Extensive, long-term Agrii trials on fields with particular black-grass problems are revealing the best ways of consistently driving down weed populations despite the vagaries of the weather and growing levels of herbicide resistance.

At the company’s primary black-grass management site at Stow Longa near Huntingdon where trials have been conducted on the same fields for more than 10 years, major differences in weed control have been obtained from different combinations of cultivation regime, wheat variety, seed rate and drilling date as well as herbicide programmes. And the influence of previous cultivation regimes, in particular, on weed populations is becoming increasingly clear in a pioneering five year study across a typical heavy land rotation.

“Our cultivation systems work clearly shows ploughing and delaying drilling are the best cultural techniques for reducing black-grass ear numbers in a single season,” observes Agrii R&D manager, Colin Lloyd. “But with the right herbicide programme, some reduced tillage and direct drilling regimes can be almost as effective in controlling black-grass, deliver similar yields and generate higher margins over establishment and chemical costs.

“The extent to which ploughing for a second season in a row brings up black-grass seed buried the previous year has also become very obvious. In contrast, ploughing after direct drilling and most min-till regimes resulted in far fewer black-grass plants in the following crop of oilseed rape; differences which had a dramatic effect on the herbicide regime it required.”

Parallel trials at Stow Longa have shown the particular value of a two pass min-till system in encouraging large amounts of black-grass to germinate early even in a dry autumn and where the seed is considered relatively dormant. They’ve also demonstrated the considerable potential for boosting non-chemical control by choosing more competitive wheat varieties and increasing sowing rates.

“This and our work at satellite sites with black-grass problems in Buckinghamshire and Lincolnshire, amongst others, has underlined the crucial importance of dealing with black-grass on field-by-field and season-by-season,” stresses Agrii eastern regional technical advisor, Will Foss.

“What works on one field with one population of black-grass in one year won’t necessarily work in the next field, with a different weed population or in the following season. Reliable control demands a combination of cultural techniques (which are too often ignored) and robust herbicide programmes – with the emphasis on maximising residual activity – carefully matched to the specific challenge.”

Base on Agrii studies and field observations to date, Will Foss suggests the following key black-grass management guidelines for autumn 2012:

• With ALS resistance widespread, test black-grass seed samples to understand whether the mechanism is EMR, TSR or (as in many cases) a combination;
• Where black-grass levels are high consider ploughing to bury seed but beware of bringing up last year’s seed if ploughing for a second successive year;
• Wherever the plough isn’t used, prepare quality stale seedbeds soon after harvesting and leave enough time to spray-off with glyphosate at least once before drilling;
• Delay drilling the worst fields until mid-October to give time for good pre-planting control with glyphosate;
• Only use dormancy forecasts as a guide – when it rains black-grass will germinate;
• Increase seed rates and/or select particularly competitive varieties to assist control;
• Apply a robust residual pre-em herbicide even  in dry seasons;
• Consider pre-em stacking and/or sequencing with an early post-em herbicide to maximise residual activity – especially where ALS resistance exists; and,
• Apply the main post-em herbicide before Christmas wherever conditions permit to target black-grass before tillering and use a residual partner to extend activity.