The Big Picture: Oilseed Rape Review

12% drop in oilseed rape area

The oilseed rape crop area has fallen by 12% over the past five seasons, but this average masks a wide range of regional differences. Not surprisingly eastern counties have borne the brunt of the fall – there has been an over 70% decline in Essex and Hertfordshire, for example.

Scotland, though, is bucking the trend – the area there is up 7%.

The fall in area is due to a combination of the market environment and the introduction of a ban on neonicotinoid seed treatments.

In 2012 oilseed rape was selling at a high price, whereas the price of wheat was declining, making second wheat less attractive within the rotation. The result was unsustainably short rotations and an untenably high crop area of OSR.

But the large area decline in some regions is almost certainly a direct result of greater cabbage stem flea beetle pressure, since the loss of neonicotinoids, making the crop difficult and risky to establish.

In Scotland cabbage stem flea beetle is less problematic, and the availability of more competitive clubroot resistant varieties have likely made the crop more attractive.

As growers become more confident about growing the crop post-neonicotinoids the crop area has increased slightly. Using hybrid varieties and/or judging the best time to drill (see below), to get the crop out of the ground quickly and overcome cabbage stem flea beetle damage have proved that it is still possible to grow the crop successfully in all but the highest flea beetle hotspots.

Hybrid crops fail less than conventional varieties

No grower wants to see their crop fail, but with the added pressure from cabbage stem flea beetle, as well as the usual issues of weather, slugs and pigeons, it is perhaps inevitable that some crops will fail in any given season.

Data on failed crops suggests that hybrid varieties are less likely to fail than conventional ones. Crop failures from the latter range from 0-4.35%, while for hybrid varieties this was 0-1.81%.  

One of the strengths of hybrid varieties, generally, is early vigour, which helps gets crops off to a faster start and established more quickly. This contributes to increased resilience in challenging growing conditions.

While there are many different strategies available to get crops off to a fast start and established, choosing a hybrid variety is one that the failed crops data suggests can help.

Hybrid varieties do not just have to be chosen from the AHDB Recommended List -  many breeders also market varieties ‘off the list’ to highlight the strengths of the variety, some of which are not assessed within the RL system, such as vigour, pod shatter resistance and resilience in challenging growing conditions.

If you are interested in a variety that is not on the Recommended List, there are multiple sources of information such as NL trials, the breeder’s website, distributor agronomists and data. Where on-farm trials aren’t an option, visiting local trials which benchmark varieties against each other is essential to understanding how a variety might perform in your locality. 

Crop is drilled around two weeks earlier than five seasons ago

Nationally there has been a trend for growers to drill oilseed rape earlier. In autumn 2016 peak drilling was in the third week of August, while in 2012 it was around two weeks later.

Scotland has followed the national trend to drill two weeks earlier, but typically the region appears to have two clear spikes in drilling time. In 2016/17 these were the second and last weeks in August.

National overview 

While the national trend has been to drill earlier there is some variation between years, which is likely correlated with weather patterns. That’s almost certainly the case in Scotland, with harvest demands meaning growers prioritise that activity over oilseed rape sowing, causing the two peaks in drilling time.

However, the general pattern of drilling earlier seems almost certainly to be a reaction to cabbage stem flea beetle pressure, with many growers trying to get the crop established before pest pressure hits.

Scotland

While drilling earlier to get crops away quickly can be a good tactic to overcome cabbage stem flea beetle attack, it is equally important to make sure soil conditions are optimised for establishment.

So if it is very dry, for example, in the earlier drilling slot, it might make more sense to wait for some moisture before drilling to make sure crops get out of the ground quickly, rather than the crop struggling to germinate.

Remember though, adjusting sowing dates can have an impact on subsequent disease and pest pressure. For example drilling oilseed rape earlier would leave it susceptible to light leaf spot for longer, and for those growing the crop in northern regions would increase the risk of clubroot infection. However earlier drilling would allow the plant to grow away from Phoma thresholds.

Sowing dates are a balancing act, and also rely on in-season conditions. The important thing is to be aware of the impact your changes could have, and adapt accordingly.

Decline in seed rates over the past five seasons

Seed rates have declined nationally over the past five seasons. Almost twice as much oilseed rape was sown at 2.5 kg/ha or less in 2016/17 compared with 2012/13.

This trend is true of both hybrids and conventionals nationally, with hybrid seed rates declining 6% to 3.11 kg/ha, and conventional sowing rates declining 5% to 4.00 kg/ha.

The data also shows that farm saved seed is sown at 1 kg/ha more than certified seed.

The national data masks to some extent a north/south divide. Seed rates have dropped markedly in the north – by around 20% in both conventional and hybrid varieties. But in East Anglia and the South East both hybrid and conventional seed rates actually increased in autumn 2016 compared with autumn 2012, while other regions showed little change.

Again, this shows the impact of the neonic ban and the fear of cabbage stem flea beetle. In the north where it is not an issue currently, growers continued to follow research suggesting lower seed rates result in a better canopy structure and ultimately higher yields.

But in the south that movement has been slowed by the counter argument of making sure there are enough plants to survive pest attack in the autumn for a productive crop.

