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Nitrogen requirements

Date Added: 27/01/2011

This year's crop of winter-sown oilseed rape is looking more advanced compared to the same time last year, but that doesn't mean that growers should take their eye off the ball in terms of ongoing crop and canopy management.

The advice of Richard Elsdon, Technical Manager for United Oilseeds, who believes that there is a degree of optimism amongst growers that must be tempered to ensure final yields live up to expectations.

"With ex-farm oilseed rape prices recently breaking through the £400/tonne ceiling, it is particuarly important for growers to manage their crops properly this spring in order to maximise their returns from investment", Mr Elsdon states.

"OSR growers should carry out at least one Green Area Index (GAI) assesmsent and preferably two or more at different stages throughout the spring to get the best understanding of how the leaf area of their oilseed rape crops is either progressing or regressing.  This information can then be used to amend spring fertiliser application rates accordingly".

Mr Elson also warns growers that they shouldn't be complacement about the ability of forward crops to produce high final yields.  "Crops that are ahead coming out of winter can just as quickly go backwards if damaged by a spell of severe spring frost", he warns.

"Conversely, forward crops that remain unscathed by cold weather can grow away too quickly in the spring and therfore shouldn't receive any additional nitrogen until the plants are well into the extension stage.  Growers  must therefore understand the true position of their crops to ensure the correct timing and rate of nitrogen application".

"Applying too much spring nitrogen to crops that are already at an advanced stage will be counter-production and will lead to overly vigorous leaf canopy growth", he adds.  "This will result in poor light interception later in the growing season".

Excessive canopy cover can often make crops look as if they will produce an abundance of flowers and seed pods, but crops with too many flowers will reflect too much sunlight away from the plant and have a detrimental affect on the survival of lower leaves Mr Elsdon warns.

"When petals subsequently fall from the plant, there will be a shortage of leaves and the plant will suffer reduced rates of photosynthesis. This will have a knock-on effect on final yields with fewer and smaller seeds collected at harvest. Oil content will also be adversely affected.

"Farmers must therefore do everything they can to apply the right amount of nitrogen and to manage plant development for optimal yield and oil content. For forward looking crops, the application of spring nitrogen should be delayed until plants have produced at least six inches of stem extension. This will minimise excessive vegetative growth later on and avoid unnecessary canopy development," he advises.

The PLANET website at is a computerised version of Defra's ‘Fertiliser Recommendations' booklet and can give growers a clearer indication of how much residual nitrogen there is in the soil so that they can adjust their fertiliser applications accordingly.

In the most extreme circumstances, Mr Elsdon expects growers that fail to properly manage their oilseed rape through the late winter and early spring could see final yields drop by as much as 30%.

"With crop prices finally reaching a sustainable level, it would be a shame for growers not to take full advantage of the market," he concludes.



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