Double up on lush oilseed rape canopies to hit high yields

Following a mild autumn and winter, many of the oilseed rape crops in the UK are further forward than normal, and will need to be carefully managed in order to achieve high yields.

Last season, while some oilseed rape crops promised high yields throughout the growing season, they ultimately failed to deliver, with some experts pointing to oversized canopies as a major reason.

This season growers are being urged to double up on their canopy management to unlock the true potential of the crop, by delaying nitrogen applications and ensuring they use a plant growth regulator.

See also: Soil nitrogen testing is critical this season, farmers told

A fall in prices, high pest pressure and the loss of key pesticides mean attention to detail has never been more important for oilseed rape growers, says Adas senior research consultant, Peter Berry.


In 2012 a Adas report suggested growers were underestimating the extent of the problem and the impact it was having on yields.

The Basf-funded survey carried out by Adas showed more than 99% of fields showed some sort of lodging, with much of it in the centre of the field out of the view of the farmer.

On average 35% of oilseed rape cropping area lodged and it is estimated that lodging could reduce yields by up to 50%.

“Even leaning 25deg from the vertical, it can still give yield losses of 15-25%, due to the very top of the canopy intercepting the all the light,” he explains.

He points out aside from crops struck by flea beetle damage in Eastern regions establishing slightly later, the majority of the UK is seeing lush forward crops.

“Oilseed rape crops are very forward at the minute with above average green area indexes (GAIs) for this time of year, so will need reining back in,” he explains.


He adds that a combination of nitrogen manipulation and PGRs will be needed this season, when other seasons a focus on one would have sufficed.

His formula for a high yielding oilseed rape crop is to use the nitrogen already in the crop, before applying the first fertiliser split in the programme.

“Nitrogen has the biggest effect on canopies, so reducing the nitrogen will be the first port of call for many growers, with forward crops,” he says.

Work funded by GrowHow has shown that delaying N into April for a crop with a GAI of 1.4 increased yield by 0.4t/ha.

Growers need to assess how big the canopy is and how much nitrogen is in the canopy to tailor their nitrogen accordingly.

“The target is to achieve an optimum-sized canopy at flowering and then prolong canopy greenness as long as necessary to fill all the seeds.”

Despite the latter part of the winter still posing the threat of pest damage and cold weather, measuring the GAI of the crop will be important to gauge when and where to apply both nitrogen and plant growth regulators.

There are number of options available in order to measure the GAI of a crop including BASF’s smartphone app and HGCA Green Area Index Tool, available online.

High crop density

While growers typically aim for even plant populations of between 40 plants/sq m, to maximise light penetration into the canopy, plant populations will be high on some farms, following a knee-jerk reaction to combat the threat of pests such as flea beetle and slugs.

Most crops are well above the GAI of 1.0, with many hovering around the 2.0 and 3.0 mark.

Growers who still have these forward crops, in mid- to late-February should consider delaying or cutting out their first nitrogen split altogether.

“The big canopies can remobilise the nitrogen from their leaves and stem, into the pod and achieve a good yield so there are potential cost savings by applying less fertiliser,” he says.

Each unit of GAI requires 50kg/ha of N, so to achieve a GAI of 3.5 at mid-flower a 3.5t/ha crop needs about 175kg/ha.

An extra 30kg/ha for each additional 0.5t/ha of yield expected (above 3.5t/ha) should be added back to the adjusted total to achieve the final N rate, although growers need to assess crops on a field-by-field basis.

Canopy management

  • Canopies must be large enough to intercept most available light, but no bigger
  • Over-large canopies cause increased lodging and poor penetration of light to lower pods, affecting seed numbers and filling
  • Causes of large canopies include early sowing, high plant density, warm autumns and mild winters, high levels of soil N and early N application
  • Triazole fungicides such as metconazole regulate growth and can benefit yield in crops with large canopies
  • Canopy size is measured by green area index which shows the ratio of green tissue area to ground area

Soil mineral nitrogen testing

In addition, soil mineral nitrogen (SMN) testing can also be used in conjunction with the GAI figures and RB209 Fertiliser Manual values.

Soil nitrogen levels are notoriously variable, and experts suggest, testing could be worthwhile to help tailor fertiliser applications to crop needs.

Growth regulation

Independent AICC agronomist Peter Cowlrick of CCC says growth regulators are likely to play a crucial role in oilseed rape this year, to help manage some of the denser canopies coming through the winter.

He explains that lodging in oilseed rape crops can cut yields in half and points to some of the newer plant growth regulator (PGR) products to help combat this, having performed well in trials.

“By using a PGR you generate a more open canopy, through suppressing apical dominance resulting in stronger lower branches and allowing more light through to these lower branches,” he says.

AICC trials in 2014 showed that applications of 0.8-1.2 litre/ha of BASF’s metconazole + mepiquat based Caryx, helped reduce lodging by up to 80-100% where lodging was evident.

While it is one of the more expensive growth regulator options in oilseed rape, it will pay for itself by securing yield potential in scenarios where lodging potential exists, such as a GAI greater than 1.5-2 in early March.

“In these thick canopies, it should be the first product you use because it is more active in the fluctuating temperatures you can get in early spring, with typically cold nights and warmer days,” he explains.

He points out timing will be key with PGR, and should not be applied before mid-stem extension, to maintain their efficacy and achieve the greatest height reduction.

Applications should ideally be made when the green bud is extending above the canopy, with the last application at yellow bud, providing a tight window for growers, he says.

Caryx has three key effects on the plant; it helps promote a better canopy, protects against lodging and promotes rooting.

Rooting benefits of up to 25% at a depth of 40cm may be especially beneficial in a dry year, with yield benefits of up to 0.25t/ha, according to Adas.

Disease threat

While PGR’s remain crucial to prevent lodging, the disease threat will once again loom large, with light leaf spot, one of the main yield robbers last season.

“The weakness of metconazole products are that they are not very good on light leaf spot, so if you have got it in the crop then a tebuconazole and/or prothioconazole approach would be sensible,” he explains.

Tebuconazole products such as Folicur have also performed well in PGR trials and in the field, and still offer a cost effective alternative on the more average canopies (GAI 1-1.5) and for controlling disease.

He reminds growers that yield reductions of up to 0.4t/ha can occur, when PGR are applied on crops with a GAI below 0.8 in March.

“It needs to be actively growing and well nourished with a reasonable canopy to justify use,” he says.


Mr Cowlrick warns a shift to taller varieties such as Picto, Campus and Trinity, will also have to be taken into consideration by growers.

“They would place a greater emphasis on some sort of growth regulation on stronger crops within the programme, and growers need to be careful not be caught out,” he adds

He advises considering the standing power and height of new varieties and benchmark these against previous varieties grown.

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