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Oilseed Crops Require Careful Spring Canopy Management

15 February 2012

UK - Following near perfect conditions for oilseed rape crops to establish during the autumn of 2011, many arable farmers will need to deploy a careful canopy management strategy to ensure that crops which have grown too far forward don’t suffer reduced seed yields at harvest.

That is the cautionary advice from Richard Elsdon, technical manager for United Oilseeds, who warns that history proves the UK farmer is not good at managing forward crops of oilseed rape.

“The exceptionally cold conditions last winter ensured that growth rates of crops which had got too far ahead were curtailed, resulting in plants of a more manageable size going onto the spring,” Mr Elsdon describes. “The ensuing dry spring limited growth further, so that crops in many areas were typically 30-35cm shorter than normal. This reduced growth, and a thinner canopy, ensured that light could penetrate the canopy and keep leaves lower down the stem green while flowering took place.

“After flowering, plants with an optimal amount of canopy were able to photosynthesize to a far greater level than those with extensive canopy growth, enabling them to produce more energy. This in turn allowed the development of a greater quantity of oil-rich seeds. As a result, last year’s harvest gave some exceptionally high yields.”

In contrast, the 2006-07 season produced one of the poorest yielding crops of the last ten years. As was the case last year, crops grew vigorously in the autumn producing huge plants going into the winter. However, a relatively mild winter meant that crops continued to roar away into the spring, producing a massive top canopy and poor yields.

“With weather patterns becoming increasingly unpredictable we simply cannot depend on nature to come to our rescue again this season,” Mr Elsdon warns.

“Last autumn was characterised by a prolonged period of conditions that were well suited to vigourous establishment and excessive plant growth,” he adds. “By late November many current crops had already achieved a Green Area Index (GAI) of 3.4, whereas the ideal for that time of year should typically be much lower at around 1.5.”

Mr Elsdon warns these large crops will need very little in the way of nitrogen prior to the spring, but will still need their full allowance of sulphur if they are to yield well. “Work carried out for HGCA by Dr Steve McGrath from Rothamsted Research has shown that 70-80kg per hectare of sulphur as SO3 can be applied as late as April without compromising yield.

“As most farmers apply sulphur as ammonium sulphate, a later than usual application will reduce the nitrogen growth surge as the crop will by then be 30cm or so tall.”

In an ideal world, Mr Elsdon believes that forward crops should receive sulphur as magnesium sulphate so that nitrogen calculations and applications can be carried out separately.

Mr Elsdon also states that no matter how large crops have grown, all should have their GAI measured in late February – just before the spring growth spurt takes place. This can either be done using a digital camera and downloading the image to a GAI website, or by using a dedicated smart phone application which can give instant results from the field. A more accurate, but more onerous method of calculating GAI, is to measure the weight of crop growing in a square metre. The weight in kilos multiplied by 0.8 gives the actual GAI of the crop.

“At the end of February, a GAI of up to 2.5 is fine and shouldn’t give cause for any alarm bells to ring. Above this, farmers will need to think carefully to ensure their spring nutrient strategy doesn’t provide too much nitrogen to already advanced crops.”

TheCropSite News Desk

Rape, Growing/Soil conditions, Risk Management

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