GROWERS received expert advice on oilseed rape canopy management at a meeting near Northallerton on Thursday of last week.
Achieving high seed set rates was crucial to maximising yields, explained Pete Berry of ADAS. This stage occurred two to three weeks post-flowering and would require careful management.
“A crop with a large, uniform canopy may look promising, but an over-sized, thick flower layer will intercept light and prevent it from reaching other photosyntheticallyactive areas of the plant,” said Dr Berry. “Therefore, yields are likely to be disappointing. It can also greatly increase the risk of lodging, which can cause losses as high as 50pc.
“It’s best to aim for a Green Area Index (GAI) of 3.5 at flowering, to maximise the number of seeds set per square metre. Phone apps which use a photograph of the crop to estimate GAI are useful. However, they rely on measuring the percentage of ground that is visible, relative to the crop, so they lose accuracy as crop size increases.”
The correct use of nitrogen (N) to manipulate crop growth pattern was an essential management tool, he told delegates. Timing was critical, with studies suggesting later applications would normally prove more effective.
He cited a trial in which two treatments of 120kgs/nitrogen/ha were applied on March 10 and April 12, with yields averaging 4.47t/ha.
Meanwhile, the same total amount of fertiliser, but with 140kgs/N/ha applied on April 12 and 100kgs/N/ha on April 25, produced yields of 4.78t/ha.
“It may require some courage to hold off N applications until later in the season, but research shows that it does benefit the crop. If N is applied too early, it will simply increase stem length and heighten the risk of lodging,” said Dr Berry. He reminded growers that the crop must take up approximately 50kgs/N/ha, to produce one GAI.
In general, N fertiliser was 60pc efficient, but the figure varied widely, depending on water-logging, soil type and other factors. Achieving an optimum GAI of 3.5 at flowering was generally considered sufficient to achieve 3.5 t/ha. An additional 30kgs of applied N were required to produce every extra 0.5t of oilseed rape, above a 3.5t/ha yield.
Dr Pete Berry of ADAS
“As the seeds develop, they take N from the pod wall and the canopy starts to die off. If there is insufficient nitrogen available, the crop will die off prematurely and fail to achieve its yield potential.
“It’s important not to over-estimate N levels in the crop, because in a mild autumn and winter, the plants will keep growing and the concentration of N in the tissue can be diluted,” he said.