Winter oilseed rape needs nitrogen now

Most winter oilseed rape crops could benefit from nitrogen fertiliser soon, if they haven’t already had their first dressing, according to specialists.

Ideally soil mineral nitrogen tests and green area index (GAI)* assessments should be used to fine-tune the amounts applied, they add.

In marked contrast to last season, when many over-lush crops went into the winter, this year’s stands are more “normal” after last autumn’s relatively poor establishment.

“This year’s crops vary from normal to small,” said ADAS‘s John Spink. “Last year we had to deal with some very big crops.”

By this time last season some had already absorbed up to 100kg/ha on N from the soil, he noted.

“The N content in reasonable crops this year is 50-60kg/ha, but some with plants only 2in high will have next to nothing in them – say 5kg.”

On the plus side ADAS soil mineral nitrogen tests in the past month show levels are 20kg/ha up on the previous two years.

“It’s been wet but there’s also been a lot of mineralisation as soil temperatures have been 2-3 degrees above normal.

“If the GAI is over 1.5 you definitely need to hold back or you’ll end up with excessively thick canopies.

“But small, backward crops, where there’s still good yield potential, need a kick now to help produce sufficient pod numbers. I’d be thinking about 30-40kg/ha of N, with the bulk in the main dressing in another two weeks’ time. But I’d probably hold back a bit for later to keep the crop going during pod fill.”

Given no leaching risk, a reasonably good dose of N, say 50-100kg/ha, should be applied soon, suggested Masstock‘s David Langton. But growers should use soil N-Min tests and make allowances for the results, he urged.

“Some crops are starting from quite a small base and need at least 150kg [in total] to build the canopy. Most GAIs at the moment are 0.5-1.”

Plants might look small but there had been good rooting which would help take up the N and avoid leaching, he added.

“Nobody wants to lose expensive N down the drain.”

Most crops appeared more backward than a year ago, acknowledged Syngenta‘s Nigel Padbury.

“I don’t think recent frosts will have affected them too much as most plants have reached the six to eight true leaves ‘safety’ stage. But they’re wide open to pigeon and other animal grazing.

“So it’s certainly worth getting some N on soon, perhaps 30-40% of the total – say 50-80kg/ha.”

United Oilseeds‘ Richard Elsdon was more cautious.

“I’m a great fan of the GAI approach.

“I agree with applying a small amount now, but probably no more than 15-20kg/ha. The soil’s still cold, the plants are small and fertiliser is very expensive. Let’s put most on when the plants can really use it.”

* GAI can be assessed by weighing 1sq m of growth in kg and multiplying by 0.8. The alternative is to visit and upload a digital photo of the crop for guidance.

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