Oilseed rape (OSR) is the only canopy crop grown in Ireland that takes up surplus nitrogen (N) from the soil in the early stages of development, which can be used to meet its future growth requirements.
As a consequence, calculating the Green Area Index (GAI) of winter OSR at this time of the year can be used to calculate the amount of N available within the plant’s own tissues to promote growth over the coming weeks.
It is then a case of subtracting the GAI-estimated figure from the crop’s total N requirement to work out the amount of artificial fertiliser that should be added to meet its full yield potential.
What is the Green Area Index of oilseed rape?
It is the ratio of green leaf and stem area to the area of ground on which the oilseed rape crop is growing. A crop with a GAI value of 1.0 contains the equivalent of 50kg N/ha.
Seedtech technical director, Tim O’Donovan has confirmed that GAI values are consistently accurate, when it comes to assessing how much available nitrogen is already stored within OSR crops.
“Trials undertaken by Teagasc verify this. In fact, the work undertaken in this regard was fully reported on at the 2022 National Tillage Conference,” he said.
“There are two options available to growers when it comes to determining a GAI value.
“The first is to download an app on to a phone or tablet. Initially developed by BASF, it takes a photograph of the OSR crop and converts the information it uploads into the required GAI value.
“The second option is for growers to physically harvest 1m square quadrants of crop, down to ground level, and physically weigh the biomass they collect,” he added.
According to O’Donovan, multiplying the weight of material (kg) by 0.8 generates the required GAI value.
“Seedtech commits to provide an up-to-date agronomy service to our customers. We market two OSR varieties: Aurelia and LG Aviron,” O’Donovan said.
“Our own sales of winter oilseed rape seed have grown significantly over the past year.”
It is estimated that the total area sown out in oilseed rape for 2022/2023 was approximately 19,000ha, up from 15,000ha in 2021/2022.
“Winter rape crops are looking good at the moment, very much on a par with last year. If the weather plays its part, then we could be looking at final yields in the region of 5t/ha,” O’Donovan concluded.