How to get the best from the GAI tool in rape this year
Oilseed rape crops are coming through the cold winter conditions and many are past the midway point in the growing season. Unlike the last two years, the majority of oilseed rape crops this year will benefit from a PGR fungicide such as Sunorg Pro in order to create the optimum sized canopy. Many growers and advisors have and will be using the web-based GAI* tool on www.totaloilseedcare.co.uk to help.
"The BASF GAI tool has become something of an industry standard, helping you get your eye in on which crops are appropriate to treat and when. It is very straight forward and easy to use - you upload a photograph of the crop taken from above the crop on an overcast day and instantly will receive a GAI score," says Will Reyer of BASF.
"This year we have had a few enquiries relating to some surprising GAI indices, which clearly don’t match the crops in question. For example, high scores have been given when a crop has dying leaves lying on the soil. In these cases, sometimes the tool can have difficulty in distinguishing between dying tissue and green tissue."
Dr. Pete Berry of ADAS who helped develop the tool says that this is because the shade of green in dying leaves can be similar and sometimes difficult to differentiate from the shade of green in healthy crops. "In situations where you are unsure about the result from the GAI tool, then we recommend comparing the photo with the image on screen after it has been analysed by the online tool. Non-green areas should appear blacked out in the resulting image. If this is clearly not the case, then there may be a colour recognition issue. It will be best to wait for a couple of weeks, when hopefully the dying tissue has gone, and then repeat the test as normal," explains Pete.
Another query this year is where growers have established crops in wide rows. "To help get a good indicative score, we advise taking the photo at a 450 angle to the rows to minimise any error," points out Will Reyer.
"We advise that digital photographs in a JPEG (jpeg) format should be taken looking down on the crop and on a cloudy day when the light is more subdued in order to avoid shadows. Taking photos in direct sunlight introduces shade or darker areas in the photo and could affect the GAI measurement. It is also best to set the resolution low (less than 2MB) on a digital camera; mobile phones are fine as long as more than 1 megapixel resolution."
"You should measure GAI twice, once for nutrition in February and again from mid-March (growth stage range - green bud visible through to mid-stem extension) for metconazole. The recommendation is when the GAI of the crop is greater than 1 in mid March, then apply up to 0.8 l/ha of Sunorg Pro to the crop. This can be done in March or April depending on the crop. A high plant population with high soil fertility will require an application at mid-stem extension to maximise stem shortening and reduce lodging risk. Used at late green bud onwards (early April) Sunorg Pro will maximise canopy effects and produces the greatest yield benefits ; it will also reduce lodging risk, improve rooting and contribute to Sclerotinia control. The April application is the most common, in part due to the need to wait for sustained active growing conditions before application," he says.
