Talking Agronomy: Chris Martin - Creating the optimum OSR canopy

Taken from Arable Farming, March 2017


Most oilseed rape crops across the region have come through early winter fairly well. Phoma lesions are easy to find on the usual suspect varieties, but of more concern, visible symptoms of light leaf spot (LLS), which had a major impact on last year's rape crop yields, have been appearing in many crops from early January.


Even leaves from crops which appeared clean and had received a robust autumn fungicide soon showed the symptoms after a couple of days in a plastic bag in the airing cupboard. Any fields which have not yet been treated since the New Year should now be sprayed as a priority.


With most crops relatively forward, nitrogen quantities and timings, as well as appropriate plant growth regulators, could have a major role to play this spring if we are going to maximise potential.


The biggest influence on rape yields is the number of seeds harvested, and the seed number is principally determined in a period of only two to three weeks at the end of flowering. During this critical period, it is important we have built a crop canopy which is going to photosynthesise as efficiently as possible, in order to maximise seed number.


Too thin a canopy, and there will not be enough green material to intercept sufficient amounts of our most precious resource, solar radiation. Conversely, too thick a canopy will also lead to a suboptimal number of seeds as flowers will reflect sunlight, essentially wasting it. Thicker canopies also tend to create a more humid microclimate which encourages disease, which can both steal photosynthetic area and also reduce the efficiency of other vital plant functions.


The aim, therefore, is to build a canopy with a Green Area Index (GAI) of about 3.5-4 at mid-flowering. From above, you could probably be able to see up to 5% ground, so significantly smaller than most of us would have historically strived for.


As each GAI of OSR contains around SOkg/ha N, the optimum crop at the end of flowering should contain around l 75-200kg/ha of N in total during this critical period. Nitrogen applications and timings should therefore be tailored to build up to this optimum canopy without  overshooting it.


Once we have successfully built this optimal can­opy, in order to maximise seed numbers, it's then important we continue to keep the crop green and healthy for as long as possible, in order to maximise seed fill.


While this trait is determined by a thermal period of time, we still need to ensure the crop has access to all the nutrients and moisture it needs to make photosynthesis as efficient as possible during this period. Soil structure, establishment choice and inclusion of companion plants at sowing can all have a big role to play in encouraging rooting of the rape crop to help it achieve this.


Additional nitrogen to that required to build the optimum canopy at flowering will also have a big role to play. Solid N applications as late as farm machinery will permit an even spread, or foliar nitrogen alongside the main sclerotinia spray have both proved useful in maintaining the crop's Green Area Duration (GAD), thus maximising seed fill.


Nitrogen stabilisers such as N-Lock applied in early spring could have a major role to play here by helping keep nitrogen available for longer so it prolongs GAD, rather than overcooking the canopy at flowering.


09 March 2017

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