Big crop canopy differences from nitrogen timings

Dramatic differences due to nitrogen timings are showing up in oilseed rape trials on drought-prone land at the meeting’s host farm.

The first of a range of N dressings on the GrowHow/HGCA/Bayer experiments at Fawley Court Farm, designed to see if semi-dwarf varieties might need specific N management, had been applied at the end of February. Manager Mark Wood’s first treatments, 80kg/ha plus sulphur, had not gone on until 12 March.

Dramatic differences were already showing between the trial treatments which include one with an initial half total dose of 180kg/ha intended to stretch the two varieties – Excalibur and semi-dwarf PR45D03 – to their limits.

“It’s amazing how similar they all looked only eight days ago,” said John Spink, then of ADAS but now with Eire’s Teagasc.

By contrast the surrounding Castille on the mainly sandy loam appeared relatively thin and not far off flowering. “We didn’t have any rain in March, and I think the roots just haven’t found the Sulphan yet,” said Mr Wood.

With hindsight it was easy to suggest that the commercial dressing should have gone on sooner, said Mr Spink. “But you wouldn’t have expected the whole of March to be dry. The trials have clearly got hold of the N and we’re seeing some massive responses.”

Nevertheless the farm crop still had plenty of potential, and was certainly more promising than the lush dark green Excalibur which would eventually receive 360kg/ha of N and so overshoot the optimum GAI of 3.5 by flowering, he explained.

Despite appearing relatively thin, Mr Wood’s fields drilled in late September were mainly uniform thanks to good establishment. Only rarely had trials shown any advantage in sowing at the start rather than the end of September, and good yields were possible from plant populations as low as 9/sq m (1/sq ft), noted Mr Spink.

One of team member Innes McEwan’s crops, which according to its GAI (Arable 20 Feb) needed no bag fertiliser, posed a potential problem with sulphur supply, noted GrowHow’s Tom Land.

“We need to get everything else right before knocking back the N,” he said.

Malate sulphur testing suggested Mr McEwan’s crop needed 100kg/ha of SO3. In theory that could be supplied by the firm’s Double Top, but applying enough to do so would supply far too much N, risking an overlarge canopy and reduced yield.

Using kieserite (magnesium sulphate) would be too expensive, so Mr Land’s recommended an alternative. “I’ll get into trouble for saying it, but I’m advising ammonium sulphate.”

Using enough to supply most of the necessary sulphur (allowing for the fact that the field had a history of organic manure use) would add only 27kg/ha of N. And because fertiliser N is used with only 60% efficiency that reduced the risk of over-supply, he explained.

Pre-harvest management

Given the wide range of oilseed rape growth stages within fields this season growers planning to desiccate should consider treating patches of different maturity separately, suggested Syngenta’s Iain Hamilton.

“They may well need to this year.”

A key benefit of desiccation was reduced drying costs – a big issue last harvest, he said. The main choice was between “true desiccation” via Reglone (diquat) or “slow death” using glyphosate, as in Touchdown, for about half the cost.

Each offered specific yield-saving advantages, while swathing, traditionally a more northern operation, fell somewhere between the two in terms of speeding harvest.

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