All seed crops are managed in a way to optimise yield. One component of doing this for oilseed rape is to develop an appropriate canopy size to capture the sun’s energy and convert it into yield. The canopy of winter oilseed rape can vary in size and development rate, and it can capture significant soil nitrogen before spring.
However, too large a canopy can result in crop lodging and impeded pod development if they are shaded by excessive flowering. OSR canopies in spring can vary depending on sowing date, available soil N, weather conditions and pigeon grazing.
So, how do we manage these variable crops?
Until recently, fertiliser N rate was based solely on the previous crop grown in the field (N Index system). Different N strategies are used for oilseed rape across Europe.
Some are tailored for very cold winters, but the UK canopy management system is more suited to our growth patterns. It determines the amount and timing of N application based on measurements of N in the crop and soil post-winter, a target leaf area at flowering and an estimate of N use efficiency. But are the principles that underpin this system accurate for Irish conditions?
What are you doing in this experiment?
This work at Oak Park is testing the principles of canopy management in oilseed rape under Irish conditions. Trials over the past three years tested different nitrogen strategies, including fixed and canopy management approaches.
Three sowing dates (mid-August, end of August and mid-September) were used to produce varying growth levels in crops post-winter. Some plots were defoliated to simulate pigeon damage.
Canopy management (CM) strategies were largely based on the UK approach, using N rates to produce a canopy of 3.5 units of GAI (green area index) at flowering. ‘Nominal’ yield targets of 4.5t/ha, 3.5t/ha and 2.5t/ha were chosen to give a range of N rates.
An ‘Irish’ canopy management approach was added in the final year, based on the findings of the first two years. All canopy management approaches were based on soil mineral N levels and canopy measurements in February.
What have you observed/recorded to-date?
In all three years, canopy management approaches allowed optimum yields to be achieved with lower levels of nitrogen than the fixed N rate approach.
During these experiments, an extra 50kg N/ha became available from Irish soils versus the amount assumed using UK data. This may be due to higher organic matter soils and warmer winters.
What does this research mean to a grower?
These results show that canopy management approaches allow more accurate targeting of fertiliser N, reduce N rates and give lower environmental risk. They also help avoid excessive canopies, lodging and reduced pod fill.
They could be even more accurate if we had more accurate estimates of available soil N and yield potential, as individual sites and seasons will result in different optimum N values.