Brassica napus L. ssp. oleifera (DC). Metzg.
Volunteer oilseed rape infestations result from the significant seed losses that occur in both spring and winter oilseed rape crops due to shedding before and during harvest. The seed pods become fragile and split open as the seed ripens but the lower pods ripen before the upper pods start to mature. The crop is harvested at a relatively late stage to avoid problems with the drying and storage of immature seeds. Seed losses of 8-12% during crop harvest can mean several thousand seeds per m² being shed onto the soil. In adverse conditions losses can exceed 20%. Shed seed numbers often reach 10 million per ha.
Volunteer seedlings can emerge in any crop that follows oilseed rape. Volunteer oilseed rape can be a problem even when the original rape crop was grown 3 years earlier. Feral or volunteer oilseed rape can cause quality problems in subsequent oilseed rape crops that are grown. The volunteers may reduce oil quality where glucosinolate levels are affected. Volunteers from genetically-modified (GM) oilseed rape are a particular problem and may cause a non-GM crop to exceed the EU threshold for the allowed level of GM contamination. Out-crossing of herbicide tolerant (HT) oilseed rape into non-HT crops grown in close proximity can occur. Cross pollination between HT GM volunteers with tolerance to individual herbicides may produce volunteers that have resistance to more that one herbicide. This phenomenon is known as gene stacking. Ploughing delays the risk of gene flow to the future, while minimal cultivation provides an immediate opportunity for it to occur.
Oilseed rape seed that becomes buried develops a light requirement for germination to occur but will respond to just a short flash of light. Germination levels are also related to the amplitude of temperature alternations in soil.
Seedlings are able to emerge successfully from 5 to 7.5 cm deep in soil but few emerge from 10 cm or deeper.
Persistence and Spread
Persistence and Spread: Shed oilseed rape seed can remain viable in soil for several years. Volunteer seedlings have emerged 9 years after the original rape crop was harvested. Light and temperature are thought to affect seed persistence. The seeds do not exhibit dormancy at the time of seed shed and only develop secondary dormancy after exposure to certain environmental conditions. Field studies have shown that freshly shed seeds exposed to dry, dark conditions after an early cultivation exhibit the greatest persistence. Seeds left on the soil surface for 4 weeks before cultivation do not build up into a large seedbank. If the soil is left uncultivated the seeds rarely persist. In set-aside land in Scotland, volunteer rape was recorded in the first year but gave way to other species in subsequent years.
Persistence is also associated with the development of secondary seed dormancy. Secondary dormancy levels range from almost zero to 60% for winter cultivars and to 85% for spring ones. Genotype is the principal factor controlling secondary seed dormancy. Within genotypes, variation can occur between seed lots and year of harvest. It appears that cultivars that are slow to germinate have a higher potential for developing secondary dormancy.
In a study of the effect of duration and level of drying temperature on the viability of oilseed rape seeds in pig slurry, seed survived up to 9 minutes at 50°C but did not survive even 3 minutes at 75 or 100°C when imbibed in pig manure and heated at different temperatures.
The choice of a low shedding cultivar and timely harvesting will reduce but not eliminate seed shed. The optimum stage for oilseed rape harvest is when seeds in the bottom pods are dark brown, those in the middle pods are reddish brown and seeds in the top pods are green but starting to turn brown. Rape may be harvested by direct cutting and combining when ripe, the alternative is to swath the crop at an earlier stage of ripening and combine from the swathe after 7-14 days. Shorter cultivars are usually direct cut while taller ones are swathed. Overall losses in winter rape were 11% in a direct cut crop of which around half was due to natural shedding. In the swathed crop, losses ranged from 11 to 25% of which less than 2% was due to natural shedding. The later the swathing the greater the losses. In spring rape, direct cutting losses were up to 5% for the direct cut and for the swathed crop.
Up to 29% of the shed seed enters the soil seedbank when the stubble is ploughed immediately after oilseed rape harvest. If tillage is delayed this is reduced to less than 3%. Experiments suggest that the germination of shed oilseed rape seed can be maximised by keeping seeds in the light and exposing them to alternating temperatures. Freshly shed seed is not dormant and exposure of seeds on the soil surface does not induce dormancy. Cultivations that bury seeds deeply in soil increase the risk of inducing seed dormancy and hence persistence. To avoid this, freshly shed seed should be retained for a period on the soil surface. If conditions are wet after harvest a 2-week delay in cultivation should be sufficient to encourage rape seed germination.
Updated November 2007.