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N timing and application rates key to OSR canopy management

Measuring green area index either digitally or by more traditional means is important to inform OSR N fertiliser and PGR regimes and minimise the yield impact of lodging and leaning.


Marianne   Curtis

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Marianne   Curtis
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Oilseed rape canopies can be managed for optimum yield through N rate, timing and application of plant growth regulators (PGRs).

 

The optimum green area index (GAI) at flowering is 3.5, with larger canopies having poor light use efficiency and a greater risk of lodging, according to AHDB.

 

ADAS crop physiologist Dr Pete Berry says various tools and more traditional methods are available to estimate GAI in OSR crops, which is best done in February, just before stem extension. These include digital photography via apps such as the BASF GAI Tool, for GAIs up to 2.5.

 

Determining crop freshweight is necessary to calculate GAIs of more than 2.5, says Dr Berry. “Record freshweight in kg per sq.m of OSR and multiply by 0.8 to give GAI.”

 

 

 

 


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GAI estimate

An estimate of GAI can also be obtained by looking at the fraction of soil covered by crop. One-third covered equates to a GAI of 0.5, half equates to GAI 1 and three-quarters to GAI 2.

 

The crop’s nitrogen content – crop N – can be estimated using GAI, with one GAI unit equating to 50kg N/ha. Alternatively, crop height can be used as an indicator of crop N, with 10cm equating to 35-45kg N/ha, 15cm to 55-65kg N/ha and 20cm to 75-85kg N/ha, says Dr Berry.

 

This enables N fertiliser rates to be calculated (see table). Fertiliser timings also need to be adjusted to reflect canopy size (see panels).

Yield response

Using PGR metconazole at GAI 0.8-2 in March gave a yield response of about 0.19t/ha and where GAI was greater than 2, a yield response of about 0.28t/ha, according to trials done by AHDB and BASF.

 

Dr Berry says: “Crops with a GAI above 1 before stem extension are worth targeting with PGRs – you might just want to target part of the field. N amount and timing is the best tool against lodging with PGR behind it.”

 

Work done by ADAS looking at lodging (flat) showed a 50 per cent yield loss when lodging occurred at flowering and a 10-15 per cent yield loss when lodging occurred at seed fill.

 

When lodging was at an angle of 45 degrees, yield loss at flowering was 20-25 per cent and 0-25 per cent at seed fill.

 

When leaning was at 20-25 per cent, yield loss at flowering was 0-5 per cent at flowering and 15-25 per cent at seed fill, says Dr Berry.

 

“We were surprised by this level of yield loss, but it should not have been too unexpected as we want the canopy open to maximise photosynthesis. Anything which changes the canopy structure is bad news.”

Effect of canopy size on N rate

GAI end of Feb GAI = 1- Kg/ha N GAI =2 Kg/ha N
Target N needed in crop 175 175
(above: 3.5 GAI x 50kg N for each GAI)
Crop N 50 100
Soil mineral N 25 25
Shortfall 100 50
(Target N - crop and soil N)
Fertiliser requirement 167 83

 

Source: ADAS

N timing – small canopy

GAI = 0.6 (30kg N/ha)

Soil N = 30kg N/ha

190kg N/ha needed for optimum canopy

Extra 60kg N/ha to target 4.5t/ha yield

Application timings:

100kg in late Feb or early March

90kg at green bud (mid-late March/early April

60kg at yellow bud/early flowering (early to late April)

Source: ADAS

N timing – large canopy

GAI = 2 (100kg N/ha)

Soil N = 20kg N/ha

90kg N/ha needed for optimum canopy

Extra 60kg N/ha to target 4.5t/ha yield

Application timings:

90kg at green bud (mid-late March/early April)

60kg at yellow bud/early flowering (early to late April)

Source: ADAS

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