In East Anglia it appears that growers have looked to utilise hybrid vigour and a higher seed rate to guarantee an established crop, while in the south east there has been an increase in seed rates for conventional varieties.

The higher seed rates for farm-saved seed also suggests that this tactic is more widespread for these growers.

Sowing rates for oilseed rape in the UK remain high compared with the rest of Europe. In France for example, certified seed is sown at 2 kg/ha, showing that there is potential to reduce seed rates in the UK further.

While it is understandable given the pest pressures that growers in some regions are more reticent to decrease seed rates, they can be detrimental to the final canopy structure in some seasons. The balance between the two competing factors is not an easy one to reconcile.

For those considering reducing sowing rates, a trial on-farm is a valuable way to understand how changes to seed rates affect your crop, before making any changes to your commercial crop.  

Fungicide sprays have remained stable, but are you missing a trick?

Over the past six seasons fungicide use has remained broadly stable, with oilseed rape crops receiving on average three disease control sprays per season – most likely broken down as one autumn application, an early spring spray and another application at flowering.

Light leaf spot has been identified as the number one disease threat in oilseed rape by the DEFRA winter oilseed rape disease survey. Surveys have also shown light leaf spot is a threat across the whole of the UK, not just the traditional northern climes – a fact also highlighted by our free SpotCheck service this season.

A disease that cycles constantly through the season, light leaf spot is notoriously difficult to identify. Our recent survey found while 64% of growers could recognise classic light leaf spot symptoms, only 14% could identify the disease on the stem.

Three disease control sprays is probably the minimum requirement to control phoma, light leaf spot and sclerotinia.

In some seasons three applications will be perfectly adequate, but in other more challenging conditions it could be a missed opportunity to maximise yields, as shown by a recent AHDB-funded trial, which demonstrated that an additional disease control spray in March had a positive impact on yield. 

The challenge is getting the timing spot-on. An early spray in the autumn to combat phoma could leave the crop exposed to light leaf spot, while delaying in the spring to combine light leaf spot control and a growth regulator application could leave you susceptible to the disease too.

What is important is being flexible according to the variety you are growing and the conditions of the season.

Consider if your disease control programme is maximising your yield and be prepared to, if necessary, spray twice in the autumn, or twice for sclerotinia if flowering is extended. It could be an easy way to reduce your unit cost of production and maximise profitability.

Ban on neonicotinoids has led to increased use of foliar insecticide because of cabbage stem flea beetle

Foliar insecticide use before and after the neonicotinoid ban was introduced is an interesting area. When you look at total insecticide use in OSR crop in the three years before the ban and the four years after the ban, judged by superdeveloped hectares (SDA), then the amount of insecticide has actually dropped post-ban.

But of course, so has the area grown – quite significantly. So, dividing the superdeveloped hectares by the area grown in each year starts to show a slightly different picture – one in which insecticide use has increased from 2.8 treatments/year to 3.0.

Not much difference? We did a bit more digging. Breaking down the data in smaller chunks, you can to some extent break the season into three sections – August & September for flea beetle, October to December for aphid control, and March to June for spring pests, such as pollen beetle and pod midge, etc.

This starts to show a slightly different picture – in the three years pre-ban, there was just over one treatment (1.13) for flea beetle on average across the area, since the ban it is now just under two treatments/total area (1.87). At the same time, the area treated for aphids that transmit turnip yellows virus has decreased (October–December) slightly from 0.99 to 0.72 treatments/total area.

Of course, there will be some noise in this data, some crops in September will be treated for aphids, others in October for flea beetle potentially, and some for both, which with the increase in area treated earlier may account for the reduction in area treated in the later months. But the data is clear – foliar insecticide use in the autumn has increased post-ban.

When this data is broken down regionally, it is also clear that more insecticide is being used by growers in East Anglia, the South-East and East Midlands. Typically, they are applying over the average 3 treatments/area in a season, whereas growers in the other areas are below the 3 treatment national average.

There is some good news: the data appears to suggest the drive to minimise pollen beetle sprays in the spring, and the change in thresholds has had an effect. Since harvest year 2012, the amount used has steadily dropped to a low of just half the OSR in total receiving an insecticide spray between March and June.

The ban on neonicotinoid seed treatments has made the crop more risky and less viable in some parts of the country.

Where growers have persevered, alternative techniques such as earlier drilling, higher seed rates of conventional varieties or drilling hybrid varieties, have started to be used after the resistance to pyrethroids became more widespread.

The data suggests that despite resistance issues, foliar insecticides are still being used to combat flea beetle and aphids in theautumn, although there is variation in intensity depending on flea beetle pressure.

It is excellent, though, that the new thresholds and a concerted effort by those in the industry to help growers understand where to use insecticides for pollen beetle appears to be paying off.

Oilseed rape is still the best entry for wheat, so consider the use of hybrid seed varieties where cabbage stem flea beetle threatens, to make sure the crop establishes in all conditions as quickly as possible.

For pollen beetle, keep using the thresholds and tools, such as the Bayer Pollen Beetle predictor to help time monitoring, and to make sure any insecticide use is justified – in many cases it may not be for this pest.