In the second of the series, Tom Allen-Stevens finds out how one distributor is using trials results to determine the best way to overcome resistance.
A FIELD riddled with resistant blackgrass is an unsavoury nightmare for most growers, but for those wishing to carry out herbicide trial work its a gift. United Agri Products (UAP) has descended on one such field at Caenby in Lincolnshire, where five-star resistance to fops and dims has been confirmed. Various trials have been carried out to tackle the problem, looking at both chemical and cultural control.
"We have other sensitive blackgrass trial sites in various locations in the UK where we have been looking at lower doses of IPU and its alternatives. At Caenby, we are taking the opportunity to pitch a number of resistant blackgrass control strategies against each other and assess the outcome," says Eastern regional technical manager Brin Hughes.
Three cultivation programmes are being explored:
• Ploughing late to enable chitted seed to be buried
• Stale seedbed technique using glyphosate to kill chitted seed prior to drilling
• Stale seedbed technique with no total herbicide used.
"It became apparent early on that a stale seedbed technique was a fairly hopeless approach unless glyphosate was used. Ploughing is a useful way of burying chitted and unchitted seed, but what tends to happen is any remaining blackgrass will emerge over a relatively long period of time. The best opportunity for following up with chemical control came after making a sterile seedbed using glyphosate," reports Mr Hughes.
The recommended way to produce a stale seedbed is to disc and press the field shortly after harvest. If a straw-chopper is used, care should be taken to ensure the chaff is spread as evenly as possible behind the combine. After discing, the field should be left for 5-6 weeks so that seeds can chit. The green material is then sprayed off with Round-Up (glyphosate) for example, and the next crop drilled using a power-harrow/drill combination. At Caenby, the initial seedbed was brought about with a plough, followed by power-harrow and rolls – more expensive, but just as effective.
Eight post emergence spray treatments were then applied to the plots (see table). Most of the treatments were applied on November 25. The straight Lexus (flupyrsulfuron-methyl) was applied early (November 3), when the weeds were at GS11, and the Topik (clodinafop) on March 16.
In the plots that were late ploughed, control of the blackgrass varies considerably under the different treatments. "Isoproturon (IPU) proves to be a useful chemical still, but bear in mind the predominant resistance on this sight is to fops and dims and you have to get a good seedbed for it to work right. Reducing the rate and adding Stomp (pendimethalin) appears to have little benefit. Of the residual herbicides, Harlequin (IPU and simazine) and trifluralin is the best here," sums up Mr Hughes.
The late Topik has very little effect and the residuals work better than the contact herbicides overall. But Mr Hughes warns that a blackgrass control programme can only begin to be effective if it knocks out 98% of the weeds, and the Harlequin/trifluralin mix takes out only 78%. If more than 2% of the population goes to seed, enough will be returned to the seed bank to bring about a full population in the following year.
A pre-emergence dose of Avadex (tri-allate) helps all treatments enormously: "You can actually see the 12m stripe in the field where it was applied," notes Mr Hughes. But all plots still elude the magical 98% control – 96% is the best achieved with the Harlequin/trifluralin mix.
So the only way to control blackgrass effectively is to start with a sterile seedbed, using glyphosate to burn off any chitted weed seed. Overall the treatments prove to be much more effective following this cultivation programme, with Lexus + Hawk (clodinafop and trifluralin) + oil being the best performing with 98.8% control. Across the board, pre-em Avadex improves results further, though offering minimal change to the Lexus/Hawk/oil mix. Following Avadex, the Puma(fenoxaprop and IPU) + trifluralin + oil mix also gives more than 98% control.
UAP has also carried out trials on Cyanamids pendimethlin-based AC210. The new chemical performed at least as well as the best of the old, but its early days.
Mr Hughes is also impressed with Monsantos new brome killer, Mon375, out next spring. UAP trials have shown that the best of the current chemistry (Avadex, IPU and Fortrol – cyanazine) manages to quell less than half the population of rye brome in the plots. A full treatment of Mon375 in the spring has succeeded in giving an impressive 100% clear-up rate. "It looks really good; its far less affected by soil type and trash content and does surprisingly well in difficult conditions. But again its early days and it needs more evaluation particularly against the more difficult to control species – sterile brome. We need to try sequences with Avadex and split timings," adds Mr Hughes.
However, Mr Hughes sounds a timely warning for those growers wishing to mix and match new chemistry, "Mon375, along with Lexus, Eagle (amidosulfuron) and Ally (metsulfuron-methyl) are all sulphonylureas. Only one of these chemicals can be used in a crop each year."
How eight post-em herbicides fared on blackgrass
Treatment Rate(s) Cost % blackgrass controlled
(litres/ha) (£/ha) Late plough Sterile seedbed
Pre-em Avadex – 22 + – + –
IPU 5.0 16 71 80 97 94
IPU + Stomp 2.5 +3.3 36 17 33 81 86
Harlequin + trifluralin 4.0 + 2.0 26 78 96 78 86
Hawk + oil 2.5 +1.0 30 44 86 33 75
Lexus 20g 16 22 89 89 96
Lexus + Hawk + oil 20g + 2.5 + 1.0 46 78 94 98.8 98.8
Puma + trifluralin + oil 4.0 + 2.0 + 1.0 36.50 58 86 94 98.2
Topik + oil 125ml + 1.0 40.40 6 86 0 50
Key: + = with Avadex; – = without Avadex. Source: UAP trials