Three years into a comprehensive rotation-wide black-grass management trial, dramatic differences between establishment regimes are providing valuable intelligence
Three years into a comprehensive rotation-wide black-grass management trial, dramatic differences between establishment regimes are providing agronomists with a wealth of valuable intelligence to improve cultural control strategies.Across plots with 96 different combinations of wheat and oilseed rape establishment over three seasons at Agrii’s 16ha (40 acre) Stow Longa trials site near Huntingdon, this summer’s black-grass populations vary from just four to over 400 ears/m2.“Our work is showing the vital importance of cultivation strategy in taking the pressure off chemistry across a typical heavy land rotation under acute pressure from resistant black-grass,” explains Agrii trials manager, Steve Corbett (left).”Triple R enhanced metabolism resistance to Atlantis (mesosufuron & iodosulfuron) means the best black-grass control it’s giving on the site these days is 25 per cent. And resistance to pendimethalin as well as fops and dims adds to the challenge. So, effective cultural control is critical.”Over the 13 years we’ve been conducting trials on the same fields here we’ve recorded major differences in weed control from different combinations of cultivation regime, wheat variety, seed rate and drilling date as well as herbicide programme. And our five-year study with Lemken is highlighting the major influence of cultivation regimes throughout the rotation on weed populations. Indeed, with the site’s resistance issues, cultivations are having a greater effect on black-grass control than current pre- and post-em chemical options.”The first year of the Agrii study highlighted quality ploughing and delayed drilling as the best cultural techniques for reducing black-grass ear numbers in wheat in a single season. But with the right herbicide programme, some reduced tillage and direct drilling regimes proved almost as effective in controlling black-grass, delivering similar yields and generating higher margins over establishment and chemical costs.Although producing 0.25t/ha less oilseed rape on average than direct drilling the following year, ploughing gave noticeably better black-grass control across all the previous cultivation regimes but one – an average of 78 per cent against 36 per cent. The exception was ploughing after ploughing which brought-up large amounts of black-grass seed buried the previous year to leave a higher weed population than all but the worst direct drilled plot. Eight different establishment treatments were applied to the most recent wheat crop in the rotation-wide trial. The full range of techniques from ploughing through various forms of minimum tillage to direct drilling were employed ahead of sowing at the main window in the first week in November. At the same time, despite the atrocious weather, crops were also established in the last week of September after both ploughing and direct drilling.Early (right) and later drilled wheat plots (left) in Agrii trials this season highlighting the importance of not drilling too early where black-grass is a problem.“This season has really underlined that early drilling is about the worst thing you can do with bad black-grass,” Steve reports. “We’ve obtained 90 per cent-plus levels of control in most of the later-drilled plots regardless of establishment technique, mainly because the black-grass emerged late so we could control it well with pre-planting Roundup Max. “In complete contrast, so many seedlings escaped the autumn glyphosate with early drilling that 60-70 per cent control was the best we could manage here. Indeed, in five of the six plough after plough plots control was virtually non-existent at less than 25 per cent.”Apart from never drilling fields with bad black-grass early then, what do the Agrii results so far tell us about the best control strategies; which cultivation regimes are likely to make the most positive contribution; does rotational ploughing offer a practical solution and are there other measures that can be taken to reduce the pressure placed on chemistry?
“The key lesson from our rotational establishment work is that there’s no right cultivation system,” stresses Steve Corbett. “It’s very much horses for courses. Black-grass control is a numbers game. Providing you get a good kill before you drill, any system can give high levels of control.”Some are more reliant on effective pre- and post-em activity than others, though. And those that are less chemistry-dependent tend to be more expensive and time-consuming. So, the best approach depends on your attitude to risk, and will almost certainly involve maintaining sufficient flexibility to address problems field-by-field, season-by-season. For which you need to know exactly where your black-grass is, both across the farm and within the soil profile.”In fields where black-grass becomes problematic ploughing the seed down can give you instant relief. But consistent full-inversion ploughing is essential. Equally, once you’ve got the bulk of the seed buried out of harm’s way, leave it there. Don’t bring it back up again with the plough – or with deep discs or tines, for that matter. Revert to shallow tillage and minimal soil disturbance to keep any remaining seed near the surface where manageable populations are easiest to control.Ploughing has given noticeably better black-grass control across all the previous cultivation regimes but one in Agrii trials.“Alternatively, keep the bulk of the weed seed up here the whole time and hit it hard with repeated stale seedbeds and delayed drilling,” he adds. “Either strategy can do the job. The most important thing is to avoid half-way measures which mix the seed throughout the soil profile. This means persistent problems every year and the greatest risk of failure should the weather get in the way of good pre-planting or pre-em activity.”Just like their trials work, Steve insists that a strategic approach is essential with farm black-grass control these days. It may involve sacrificing performance in some fields in some seasons to get on top of a bad problem. With every 100 ears/m2 of black-grass known to knock 1t/ha of wheat yields at Stow Longa, he is adamant that the long-term gain can be well worth a considerable amount of short-term pain.Based on parallel trial work at the site showing the substantial effect wheat variety as well as seed rate can have on yield through better black-grass competition, he also strongly advises growers to pick varieties with a four star rating for competitiveness on the research-based Agrii scale. At the same time, he points out that fluquinconazole treatment of the seed at T(-1) has proved extremely valuable in bolstering the competitiveness of less well-rated varieties under high black-grass pressures (see Farmers Guide July edition).”In-crop herbicides need to be everybody’s last – not first – consideration in black-grass management these days,” concludes Steve Corbett. “The only way to deal with the growing challenge of this and other problem weeds is flexible and thoroughly integrated, agronomist-led strategies that take the greatest advantage of every cultural control opportunity on the farm every year.”