmare but for Richard Hinchliffe this was reality. Twelve years after purchasing the land he has successfully reined in populations. The worst fields have only patches of black-grass and the highest populations stand at 200 plants/m2.
It’s an example that demonstrates what can be achieved when an appropriate long-term management strategy is used, says Dr Gordon Anderson-Taylor development manager at Bayer CropScience.
“If an effective strategy is adopted before the situation gets out of hand, dramatic increases in populations can be halted and the development of resistance slowed.”
“Knowing the resistance status of the population is a useful starting point,” explains Dr Anderson-Taylor. “Getting seeds or plants tested is really useful – it can inform herbicide programmes and indicate to what extent the emphasis should be on cultural control.”
“Where glasshouse pot tests show the presence of individuals with an RRR rating and these represent a significant proportion of the field population, cultural control should be the focus of the control programme,” he says. “An RRR rating means resistance has been confirmed and the effectiveness of affected herbicides is highly likely to be reduced."
“RR means resistance is confirmed, probably reducing herbicide performance. With R?, resistance is not confirmed but may be developing, while S means that the black-grass is susceptible to herbicides.”
Other types of testing can reveal whether plants have enhanced metabolic resistance (EMR) or target site resistance (TS), as well as identifying the specific chemical groups that the black-grass has grown to resist.
“Knowing which chemical groups to avoid in herbicide programmes is particularly useful – it can reduce the likelihood of using costly but potentially ineffective programmes,” says Dr Anderson-Taylor. “The most effective weapons in the growers’ armoury are cultural controls – particularly where they eliminate black-grass early, protecting yields from the start and reducing the pressure on in-crop control methods.
“To control black-grass with the plough, use it whenever there has been a poor year for control and a heavy seed shed. However, remember not to plough again in the next few years, otherwise viable seed will be returned to the surface,” he explains. “Stale seedbeds combined with delayed drilling are very effective. Introducing spring crops not only increases the opportunities for control in the autumn but can also provide opportunities to use alternative active substances. Higher seed rates and more competitive varieties can also add a few extra percent control.”
Mr Hinchliffe’s black-grass population has EMR and is resistant to fops and dims. Like Dr Anderson-Taylor, he feels that ‘cultural control is king’.
“Glyphosate has been key to getting on top of black-grass,” says Mr Hinchliffe. “It’s the most cost effective chemical we have. Two applications go on before a wheat crop is drilled and we repeat applications throughout winter on fallowed fields and those in spring cropping.”
“We’ve introduced two spring crops: spring beans and linseed - they help by giving us extra time to apply glyphosate. Only the worst fields are fallowed for a year.”
Protecting yields and herbicides justifies the financial sacrifice Mr Hinchliffe makes when he fallows a field. “Black-grass populations gather momentum so quickly. In the first year 1t/ha can easily be lost rising to 2t/ha in the second year and so on. I’d still be applying a full plant protection programme and not getting the return, while increasing the selection pressures and making future control harder too.”
Based on an average yield of 8t/ha and taking into account a full plant protection programme, Bayer CropScience estimates that the payback time for fallow can be under three years in difficult black-grass situations.
Competitive varieties such as Relay, sown at rates no less than 350-400 seed/m2, complete the cultural controls used by Mr Hinchliffe. For chemical control he chooses Liberator (flufenacet and diflufenican). “Flufenacet is one of the most effective in-crop actives and I like to keep pendimethalin to partner with Atlantis WG (mesosulfuron and iodosulfuron). I thought about stacking and although I’ve heard and seen good results with flupyrsulfuron and Liberator I’m reluctant to add another SU into the mix.”
Autumn Atlantis WG and pendimethalin applications are timed to go on when the residual action of Liberator is tailing off. “This way we have autumn-long residual control,” says Mr Hinchliffe who applies Atlantis WG in mid-November. “In spring the canopy is bigger so it’s harder to hit the target. The black-grass is stronger too and is more able to metabolise the actives,” he explains.
Last year, for the first time in 12 years, spring applications performed better than autumn. “It reminds us how important soil moisture is. I thought I had timed applications perfectly; the black-grass was green and looked like it was growing but the black-grass’ metabolism must have been very slow due to the lack of moisture.”
“Application technique is really important. Atlantis WG must be applied to a dry leaf. In November spray days are short; you’ve got to wait until the dew has burnt off and allow time for it to dry on before the dew comes back down. Generally we only get a single tank applied in a day. Actively growing weeds, a fine spray and a travel speed of no more than 12km/hr are all key to a good application.”
Finally, any late germinators are hand rogued, an action which neatly demonstrates Mr Hinchliffe’s zero tolerance approach to black-grass.
Enhanced Metabolism Resistance – EMR results in herbicide detoxification and is the most common resistance in grass-weeds in the UK. It affects most herbicides to some degree, but only in very severe cases does it result in complete loss of control. As it sounds, an increase in metabolism allows weeds to detoxify themselves of the herbicide.
ALS target site resistance (ALS TSR) blocks the site of action of sulfonylurea and other ALS inhibiting herbicides (eg Atlantis WG, Lexus SX (flupysulfuron-methyl), Unite (flupyrsulfuron+pyroxsulam)). It only affects this group of herbicides and can result in complete loss of control. Currently, it occurs much less commonly than ACCase TSR, but is increasing.
ACCase target site resistance (ACCase TSR) blocks the site of action of "fop" (e.g. Topik, Falcon), "dim" (e.g. Laser) and "den" (e.g Axial) herbicides in grass weeds. It only affects these groups of herbicides and can result in total loss of control. As this form of resistance can increase rapidly it is now present in the majority of black-grass populations.
Both Target Site Resistance (TSR) and Enhanced Metabolism Resistance (EMR) are threatening the efficacy of cereal herbicides. There has been a steady increase from both mechanisms but EMR is currently the major form of resistance threatening the most important post-emergence cereal herbicide, Atlantis WG. Protecting against the development of both TSR and EMR should be the focus of all growers with black-grass to control, according to the experts