Oilseed rape market seeks growers: Any ‘volunteers’?
Winter oilseed rape can be a precious beast when it comes to drilling to establish a good crop. On the other hand, volunteers seem to take on a whole different character surviving all sorts of rigours. They are also able to germinate and grow for years to come. It has been estimated that the crop can shed around 2,000 to 10,000 seeds/m2. At this time of year, localised thunderstorms can also bring hail, which can cause severe pod shatter and increase the seed-shed burden. Although there is a rapid decline in viable seed numbers in the first few months after harvest, 5% of this shed seed may still be viable after nine years.
But why do volunteers matter so much if you are just drilling oilseed rape? The trouble is that not all varieties produce the same oil quality.
Double zero v HOLL
Most varieties are now double zero – with low content of erucic acid and low content of glucosinolates. But on the AHDB Recommended Lists there are now some high oleic, low linolenic (HOLL) varieties. The oil from these varieties is a low trans fatty acid and low saturated fat vegetable oil that is stable and performs well at high temperatures. Double zero volunteers are unlikely to cause too much of an issue to HOLL oil quality, if growers follow the recommendation to include a minimum gap of three years between double zero and HOLL rapeseed crops.
HEAR v HOLL
Greater care must be taken if you have gone down the route of growing speciality high erucic acid rape varieties (HEAR). HEAR varieties are used in industrial processes, such as inks, lubrication and as a slip agent in the production of polythene. HOLL varieties should never be grown on land previously used for HEAR varieties.
Choosing an established variety, which you have experience of, is a good start but it is always good to explore all of the opportunities. Plant breeders often make great advances breeding better varieties with higher oil gross output in combination with good resistance to light leaf spot and phoma stem canker, so the Recommended List is a good place to look when considering a new variety. Our recommended varieties have at least three years of information behind them and have shown to provide an advantage over the current best varieties. New candidates, as the name suggests, are still unproven and information is limited. Some maybe the next best thing, but some may not be.
There are some varieties with useful resistance to clubroot. If your current oilseed rape has disappointing patches with poor pod development and yield, then you can check the roots after harvest to see what lies below. There are tests which can also be done on the soil to check for levels. Growing brassicas in short rotation in compacted waterlogged soils, where the pH maybe below optimum, can increase clubroot levels. It is possible that flooding may also have brought in the soil-borne pathogen. Where possible, extend rotations between brassicas as long as you can, otherwise each brassica crop will build up levels. Using clubroot resistance should be seen as part of an integrated approach and not the solution.
Which varieties are more vigorous and can they grow away from pests? This is not easy to define or measure. With new developments in sensors, getting an accurate measure of vigour both at emergence, during the autumn and early spring would be a valuable tool to understand more about current varieties. An even greater challenge would be to measure varieties with more vigorous root systems (which may be equally advantageous to producing good yields). Look out for future research calls in this important area.
Oilseed rape seems to be rather tasty to a wide range of pests. Cabbage stem flea beetles take advantage where dry seedbeds lead to slow emergence, slugs if it is wet, and pigeons at any time. Aphids also play a part by transmitting Turnip Yellows Virus – to which some specialist varieties have resistance.
Cabbage stem flea beetles migrate into crops during emergence and can feed on and destroy the growing point. They then bite ‘shotholes’ in the cotyledons and early true leaves. The beetles lay eggs in the soil and the larvae bore into the leaf petioles and, later, into the main stems. There have already been reports of the presence of adult beetles in harvested oilseed rape. To establish and manage your local pressure, follow the top tips:
> Check the number of cabbage stem flea beetle in the previous crop’s harvested seed
> Assess damage to volunteer oilseed rape plants
> Monitor larval numbers in late October/early November
> Only spray if thresholds are exceeded or there is evidence of high pest pressure.
> Only spray pyrethroids when absolutely necessary (to minimise the spread of resistance)
Where Recommended List trials are decimated by the pest, we keep them going for measurements – just in case some patterns in varietal differences arise. The same is true for sites affected by verticillium wilt. For this disease, research is underway to identify varietal resistance, but where trial sites badly affected with the disease they can form part of the data used to evaluate varieties.
I’m writing this with a cup half empty attitude, but careful planning of crop rotation, variety choice, soil structure and nutrition are all things within your control before the weather dictates which horrors will attack your crop.
AHDB has many useful publications and web tools to assist in variety choice, pest weed and disease management and resistance, plus market intelligence on profitability. Some of the challenges are the focus of current of future research, so let’s be positive that the combined ingenuity of growers, agronomists, breeders, agchems, biopesticide industries and researchers resolve some of these issues so we can look at a cup three-quarters full in the future.