Match OSR Management to Plant Population as well as GAI

By George Chancellor

14 February 2012

Big, particularly large-leaved winter OSR crops and excellent establishment, courtesy of one of the mildest early winters in recent memory, make it essential for growers across England and Wales to match their spring management to accurate assessments of plant population as well as Green Area Index (GAI), warns Agrii oilseed rape specialist, Philip Marr.

“Having gone into the winter with GAIs as high as 2.5 in many cases, most crops are coming through February with more reasonable levels of around 1.5,” he reports. “However, good establishment combined with relatively few early winter losses means that those sown with higher-than-ideal seed rates are very thick as well as big. And our extensive Best of British Oilseed (BoBO) studies have shown these crops require quite different management to those with exactly the same GAIs but more optimal plant populations.”

In both cases, Phillip Marr stresses that every effort should be made to encourage maximum spring root biomass development through good micro-nutrition and tactical applications of Nutriphite PGA. With many soils coming out of the winter with the most marked moisture deficits for many years, this will be especially important to minimise crop risk from another dry season.

Equally, first nitrogen applications for canopy development – with accompanying sulphur, of course – must be planned strictly on the basis of crop GAI and soil N-Min status to deliver the optimum 3.5 GAI at flowering, regardless of plant population.

“When it comes to fungicide treatment, though, crops with populations of 50 or more plants/m² need particular care,” Philip Marr insists. “As well as being far more susceptible to lodging, they’re also at very much greater risk from phoma and light leaf spot than more open crops. So it will be especially important to get the balance between growth regulation and disease control right.

“Full rate metconazole applied at the start of stem extension from mid-March during active growth will be vital here for the best growth regulation, promoting more branching and more even flowering by reducing apical dominance as well as cutting lodging risk by limiting plant height.

“ In contrast, a lower rate of metconazole or tebuconazole applied at mid-stem extension should be quite sufficient to even-up crops with more ideal 20-30 plants/m² populations.”

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In addition to a different fungicide/growth regulator strategy, Phillip Marr is adamant that later nitrogen applications for yield need to be flexed to reflect the extent to which yield potential varies with plant population.

“Far more efficient light interception means crops with lower plant populations have a very much higher yield potential than those that are too thick,” he explains. “HGCA-funded trials at Sutton Bonnington way back in 1997, indeed, showed a difference of around 1 t/ha. Which means thicker crops are likely to require around 60 kg/ha less nitrogen to yield to their full (lower) potential. This should be reflected in April applications.

“The standing power and height of varieties are obviously important considerations in spring management to maximise performance and minimise lodging risk. But it’s vital not to under-estimate the crucial importance of plant populations in both respects.

“High levels of plant losses in last season’s very cold winter left crops which really underlined the benefit of less dense winter OSR populations by delivering exceptional performance for many, ” Philip Marr concludes. “Those who followed our BoBO advice and sowed their crops at the correct seed number for the variety, seed batch and conditions this season are well-placed to maintain or even build upon their 2011 achievements. But particularly low winter losses and strong early season growth this time around mean those continuing to sow too many seeds are going to have to work much harder to manage their crops for the same result. ”