Big OSR crops will benefit from flowering N

Hitting the right timings for growth regulators and fertiliser splits this spring will be even trickier given the wide range in plant sizes and growth stages.

The cost of getting canopy management wrong is high with even a modest leaning crop resulting in a 1.2t/ha loss in rapeseed yield.

This season, there is also a higher risk of premature pod senescence and yield loss in some forward crops, as large canopies run out of steam at pod fill.

Therefore, with oilseed rape margins already slim, growers cannot afford to lose out to lodging and will need to maximise seed numbers.

See also: How to assess and regulate oilseed rape growth this spring

Dekalb oilseed rape technical support Will Vaughan France highlighted that crops were about a month ahead of where they ought to be at this time of the year, but he doubted they will be a month earlier to flower.

“It could, therefore, be a challenging spring with a long time to flowering and crops will need to be carefully managed in this period.”

Yara agronomist Ian Matts said he had seen a wide range of crop sizes from huge, forward crops with a green area index of 3.0 down to those with a GAI of under 1.0, which were later drilled with no seed-bed fertiliser.

Trials at the 3X3 site near Lincoln show that the plots which received 20kg/ha each of seed-bed N, P and K have taken advantage of the milder winter and have double the green area index (2.0), measured 10 days ago, than those that didn’t receive any (1.0). Varietal differences are also being seen.

Fertiliser strategy

These contrasting sizes will require different approaches and while the big canopies will allow savings in fertiliser use, this may be the season when growers should opt for an additional flowering application.

His advice is for two application timings, the first, which goes on in two splits, is to build the canopy while the second is to maintain the canopy.

Crops with a GAI of 2.0 already have 100kg of N and those with an index of 3.0 have 150kg, so need less fertiliser.

But even these forward plants will need some nitrogen applied in the first split or the plants will bolt through their growth stages, he explains.

He suggests applying 20-40kg when plants start actively growing, normally at end of February, but is nearer the middle of this month this season.

The remainder goes on the second split and the timing depends on the size of the first split. “Plants use 2.5kg a day so when only applying 40kg first split, they need more in about 20 days.”

Flowering application

Then to maintain canopy, consider a flowering application to maintain the green area index and trials have consistently shown a 0.3-0.4t/ha benefit to this.

“This year, even after applying a plant growth regulator, you may end up with bigger canopy than desirable and you need to maintain this through the season to avoid pods being aborted later on.

“This is because the seeds start to strip nitrogen from the pod and if this happens too early, you get premature senescence and lose seeds, which means lower yield,” he told Farmers Weekly.

A flowering application is typically 40-60kgN/ha and farmers should avoid applying on a hot day to minimise scorch risk; it can be simply added to the sclerotinia tank mix.

PGRs and lodging

Maximising yields is not just about nutrition. Growth regulators have a key role and BASF agronomy manager Ruth Stanley says it’s all about maximising the number of seeds/sq m.

Using a plant growth regulator produces a canopy that has more branching and minimises lodging.

“Lodging is not just a pain at harvest trying to get the crop combined, but there is a loss of yield.”

Adas looked at the yield impact with plants at 90, 45 and 22.5deg.

“An angle of 22.5deg is not what many would call lodging, but is just a leaning crop. At mid-seed fill, this can lead to 1.2t/ha loss in yield.”

She explained that it is due to less light penetrating the leaning canopy to reach pods and leaves lower down.

This extra branching in unlodged crops treated with Caryx (mepiquat + metconazole) resulted in an extra 0.5t/ha in Adas trials. It can be applied at stem extension to early flowering.

But this season, she said there is challenge of the early light leaf spot pressure, which may need an additional spray, such as metconazole + Filan (boscalid) or prothioconazole if the pressure is really severe.

“Timing is a challenge with varying sizes of crop and growth stages, plus the ability of getting on to land means it maybe a compromise,” she said.