How to optimise green area index in oilseed rape

Applying fertiliser at drilling improves establishment

The construction of the canopy starts with decisions around the optimum plant population and producing a seedbed and soil structure conducive to rapid, even germination and emergence. Establishment can be improved by ensuring the availability of the key nutrients through fertiliser application at drilling, and the removal of weeds through timely herbicide applications.

Trials have shown that light interception is optimised at a green area index (GAI) of between 3 and 4, however the canopy structure of this crop is important for optimising photosynthetic rates. Different components of the plant have differing photosynthetic abilities, with the leaves being most efficient dry matter producers and the stems the least. It is therefore important to ensure that the GAI is between 3 and 4 and that there is the correct leaf to stem ratio in order to make the most efficient use of the light that is intercepted.

Where canopies are too dense, the number of leaves per plant is reduced and light penetration to the leaves and lower pods is reduced and can lead to abortion of seeds within developing pods. The first formed pods on the terminal raceme are those with the highest yield potential, however these are likely to abort where shading occur, having a major impact on yield. Dense canopies are also more likely to lodge as plants have thinner weaker stems. Lodging can have a severe impact on yield, with losses of up to 30% possible.

Nitrogen and sulphur are important for canopy development

To ensure continued canopy development through to the spring and beyond applications of the key nutrients should be made in line with demand. Nitrogen and sulphur are of particular importance, whilst as soils become cold and wet, soil phosphate supply can be restricted. Potassium is in the greatest demand and is essential towards maintaining the structure / architecture of the canopy and nutrient flow around the plant.

During the autumn and spring period roots and shoots are growing rapidly with maximum cell division occurring giving a high demand for boron and calcium which needs to be satisfied. Manganese and molybdenum will also be important to ensure effcient nitrogen utilisation / metabolism within the plant. Poor nitrogen metabolism can lead to a build-up of nitrate in the leaf that may act as a food source for invasive diseases.

The greatest nutrient demand per day will come as the canopy really starts to expand approximately 150-180 days from sowing.

The spring nitrogen application will be the main determinant of the canopy size and structure. To produce each unit of green leaf area the oilseed rape plant needs to have taken up 50kg/N per ha so to achieve the optimum target GAI of 3.5 the crop need to have taken up 175 kg N/ha.

This figure is somewhat less than the recommended application rates based on trials data and reflects nitrogen use efficiency of approximately 65% due mainly to environmental factors such as poor soil structure, water stress and compromised root systems.

Yara has developed two tools to help with the calculation of the optimum nitrogen rate: Yara Image-IT, the smartphone application, and the Yara N-Sensor with absolute calibration mode.