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Early disease control and nitrogen keys to preserving OSR potential

Well-judged fertiliser inputs and effective light leaf spot control will be important to maintain the good yield potential of well-established winter oilseed rape crops this season.


Well-judged fertiliser inputs and effective light leaf spot control will be important to maintain the good yield potential of well-established winter oilseed rape crops this season.


This was the conclusion of a roundtable discussion in Wiltshire organised by Bayer CropScience and hosted by arable manager Martin Smart. Overseeing 1,600 hectares (3,950 acres) for Ashton Farms, Trowbridge, he grows 350ha (865 acres) of winter rape.


Mr Smart is one of the company’s 40 InVigor Developers - farmers conducting trials examining the impact of different inputs on varieties - 14 in his case, including the firm’s Harper, Fencer and Brother.

Crop nutrition

Monitoring air and soil temperatures daily, Mr Smart does not expect to apply fertiliser to rape before the second week in February when the crop usually wakes. “I believe what actually drives it is day length,” he said.


GrowHow’s Ross Leadbeater said growth could not be expected until the soil temperature reached 5degC, but last season it never fell below 5degC. “We had mineralisation going on all through autumn and winter.”


However, high winter rainfall meant much of the nitrogen from the process had been leached and was unavailable to crops, some of which had achieved a green area index (GAI) of 2 by November.


“Last spring we had very low residual levels of nitrogen, so crops ran out and early nitrogen was a must. From what I am seeing so far, it looks as though it will be a must again.”


So far, this season’s crops were ‘not over big’, with GAIs of 0.5-0.75, depending on variety. Although Mr Leadbeater had not yet conducted GrowHow N-Min analyses on heavy land, early tests on chalk soils in Hampshire suggested reserves would not be high.


“The N-Min tests I have done found about 70kg/ha of nitrogen in the soil reserve; but only 10kg is readily available to the growing crop.”


Mr Leadbeater said the majority would not be available until soil temperatures were consistently above 5degC.


“So, early nitrogen - as soon as Martin can travel and the temperature is above 5degC - will be the order of the day.”


Last season, Mr Smart’s split tramline trial which doubled the early nitrogen dose - partly to ensure the crop had sufficient sulphur via his Heartland 24:8:8:8 fertiliser - produced the highest yield, reinforcing the view early N was useful.


Magnesium deficiency showed in the variety Compass last year, so up to 100kg/ha of kieserite will also be applied to avoid a shortage.


Target yield for the trial plots, even on this year’s brash soil, is five tonnes per hectare (2t/acre). Last year’s trial yields off similar land ranged from 3.5-6t/ha (1.4-2.4t/acre), with the highest oil content rated at 47 per cent.


Mr Leadbeater’s overall nitrogen applications plan for this season is for 100kg N/ha in late February, with the same amount at stem extension, and 50kg N/ha as late as possible, applied using the farm’s Bateman Hi-Lift sprayer.


The farm’s trials also include tests of novel spray nozzles and fungicides, including those with growth regulatory properties.


Bayer commercial technical manager Ben Giles said flea beetle had damaged crops unprotected by the recently withdrawn neonicotinoid seed dressings. One of the firm’s trial sites had been lost to the pest, while other fields had been re-drilled twice and some patchily established stands would be hard to manage.


“But the issues have been very regionalised,” said Mr Giles. “It is mainly in Cambridgeshire, Essex and Bedfordshire - there have been some big problems around Bedford.”


Crops elsewhere, despite apparently taking a hit and turning purple in the recent cold snap, remained full of promise provided they received appropriate fertiliser dressings and effective disease and pest control, the meeting agreed.


All Mr Smart’s trial varieties, grown in blocks 100 metres long by 12m wide, had come through winter well but there was a long way to go before harvest, he said. “At the moment I like Extrovert and, out of the Bayer ones, I think Fencer has the edge on Harper and Brother.


Mr Giles explained oilseed rape fungicides fall into three groups. “From now until about green bud, you have got the light leaf spot types, if you feel they are necessary. You have then got the short window fungicides - metconazole and tebuconazole, which are PGR-active for canopy manipulation - and then the sclerotinia products.”


This season’s crops are more reasonably-sized than those at the same time last year, but have well-developed roots. So the need for a growth regulatory fungicide was not pressing.


However, although light leaf spot symptoms were only just starting to show in untreated crops, controlling the disease was vital.


“If you look at the Rothamsted guide, the risk is huge - the whole map is red. So it is very important to monitor crops from this point forward,” Mr Giles said.


He highlighted two potential problems with the disease which, uncontrolled, could cut yields by up to 50 per cent; the early pale yellow circular symptoms could be confused with frost or general environmental damage and southern growers might be unfamiliar with it.


“If you see something and you do not know what it is, get it checked,” he said.


Infected leaves incubated in plastic bags in warm conditions would soon show the classic white puffiness of spores around the yellow circles.


“Until the last couple of years people had not really seen it down this far in England. Last year was horrendous. People were seeing things on stems they just did not recognise - black halos on stems and then the pods.”


The reason was they had not applied the early spring fungicide critical for light leaf spot control, he said.


“By the time you get to stem extension, it is too late. You can damp it down if you use the right products, but you will not get on top of it.”


Few varieties have more than moderate light leaf spot resistance, he warned.


Even growers who applied late autumn phoma-control sprays should be prepared to apply a light leaf spot treatment, such as Proline (prothioconazole), as soon as they detected the disease, said Mr Giles.

Round the table

  • Martin Smart, arable manager, Ashton Farms, Wiltshire
  • Ben Giles, Bayer CropScience commercial technical manager
  • Ross Leadbeater, GrowHow plant nutrition advice manager


Key points

  • Crops are likely to require early nitrogen
  • This season’s crops are ‘not over big’ – GAIs typically 0.5-0.75
  • Soil N reserves unlikely to be high
  • Light leaf spot (LLS) symptoms starting to show
  • Few varieties have good resistance to LLS
  • Early spring fungicide vital for effective LLS control
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