Winter oilseed rape crops merit extra care to preserve their early promise, and growers should try to pinpoint what lies behind any patchiness to help drive yields upward.

The advice comes from the company behind the Best of British Oilseeds initiative which aims to help lift the UK average output of 3.1t/ha (25cwt/acre) beyond Germany’s 4.2t/ha (34cwt/acre) to 5t/ha (40cwt/acre).

The clue to better performance lies in canopy tweaking, David Langton from Masstock R&D is convinced.

“Most farms will be going into spring facing their biggest ever crops.

Some roots are phenomenal.

The challenge is capturing the yield potential that offers.”

Most UK growers still produce excessively leafy crops, says Mr Langton.

“The key from now on is to manipulate the canopy, and nitrogen is the key tool to use.”

The aim should be to apply just enough N to ensure crops hit stem extension with the optimum structure, and many forward crops will benefit from delaying and reducing first dressings.

Ideally soils should be sampled to find out how much N is already available.

Nationwide guidance, based on winter rainfall, is too imprecise especially on heavy and medium land, he believes.

“They also need to assess the green area index (GAI) to see how much N the crop has already absorbed so appropriate adjustments can be made.”

This involves harvesting 1sq m above-ground representative samples.

“Weigh each, in kilograms, and multiply by 0.8 to get the index.”

Each GAI unit contains 50kg/ha of N, and many crops already have GAIs of over 1 – some up to 2.

According to how much N is in the top 90cm of soil, the balance (to the 175kg/ha needed for a target yield of 3.5t/ha) can then be made up by fertiliser, assuming crops recover only 55-75% of what is applied depending on soil type.

Higher yield potential merits more N, but growers should take care.

“Higher N applications can lead to yield reductions, particularly if you induce early lodging – plant population is an important factor.”

andrew.blake@rbi.co.uk