Oilseed rape establishment is always partly in the lap of the gods. But understanding and taking the wide range of factors affecting it into account can greatly improve the chances of success, discussions at the initial meeting of the Sigma Team* (Arable 18 July) suggest. Andrew Blake reports.
Acknowledging the wide range of equipment available to do the job, ADAS’s John Spink stressed that there was no universal sure-fire mechanical technique.
“You must think about what you are doing and trying to achieve,” he said. “Your cultivations must suit the prevailing conditions.”
Where soils were “self-structuring” and in good condition there was much to be said for maintaining or developing the surface tilth, rather than ploughing and pulling up clods, which could leave dry cobbly seed-beds.
“If you’ve got a good tilth why bury it? Seed-to-soil contact is increasingly important on dry soils.”
Oilseed rape was extremely sensitive to soil moisture and aggregate size, explained Mr Spink.
Trials in dry conditions showed that with sizes up to 17mm, 60-80% emergence could be expected within 10-14 days. Above 23mm establishment was dramatically impaired, and the larger sizes made it harder to control drilling depth.
Oilseed rape husbandry needs to adjust to climate change, Ian Rudge believes.
The impact of moisture had been clearly illustrated in a crop sown after rotational trials on first and second wheats in adjacent plots at ADAS Rosemaund.
Rape after the first wheat area was almost a complete failure. But the lower-yielding second wheat left enough moisture in the ground for successful establishment. Both areas were sown at the same time, he pointed out.
In dry condition,s minimum or non-inversion tillage or direct drilling helped retain moisture and maintained existing tilths for good seed-to-soil contact.
However, in very wet conditions, tines and discs tended to block and smear, causing compaction.
In those conditions ploughing might be the best approach to speed up drainage, dry the land and remove any compaction. It could also turn up better-structured soil.
Avoiding root-restricting compaction, usually unseen, was a must, advised Mr Spink. He believed it often lay behind some of Autocasting’s poor results. “Often the only way to find out if you’ve got it is to get out with a spade and have a dig around.”
It was important, though, to distinguish between compaction and seed-bed consolidation, the latter generally helping by improving seed-to-soil contact.
Growers should avoid drilling too deeply, Mr Spink advised. Trials showed emergence fell away fast if seed was sown deeper than 5cm (2in).
“Sowing depths of over 3cm are likely to reduce establishment.”
And leaving straw on the surface should be avoided as it increased the risk from pigeons by giving them somewhere dry to land in winter, he explained. “You must mix it in.”
Dates & rates debate
Sowing dates and seed rates generated plenty of discussion at the meeting, with Mr Spink emphasising that earlier drilling did not automatically ensure higher yields.
Welsh work in the 1980s found that October and even November sowings outyielded September ones.
More recent ARC trials in five areas showed that, apart from in Scotland, there was little yield difference between sowing in the last two weeks of August and the first two in September. Indeed, in Essex, drilling later was more productive.
“I’m not saying you should drill late,” said Mr Spink. “But it can be OK.”
However, growers intentionally delaying or getting behind because of bad weather should be prepared to increase seed rates and use insecticide treatments. “Low seed rates drilled late are risky.”
Over-thick crops had to be avoided as they led to inefficient canopies with excess flowers reflecting too much light, Mr Spink explained, and seed rates and sowing dates had a significant impact on the green area index (GAI) in spring – see table.
Impact of sowing date and rate on spring growth
GAI in mid-April
The target should be 20-30 well-established, weed-competing plants per square metre by spring, he said.
In decent seed-beds that could be achieved with as little as 2.5kg/ha of seed. But there was always the temptation, especially when using home-saved seed, to increase the apparent optimum to allow for losses from slugs, pigeons and winter kill, he acknowledged.
Ian Rudge, Bedfordia Farms’ arable manager, describing oilseed rape as probably the most “fickle” of crops to establish, felt its agronomy had not kept pace with the changing climate.
“We’ve got to learn to establish rape as well as we do cereals.”
Warmer autumns were releasing more nitrogen through mineralisation, he noted. “And we don’t seem to have winters any longer south of the border.
“I’m not worried about establishing rape in August any more. I’m now quite happy with sowing from the end of August into September.” All his 500ha (1235 acres) of rape was min-till established.
But Somerset’s Rob Addicott, who grows the crop on six soil types and part-ploughing for the first time this season to encourage rooting, felt obliged not to delay sowing on his heavy clay. “I need it to get away early.”
Growers should be prepared to be flexible in their approaches to sowing oilseed rape, says John Spink.
David Colville felt much the same for his 60ha (150-acre) Aberdeenshire crop – all established after ploughing, having a mid-August target sowing date. “For us it’s the earlier the better. If the pigeons want to eat it so be it, but at least it will have got its tap root down.”
Philip Smith of Perth, with a similar plough-based system, also aimed to start then and have all his 40ha (100 acres) in by the end of August to counter the threats of slugs, rabbits, pigeons and sometimes low germination. “Our biggest challenge is getting the populations right.”
Variety mix to cater for club root
With about a third of his oilseed rape area potentially at risk from club root, Mr Smith said his only realistic variety choice for infected fields was the relatively low yielding variety Mendel.
But the infection was patchy, he pointed out. “So we’re mixing in 25% of a conventional variety, NK Grace or Catana, to see if we can take advantage of the higher yield in those parts where there’s no club root threat.”