The strikingeffect of cultivations and herbicide use on the development of resistantblack-grass has been revealed in a survey undertaken by ADAS on behalf of BASF.
Fifty-eightfarmers who took part in free resistance tests offered by BASF in 2013 and 2014were quizzed over cultivations strategies and choice of chemistry. The resultswere compared with the resistance status of black-grass seed samples they hadsubmitted for testing.
The farmerswere selected out of more than 120 who took up the test offer because theycould provide cropping and herbicide data stretching back several years, saysADAS research consultant Dr Sarah Cook, who has collated the findings.
Herbicideresistance was widespread among the original 125 samples submitted. Almost half(46%) of the samples were RR (sub-optimal control likely) or RRR(poor control likely) to all three resistance mechanisms enhanced metabolismresistance (EMR), ACCase target site resistance (TSR), and ALS resistance.
A further 36% ofsamples had the same resistances to two of the mechanisms. Just 2% weresusceptible or single-R resistant.
To identify thekey drivers behind these results, Dr Cook first examined primary cultivationregimes and degree of autumn and spring sowing on each of the 58 case studies.This indicated the degree of cultural control employed.
Threecultivation categories were identified min-till (defined as non-inversiontillage and carried out on average on at least 90% of the land), mixed tillage(where 45% of land was ploughed and 53% min-tilled) and plough (where 90% ofthe land on average was inverted).
Drilling date groupswere defined as autumn-sown (growers who sowed most of their crops at this time 93% on average) or spring-sown (growers who sowed an appreciable proportionof their crops at this time 32% of land on average).
This resulted infour cultivation/drilling date strategies min-till/autumn sown, mixedtillage/autumn sown, plough/autumn sown and plough/spring sown. Each group wasthen scored on their cultural risk factors, based on established WRAGguidelines.
Dr Cook thenscored these four groups for herbicide risk factors by assessing herbicidechoice and use and measuring it against the same WRAG guidelines. The two auditswere then combined to draw up the overall resistance risk for each of the fourstrategies.
Generallyspeaking, the more min-till the higher the risk of resistance developing, saysDr Cook. This is especially the case in this study as few of the min-tillgroup carried out any spring drilling, which is known to be very valuable inblack-grass control and resistance management.
Min-tillers alsoused a lot of grass weed-active herbicides, the equivalent of almost threeapplications a year. Fops, dims and sulfonylureas (SUs), all subjectto TSR which increases rapidly and results in very poor control, accountedabout half the herbicide usage. Overall, these factors put the group overall athigh risk of resistance.
Like the min-tillers,only a low percentage of the mixed tillage group had spring crops. They werealso very reliant on herbicides (3.25 applications/year) and used even moreTSR-prone actives. However, the amount of ploughing (almost half of allcultivations) was enough to reduce the overall risk in this group to medium.
The plough/autumn-sowinggroup used less herbicide, about 2.5 applications a year, and importantly theequivalent of just one application of the TSR-prone actives. This reduced theherbicide risk factor although this group was still considered at medium riskoverall.
Theplough/spring-sowing group delivered the best result. Growers used on averagejust under two herbicide applications a year, and the equivalent of about 0.85applications only were fops, dims and SUs.
The combinationof ploughing, spring drilling and lower herbicide put this group into the low-riskcategory, with a combined risk score of 24, less than half that of themin-tillers, says Dr Cook.
The test resultsclearly reflected the different strategies. Well over 40% of samples from themin-till and mixed tillage groups, which were autumn-sown only, showed allthree types of resistance mechanisms.
This was slightly reduced where ploughing was introduced, although thenumber of samples resistant to two of the mechanisms dropped significantly.However, the plough/spring cropping group showed a dramatic fall in multipleresistance, with over 70% of samples either resistant to just one mechanism orsusceptible or R? to all herbicides, says Dr Cook.
Auditing thefarmer groups and analysing the test results clearly demonstrates therelationship between cultural control/herbicide practice and resistancedevelopment, with TSR particularly reduced under the ploughing regimes, sheadds.
This highlightsthe value of all these strategies in helping to stop the spread of resistantblack-grass on farms that are increasingly struggling to contain the weed.