Good-looking oilseed rape could benefit from late nitrogen

Extra-late nitrogen could be particularly valuable in making the most of some of the best-structured winter oilseed rape canopies this season.

Crop specialist Pete Berry, at crop scientist group ADAS, said some of this year’s crops are the best looking at this stage that he has seen in recent years.

He adds that about 40kg/ha of foliar nitrogen applied at the end of flowering in oilseed rape crops has given an average yield response of more than 0.3t/ha in a total of 23 ADAS and other trials that he has assessed.

This late nitrogen had followed optimum inputs of soil-applied nitrogen for both canopies and yield development earlier in the season.

See also Delay nitrogen on forward oilseed rape crops

“Oil is formed relatively late in seed fill, so anything that can be done to prolong seed filling will lift oil production at harvest, providing the canopy is actively photosynthesising and capturing sufficient light,” he said.

Dr Berry was speaking to more than 100 oilseed rape growers and agronomists attending the AgriiFocus oilseed rape day at North Farm, Aldbourne, near Marlborough, Wiltshire, earlier this month.

“Active photosynthesis during seed fill is essential to seed yield because, unlike cereals, oilseed rape does not mobilise carbohydrate from the stem to support the developing seed,” he added.

“However, the seed does extract nitrogen from the pod walls. This makes late nitrogen valuable in providing sufficient nutrition to allow the seeds to grow without unduly hastening pod senescence,” Dr Berry said.

The value of late foliar nitrogen varies widely with crop condition and circumstances, said Dr Berry. Indeed, across 11 ADAS trials, gross output responses have ranged from zero to more than 0.4t/ha.

High temperatures of more than 19C at the time of application are known to limit the crop’s response to late foliar nitrogen, while crops that had sub-optimal soil applied nitrogen were expected to respond more strongly to late foliar nitrogen.

It is also possible that good canopy structure and yield potential are important factors in the equation.

“We know that open, well-branched canopies with plenty of photosynthesising lower leaves and pods are the secret to high seed numbers and good seed fill,” he stressed.

These type of canopies allow sunlight to penetrate much better below the upper layers than dense canopies of thin plants to improve the photosynthesis process.

“The optimal 20-30 plant/sq m populations with excellent primary and secondary branching we’re seeing in far more crops than usual this season, leaves them well-placed to make the most of whatever the summer brings,” he said.

Dr Berry added that a combination of appropriate seed rates, the open autumn and good plant growth regulation also means that crops are generally well-rooted with good access to the available soil moisture.

“These types of crops, with high yield potential, may well be good candidates for late foliar nitrogen because they will need a good supply of late nitrogen to realise their high yield potential,” said Dr Berry.

He added that the value of these late nitrogen applications will also depend an assessment of all the costs involved – including any reduced oil percentages and additional application expenses – as well as seed values.

“So I would urge growers and their agronomists to assess their particular crops and circumstances as soon as possible if they wish to take advantage of what could be an excellent opportunity for many,” he said.