It may give you comfort to see a thick crop of oilseed rape coming out of the winter, but it won’t necessarily give you the yield you are hoping for.
Replicated trials have shown that the yield difference from a crop where the canopy has been managed correctly is up to 0.4t/ha. This could be even greater on many farms.
What makes a good canopy?
The ultimate aim from a crop of oilseed rape is to get the maximum number of seeds/sq m of crop. So the critical period is during seed set – the two to three-week period from mid-flowering onwards. Each pod could produce up to 40 seeds, although less than 20 is more usual (see diagram below).
The number of seeds set is determined by the plant’s ability to photosynthesise during that critical period. If there is a thick flower layer, caused by an over-large canopy, this will reflect the sunlight. The crop may look good and will appear full of pods. But less light will penetrate to the green leaf and stem and you will be disappointed come harvest.
The optimum canopy will be erect and will look fairly sparse. The leaves will still be there after flowering and there will be a reasonable amount of green leaf area. The aim is to achieve around 6000-8000 pods per sq m.
How does this relate to ground cover?
The rule of thumb is to aim for a green area index (GAI) of 3.5 by early flowering. You will think it looks thin – look down on the crop and you will still see about 5-10% of the ground.
The best time to assess the crop is in mid-March – around the point when the stem starts to extend or early green bud stage. At this time the GAI could vary from less than 0.5 (30% crop cover) to 3 (90% crop cover), but any crop can be managed to the optimum size by flowering.
There are two ways to accurately assess GAI – take a photo and load it up to www.totaloilseedcare.co.uk or cut down 1sq m of crop and weigh it. The weight in kg x 0.8 is the GAI.
How can you influence the canopy in winter?
There is a direct correlation with the number of plants established and the size of the canopy. You can generally get away with as few as 20-30 plants/sq m, but establishment varies from 0-100%, so growers tend to err on the side of caution.
The big winter dilemma is pigeon damage: If the crop looks sparse this is probably about right, but it will attract pigeons. A full crop could benefit from pigeon grazing, as long as this does not take place once buds start to grow up in spring. But pigeons tend only to land in a crop where they can see brown areas within it.
How can you influence the canopy in spring?
If your crop comes out of the winter too thin, it will need an early dose of nitrogen. If it is very thick, you could consider topping it – trials suggest a January trim of an overlarge canopy can boost yield by 0.3t/ha.
The main way to influence the canopy is through managing your nitrogen inputs (see Late Nitrogen in Oilseed Rape academy at www.fwi.co.uk/academy or HGCA Topic sheet 103). If your crop has a GAI above 1 in mid-March (green bud), it will also benefit from a plant growth regulator.
Thick canopies look good at this time of year, but not always in the barn.
There are two fungicides known to have a growth regulatory effect on the canopy and these can be applied right up to mid-flowering – tebuconazole and metconazole. Both have activity against sclerotinia and light leaf spot. But where disease pressure is high it may be worth considering tank-mixing with other partner chemicals.
Plant growth regulators (PGRs) applied close to or during flowering help by altering the structure of the canopy. They work by inhibiting gibberellic acid, that causes cell elongation.
PGRs have an effect throughout the plant – in the stem, the leaves, and importantly in the terminal raceme, or main shoot of the plant. Trials have shown they reduce the apical dominance of the plant and encourage a more branched canopy structure. This in turn lets more light penetrate through to the pods and leaves and helps seed set.
What about lodging?
If the canopy is too thick, the plants put more effort into extending the stems, making them tall and weak and liable to lodge. This can decrease yield by 30%.
Again delaying nitrogen application helps to slow down stem growth and reduce height. An application of PGR any time from green bud to flowering also reduces stem elongation, with the greatest effects coming from the late green bud to yellow bud timings.
Any lodging that takes place at flowering is particularly detrimental to yield as this will upset seed set. So any steps taken to limit crop height can be cost-effective.
Can a PGR influence drought stress?
Recent research carried out by ADAS has revealed oilseed rape is remarkably exposed to drought. Although deep-rooting, frequently there are insufficient roots at a depth below 40cm. This means the crop is prone to drought stress one in every two or three years, which can knock yield by over 1t/ha.
An application of metconazole has been found to increase roots at depth by 25%. It is not yet clear why this happens, but it is believed that through limiting shoot growth above ground, the plant switches more resources to develop its root structure below the soil surface. The crop is able to take up an extra 6-9mm of water during the seed-filling period, reducing the likelihood of drought stress to one in every five years.