How to measure Green Area Index accurately in your rape
With the widely variable sizes between oilseed rape crops this season, rape growers will need to assess each crop individually for growth regulation and nitrogen in order to optimise canopy and maximise yields. Developed by BASF and ADAS, the web-based Canopy GAI tool on the www.totaloilseedcare.co.uk web site, will make this assessment much easier and more accurate.
"The aim of canopy management in oilseed rape in the spring is to create an optimum sized canopy that is able to maximise the crop’s yield potential. It involves tailoring spring inputs to the state of the crop, its canopy size, its plant population as well as seasonal and environmental factors. But you need to be able to accurately assess when and where inputs are required. This is quite a challenge this spring as there is enormous variation between crops. Some rape crops are very small and under-developed, whilst others have developed normally and have 6-7 leaves. It is clear that each crop will benefit from individual assessment and different canopy management strategies applied," says Clare Tucker, Technical Manager for BASF.
"This is where using the Green Area Index (GAI) comes in. The GAI is the ratio of green tissue area to ground area and it is a way of measuring canopy size. Economic benefits from plant growth regulators have been observed in crops that have a GAI greater than 1.0 in March or crops with a GAI greater than 2.0 in April," she says.
"In the past, growers assessed GAI visually or used reference photographs. But now advisors and growers have an easy to use on-line tool at www.totaloilseedcare.co.uk where photographs of oilseed rape crops can be uploaded and an instant assessment of Green Area Index relayed back on screen."
"This tool is a practical advancement in managing canopies effectively and makes life a lot easier. All farmers and advisors have to do is to go on-line at the www.totaloilseedcare.co.uk web site at any time, click on the Canopy GAI tool button and follow the instructions. It is very straight forward to upload photographs and, once loaded onto the site, an accurate assessment of GAI is relayed on the screen within a few seconds."
"We would advise that digital photographs in a JPEG (jpeg) format should be taken looking down on the crop and on a cloudy day to avoid shadows. Set the resolution low (less than 2MB) on a digital camera; mobile phones are fine as long as more than 1 megapixel resolution."
With the wide variation between oilseed rape crops this season, rape growers will need to assess each crop individually for growth regulation and nitrogen management in order to optimise the crop canopy and ultimately maximise yields. The aim is to build a crop canopy with a Green Area Index of 3.5 at the start of flowering. This allows the crop to optimise photosynthesis during the important yield building phase after mid flowering," says Ms. Tucker.
"If the GAI is 2 or more in April then apply Caramba at yellow bud. This produces more uniform flowering across the main and side branches; by strengthening the side branches it can help the plants ’lock’ together and reduce late lodging. It is also an excellent first spray in the Sclerotinia programme, reflected by ADAS trials in 2008, allowing good coverage of the lower canopy and protection from any early petal fall. Follow it with Filan or Compass about 3 weeks later at the start of the main petal fall period."
