Using all opportunities to reduce fertiliser requirement will be an integral part of crop management in 2022.
While growers will naturally be cautious about decreasing nitrogen (N) rate and yield, this may not be the case with winter oilseed rape.
With this crop, N taken up to produce canopy over the autumn is available for springtime growth.
As the amount of stored N is proportionate to the volume of canopy at the start of growth, this can be used as a measure of the amount of N already in the crop.
So, canopy measurement can be used to calculate the amount of N required to produce a target canopy volume of 3.5 GAI (green area index).
Measuring this can be a very useful to guide optimum spring N use, as excess N can make excess canopy which can decrease yield.
Work at Oak Park in recent years has proven this to be a very useful practice.
At last week’s Teagasc tillage conference, Dermot Forristal reported that using a measure of canopy size resulted in the lowest N use and the highest yield in the past two seasons.
Principles
Canopy management aims to control spring vegetation production through N application. The more canopy that is present, the less that must be created. The principles assume that:
Canopy N (GAI) remains in the crop.Each unit of GAI contains 50kg of N in plant tissue.A crop requires a GAI of 3.5 at the start of flowering.Applied N fertiliser is 60% efficient. Each additional tonne of seed yield above 3.5t/ha needs 60kg N/ha.Excess canopy results in too many flowers at the top of the crop that reflect light and decrease the efficiency of photosynthesis. Canopy management is about optimising light and photosynthesis to produce maximum yield.
Figure 1 shows the results of work at Oak Park where the three lower N rates were based on canopy management calculations, while the higher rates were based on flat rate application. In these trials, the lowest rate of applied N produced the highest yield.
Canopy size in spring is influenced significantly by sowing date, with earlier planting more likely to result in bigger canopies and vice versa, while autumn growth is also a factor, as is the case this year.
These findings show that large crop canopies provide potential for N savings.
In Table 1, Dermot calculated that a small canopy (GAI 0.5) would require 235kg N/ha for a 4.5t/ha crop (higher than the total amount allowed).
Where the spring canopy was very large (GAI 2), the total applied N rate can be reduced by 100kg N/ha (135 vs 235kg N/ha) with a saving of €250/ha.
In general, the bigger the canopy, the less spring N required.
The N rates are calculated based on the target yield (4.5t/ha), the amount of N in the canopy (25kg or 100kg), the likely soil N in springtime (45kg or 30kg) and 60% efficiency from applied N.
Legumes also help
Having a proportion of a farm in legumes will also help to decrease dependence on expensive fertiliser, or at least the N element. This will mainly mean beans or peas, which require no applied N, and they leave some residual N for subsequent crop.
This further helps reduce overall farm N requirement.
Beans in the rotation have the potential to decrease farm nitrogen requirement.
Given their rotational requirement, beans can only occupy a maximum of 20% of a farm area, but that can be a lot of N.
Dermot compared different rotation options and calculated that a rotation with 20% beans requires 31% less N versus 100% continuous winter wheat.
This is a very useful long-term saving as we will be increasingly obliged to reduce fertiliser usage.
Variable performance
On the issue of variable crop performance, Dermot commented that yield variability normally tends to be more local than national. Teagasc research at Knockbeg found a 19% yield advantage in following crops which gave a €208/ha improvement in the margin from the rotation.
So it is important to focus on the entire rotation, as well as a single crop.
Using all opportunities to reduce fertiliser requirement will be an integral part of crop management in 2022.
While growers will naturally be cautious about decreasing nitrogen (N) rate and yield, this may not be the case with winter oilseed rape.
With this crop, N taken up to produce canopy over the autumn is available for springtime growth.
As the amount of stored N is proportionate to the volume of canopy at the start of growth, this can be used as a measure of the amount of N already in the crop.
So, canopy measurement can be used to calculate the amount of N required to produce a target canopy volume of 3.5 GAI (green area index).
Measuring this can be a very useful to guide optimum spring N use, as excess N can make excess canopy which can decrease yield.
Work at Oak Park in recent years has proven this to be a very useful practice.
At last week’s Teagasc tillage conference, Dermot Forristal reported that using a measure of canopy size resulted in the lowest N use and the highest yield in the past two seasons.
Principles
Canopy management aims to control spring vegetation production through N application. The more canopy that is present, the less that must be created. The principles assume that:
Canopy N (GAI) remains in the crop.Each unit of GAI contains 50kg of N in plant tissue.A crop requires a GAI of 3.5 at the start of flowering.Applied N fertiliser is 60% efficient. Each additional tonne of seed yield above 3.5t/ha needs 60kg N/ha.Excess canopy results in too many flowers at the top of the crop that reflect light and decrease the efficiency of photosynthesis. Canopy management is about optimising light and photosynthesis to produce maximum yield.
Figure 1 shows the results of work at Oak Park where the three lower N rates were based on canopy management calculations, while the higher rates were based on flat rate application. In these trials, the lowest rate of applied N produced the highest yield.
Canopy size in spring is influenced significantly by sowing date, with earlier planting more likely to result in bigger canopies and vice versa, while autumn growth is also a factor, as is the case this year.
These findings show that large crop canopies provide potential for N savings.
In Table 1, Dermot calculated that a small canopy (GAI 0.5) would require 235kg N/ha for a 4.5t/ha crop (higher than the total amount allowed).
Where the spring canopy was very large (GAI 2), the total applied N rate can be reduced by 100kg N/ha (135 vs 235kg N/ha) with a saving of €250/ha.
In general, the bigger the canopy, the less spring N required.
The N rates are calculated based on the target yield (4.5t/ha), the amount of N in the canopy (25kg or 100kg), the likely soil N in springtime (45kg or 30kg) and 60% efficiency from applied N.
Legumes also help
Having a proportion of a farm in legumes will also help to decrease dependence on expensive fertiliser, or at least the N element. This will mainly mean beans or peas, which require no applied N, and they leave some residual N for subsequent crop.
This further helps reduce overall farm N requirement.
Beans in the rotation have the potential to decrease farm nitrogen requirement.
Given their rotational requirement, beans can only occupy a maximum of 20% of a farm area, but that can be a lot of N.
Dermot compared different rotation options and calculated that a rotation with 20% beans requires 31% less N versus 100% continuous winter wheat.
This is a very useful long-term saving as we will be increasingly obliged to reduce fertiliser usage.
Variable performance
On the issue of variable crop performance, Dermot commented that yield variability normally tends to be more local than national. Teagasc research at Knockbeg found a 19% yield advantage in following crops which gave a €208/ha improvement in the margin from the rotation.
So it is important to focus on the entire rotation, as well as a single crop.
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