Moves to mitigate the threat posed by cabbage stem flea beetle following the ban on neonicotinoid seed treatments could have knock-on effects on other areas of oilseed rape crop management.
An OSR ‘state of the nation’ survey of 300 growers and 50 agronomists conducted by BASF last autumn revealed 55% of respondents had increased seed rates and 42% had drilled early in a bid to mitigate the risk of cabbage stem flea beetle (CSFB) damage.
One consequence of this approach is that crop canopies can be large coming out of winter, which may not be good news for yield.
BASF business development manager Clare Tucker says: “Although forward crops are more comforting to see than backward one, they will require canopy management by reducing early nitrogen and by using PGRs.”
With the aim to maximise seed set by achieving the optimum canopy size – a green area index (GAI) of 3.5 at flowering – the need for PGR treatment can be assessed by measuring the GAI of a crop just before stem extension – typically in late February.
There are a number of approaches available, including using an app such as the BASF GAI Tool, crop fresh weight calculations or using a fraction of soil covered by crop approach.
The economic threshold for an application of a specialist OSR plant growth regulator (PGR) such as Caryx (mepiquat + metconazole) is a GAI of 0.8.
ADAS head of crop physiology Pete Berry says: “We recommend you assess the crop canopy before the stem starts extending. You don’t need a massive crop to respond positively to a PGR.”
How to estimate green area index (GAI)
Canopy management in OSR – key messages
Managing smaller canopies
Oilseed rape crops held in check into early spring by cold conditions will be poised for rapid growth as soon as things warm up. But if the main raceme then shoots away, the yield potential of lower branches could be significantly reduced, advises Syngenta field technical manager James Southgate.
However, well-timed PGR applications can beneficially hold back stem extension and enable the side shoots to develop simultaneously – establishing a stronger structure and leaf canopy to produce higher yields, he says.
More advanced crops can be expected to lose leaves in cold conditions but with well-established root structures they are well placed to recover quickly. An early Toprex (difenoconazole and paclobutrazol) application, from early stem extension (GS31) will encourage branching and even up the crop, says Mr Southgate.
Smaller crops, with limited root reserves, however, will need nurturing to increase their green area, when they may then benefit from a later PGR application, from GS51, to develop shape and branching which will make the most of potential light interception.
“With a genuine PGR you can shape the crop to improve its overall efficiency. So while the onset of flowering on the main raceme may be later, you then get all the side shoots flowering at a similar time – so the flowering is far more intense, but over a shorter period.”
The benefit for the crop is that the yellow flowers physically reflect sunlight, so a shorter duration of flowering means that green leaves are able to absorb and utilise more energy.
However, where small crops continue to be slow to get going into spring, or suffer severe pigeon damage, they may be better left without PGR treatment, says Mr Southgate.