Dermot Forristal gave a great breakdown of how to save on nitrogen (N) costs in oilseed rape at this year’s Teagasc National Tillage Conference held online on Thursday 13 January.

Most of us have a smartphone and can download an app. If you have oilseed rape, you should download an app to assess the green leaf area index (GAI) of your crops.

Dermot described how this can pay off in fertiliser savings.


The Teagasc researcher gave a number of examples of how to manage oilseed rape crops with different canopies.

Taking an example of a small canopy with a GAI of 0.5, Dermot explained that this crop has probably taken up about 25kg N/ha from the soil and has the potential to take up another 45kg N/ha from soil mineral N.

The target GAI at flowering is 3.5 and approximately 175kg N/ha is needed to get to this, adding a further 60kg N/ha to target a yield of 4.5t/ha. This means the crop will need 235kg N/ha.

Cost savings

However, if the oilseed rape crop has a GAI of 2.0, it is estimated to have taken up 100kg N/ha, with a possible 30kg N/ha available in soil mineral N in the spring.

Adding the 60kg N/ha for a yield of 4.5t/ha, the crop needs 135kg N/ha to be applied. This is 100kg/ha lower than the crop with a GAI of 0.6 and can result in a saving in N costs of €250/ha.

In response to a question, Dermot noted that if farmers have crops with a GAI of 3 to 3.5, they can afford to spread 60kg to 100kg N/ha on those crops.

To read about soil fertility advice from the Teagasc National Tillage Conference, click here.