July 23, 2015
Pioneering trials show the way in ‘stacking’ cultural Blackgrass controls
Latest results from 15 years of detailed, replicated trials across 25 ha of challenging heavy land at Agrii’s specialist black-grass technology centre at Stow Longa near Huntingdon show even the most difficult infestations can be cost-effectively overcome. But only with sufficient determination, the right combination of cultural controls and enough flexibility to adapt the cropping system to the needs of individual fields and seasons.
“The days when a single post-em spray would deal with black-grass are long gone,” insisted Agrii head of agronomy, Colin Lloyd who has presided over the trial work from the outset. “The 95%-plus control levels needed to stop the weed increasing in the rotation can still be achieved from the can…. just. However, reliable control demands increasingly complex pre-em stacking, depends on sufficiently good seedbed and weather conditions for residual activity and can cost the best part of £200/ha!
“Our extensive research reveals there’s no simple ‘non-can’ solution to the problem either. Even the best rotational ploughing, for instance, won’t deal with black-grass seeds well spread throughout the soil profile. Late autumn drilling can seriously damage heavy soils as well as limiting yields. Equally, a hefty weed seed bank may still remain after a spring crop or fallow.
“Just like chemistry, the solution has to be to stack the most appropriate cultural controls,” he stressed. “And in doing so we need be flexible enough to meet the needs of each field and season.”
Four years and 288 different establishment strategies on from the start of the pioneering heavy land rotation trials run over 6 ha at Stow Longa and the part cultivations have to play in the cultural control mix couldn’t be clearer.
Side by side under exactly the same conditions and with precisely the same herbicide programme, different combinations of annual establishment strategy over the years have resulted in 2014 black-grass populations varying from 4 ears/m2 to 206 ears/m2 and breadwheat yields from 4.6 t/ha to 12.8 t/ha.
The highest yields and lowest black-grass seed returns on the site invariably come from cultivation sequences in which a plough had been used at some stage to bury the weed seed. Indeed the benefits of a single ploughing in the rotation can still be seen four years on in both yields and seed returns.
“You can’t simply plough your way out of trouble, though,” Colin Lloyd said. “Our work reveals that repeated ploughing over the years can be every bit as bad for black-grass control as repeated min-tilling or direct drilling.
“Having said that, it all depends on when you drill. The key factor we find in controlling high levels of resistant black-grass every season under every cultivation regime is delayed drilling. It’s vital to get as many weed seeds in the upper soil level germinated and despatched with glyphosate before drilling to take the pressure off the pre-em and post-em chemistry.
“To do this effectively what you need more than anything else is time. What’s more, if you give yourself this time our results suggest you can get turn a poor performance into something far more robust almost regardless of your cultivation strategy.
“Four years of ploughing in our wheat/OSR/wheat/wheat rotation with the last two wheats drilled in late September, for instance, gave us a 2014 wheat crop of just 6.6 t/ha and a blackgrass seed return of 151/m2. In complete contrast, we brought in 11.2 t/ha with a seed return of 9/m2 from exactly the same cultivation regime but with the last two wheat drillings delayed for just over a month.”
Deliberately delaying drilling on heavy land raises obvious concerns for many, Colin Lloyd is the first to acknowledge. Which is why he is adamant the approach needs to be accompanied by sufficient agronomic flexibility. In particular, the flexibility to employ the most appropriate cultivation and drilling equipment for the season and the flexibility to switch to spring drilling should late sowing conditions prove difficult.
“In this context, our trials comparing spring Mulika with autumn Solstice drilling at Stow Longa have been very revealing,” he explained. “Late-September sown Solstice averaged 4.8 t/ha with a black-grass population of 212 ears/m2 to deliver a margin over input costs of £389/ha in 2013 while delaying drilling to the beginning of November gave 7.76 t/ha with 18 ears/m2 and a margin of £903/ha. Alongside it Mulika drilled in the first week of April averaged 6.5 t/ha with no black-grass ears for a margin of £729/ha.
“Earnings from spring Mulika may have been lower than from late-drilled Solstice, but in a season in which we couldn’t have got the Solstice into a half decent late seedbed the spring crop would certainly have paid handsome dividends. What’s more it gave no black-grass return compared to around 1800 seeds/m2 – a valuable bonus for future management.”
Current trials with cover crops ahead of both late autumn and spring sowing are providing valuable pointers for improving soil structure and drainage to give more leeway in drilling decision-making on the heavy Stow Longa ground too. While better drained soil should also help to discourage black-grass proliferation, in the short term the key challenge on the site has been to achieve the desired soil improvements without compromising immediate black-grass control.
The use of minimal disturbance tines for direct drilling wheat into the cover and two glyphosate applications – the first to eliminate the cover and the second to target black-grass beneath it – appear especially important here.
Also important in managing difficult black-grass, the Agrii work shows is wheat variety choice. Annual trials with more than 20 varieties reveal the least competitive can lose nearly 30% of their yield when the herbicide regime is compromised for one reason or another while the most competitive lose little more than 10%. This has led to the development of competitiveness scores for all varieties as an additional aid to black-grass management.
“Our 15 years of practical studies here at Stow Longa show there is a ‘beyond the can’ solution to even the most difficult, resistant black-grass,” concluded Colin Lloyd. “At the same time, though, they underline there is no single, simple solution.
“Instead, growers and their agronomists need to work together to adapt the various elements in the cultural toolbox to their own particular circumstances. And in many cases, they will need to make black-grass control an increasingly important driver of their variety choice, cultivation practice, drilling date and, indeed, whole rotation.”