Save on nitrogen with oilseed rape

Winter oilseed rape has gone down in area this year, to between 9,000ha and 10,000ha. There were fewer winter crops last year and so there were less opportunities to plant oilseed rape early after winter barley.

However, it is a crop that allows for great fertiliser savings. Oilseed rape is often referred to as a super cover crop. It’s a cash crop, as well as a crop that takes up a lot of nitrogen from planting in August or September, right through the winter.

This year oilseed rape crops got off to a great start in many areas. They have been damaged by pigeons and other pests in the mean-time, and so there are mixed crops out there.

These mixed crops should all be treated differently when it comes to nitrogen applications. There will be opportunities to apply less nitrogen than expected on some crops, while others will need more. However, nitrogen can be more targeted by measuring the green area index.

The Green Area Index

The Green Area Index or the GAI of an oilseed rape crop is a measurement of how much nitrogen is in the crop. By measuring this you can decide how much nitrogen you need to apply to the crop.

The GAI can be measured by eye, but an accurate way of measuring it is by using your mobile phone. You can use a GAI application on your smartphone or log onto some websites to estimate your GAI.

You simply need to take pictures of the crop and the app will give you a figure for the GAI.

If you get a GAI of 1.0 then there is estimated to be 50kg of nitrogen per hectare in the crop.

Teagasc has outlined a table to guide farmers on nitrogen applications when measuring the GAI of crops (see table 1).

Fuller and greener crops will require less nitrogen. Crops with a GAI of 2.0 do not need an early application of nitrogen and will only require a total of 130kg N/ha.

In comparison, crops with a GAI of less than 1.0 will require a total of 225kg N/ha. And would ideally get nitrogen in February.

Sulphur

Just like with cereals, oilseed rape will need sulphur to help with nitrogen uptake in the crop. Winter oilseed rape crops generally require 30-35kg of sulphur per hectare according to Teagasc advice.

Water quality

As winter oilseed rape takes up nitrogen over the winter and allows artificial fertiliser use to be cut, it also helps with water quality. The crops take up nitrogen that may otherwise be lost to water over the winter and cause pollution and a deterioration in water quality. This makes oilseed rape a very sustainable crop to grow from an economical and an environmental point of view.