Forward oilseed rape crops need careful management this spring to maximise yield potential and avoid serious lodging problems.
The mild winter has resulted in well-advanced crops in many regions, which combined with a trend towards earlier sowing, more vigorous varieties and higher seed rates to offset flea beetle damage, means 2017 could be a high-risk year for lodging.
Accurate timing of spring nitrogen and plant growth regulators (PGRs) is therefore crucial to optimise canopies for maximum light interception and yield development, say experts.
The optimum crop canopy for light interception, pod production and yield is a Green Area Index (GAI) of 3.5 by flowering, so this should be the target for any fertiliser or PGR strategy, says Sarah Clarke of ADAS.
Dense canopies have fewer leaves, poorer light penetration to lower leaves and pods, and reflect more light from a thicker flower layer which leads to poor seed fill and greater lodging risk, she warns.
“Lodging can reduce yields by up to 50%, yet it’s not as well understood as in wheat and can be less obvious in tall, thick rape crops. Manipulating canopies to reduce lodging and maximise light penetration has a yield benefit in most seasons, especially in moderate to large crops.”
Measurements of canopy size (GAI) and soil mineral nitrogen (SMN) help determine how much fertiliser is needed to meet the target canopy size by flowering. Testing is usually done in early February, before any N is applied, and both measurements should be taken simultaneously to get an accurate “snapshot” of available crop and soil nitrogen.
It takes 50kg N/ha to put on one GAI unit, equivalent to 175kg N/ha for the optimum GAI 3.5 at flowering, says Dr Clarke. A forward crop with GAI 2 (equivalent to 100kg N/ha) and SMN of 25kg/ha therefore requires just 50kg N/ha to reach the target at mid-flowering. Assuming crops take up all available soil N with 100% efficiency, and added fertiliser with around 60% efficiency, this gives a fertiliser requirement of 83kg N/ha.
However, this principle is based on a crop yield of 3.5t/ha and an extra 30 kg N/ha should be allocated for each 0.5t above 3.5 t/ha, she says.
“Do not apply nitrogen too early on forward crops as there’s a risk of overshooting the optimum canopy size,” Dr Clarke continues. “In the case of a crop with GAI 2, I wouldn’t apply anything until late March. Split the total dose by first applying the amount needed to achieve GAI 3.5 before flowering then apply the extra N needed for higher-yielding crops at early flowering, and at least two weeks after the first application.”
Growth regulators are a key tool in canopy management for hybrids and open pollinated varieties and should deliver clear benefits in most seasons, says Dr Clarke. Shortening crops reduces lodging, while smaller canopies have increased seed set and higher yield potential. There are also benefits for rooting and disease control as most PGRs have fungicide activity, she notes.
Indeed, Bayer-funded trials carried out by ADAS in the absence of disease have found a yield benefit from Folicur (tebuconazole) of up to 1.09t/ha.
Yield response depends on spring canopy size though. PGRs typically deliver some yield benefit when applied to crops with a GAI of at least 0.8 at green bud in March or more than 2 at green/ yellow bud in April. Trials show the greatest yield response comes from applications made to larger crops between late green bud (GS 3,6) and flowering (GS 4,5), with similar effects on hybrids and conventional varieties.
“Hybrids and conventional varieties may come through the autumn and winter differently, but the approach from spring onwards is very similar.”
Bayer commercial technical manager Darren Adkins reminds growers that spray timing for optimum PGR effect is usually several weeks later than the best timing for effective disease control.
“Don’t compromise disease control by going for PGR effects. Light leaf spot control in particular should always take precedence and be treated at the optimum time.
“Any product containing tebuconazole still gives some PGR effect alongside disease control at this earlier timing, but the results won’t be as great as when applied at green bud stage. There is a strong case for going back with a second application at the proper PGR timing to reduce apical dominance and encourage branching.
“You can lose a lot from lodged crops, so saving a few pounds early on could cost a lot more in terms of lost yield.”
Robust rates of at least three-quarters label dose should be used, with water volumes tailored to canopy size, Mr Adkins adds. “Generally bigger crops require higher water volumes to ensure good canopy coverage.”
In the ADAS trials full dose PGR reduced crop height by 7-11cm, with each 5cm reduction in height reducing the area of lodging by 10%. Splitting PGRs between green bud and flowering timings reduced average crop height by a further 2-4cm, while half doses had half the effect, Dr Clarke notes.