ABSTRACT
Oilseed rape (Brassica napus L.) is grown widely across the world as a constituent of arable crop rotations. Many of the weeds to occur in oilseed rape are common to those of cereal crops that tend to dominate rotations, but their relative importance does differ. For example, annual grass weeds are a major problem, as they are in cereals, but weeds of the Brassicaceae (eg Sinapis arvensis L., Raphanus raphanistrum L.) tend to be of greater significance in rape, as herbicide selectivity is more of a challenge. Spring-sown oilseed rape tends to have a different flora to autumn-sown rape, due to the differences in weed emergence periodicity. For example, Chenopodium album L. and Fallopia convolvulus (L.) A. Love are more significant in spring rape, whilst Papaver rhoeas L. and Galium spp. ¬are more important in winter rape. The major impact of weeds is on crop growth and yields, though they can also adversely affect harvesting and contaminate the harvested rape seeds. Cultural control of weeds in oilseed rape can be achieved with varying degrees of effectiveness by adjusting cultivation practices and crop agronomy. Shallow non-inversion tillage and direct drilling have become increasingly prevalent in recent years, for environmental and economic reasons, and potentially can reduce weed emergence in rape, though this is very dependent on minimising weed seed return in the previous crop. Sowing the crop in optimum conditions is critical to ensuring the best competitive effect from the crop, as a vigorous stand will suppress many weeds. In contrast, a poor crop will be much more vulnerable to weeds. Adjusting the agronomy of the crop to achieve good weed suppression is encouraged worldwide, for example: sowing early in the autumn when higher temperatures favour the crop, increasing seed rates (despite the added cost), using more competitive cultivars. Most weed control in the developed world is based on herbicides which in the past have delivered good weed suppression but in recent years the rise of herbicide resistance, especially in grass weeds, has made weed control more difficult. Mechanical weed control has not been widely practiced in intensive production systems. Herbicide tolerant cultivars have been developed both by conventional breeding (triazine, imidazolinone resistance) and by genetic modification (glyphosate and glufosinate resistance). The former has been adopted on a limited area in many countries but the latter (GMHT) cultivars dominate production in N. America whilst not being approved in Europe. Resistance to glyphosate is not yet common in Canadian rape crops though is commoner in the USA, where GMHT crops have been grown more intensively. Herbicide resistant cultivars have provided tools for farmers to overcome other forms of resistance, however, their longer-term success in the future is questionable. In Europe where GMHT crops are not approved, herbicide resistance in grass weeds remains a serious problem. This is compounded by the presence of alternative products used to overcome resistance, such as propyzamide, appearing in ground and surface waters, breaching EU standards for pesticides in water. Thus, weed control in oilseed rape across the world is not easy and the traditional herbicide based model is failing for a variety of reasons. There are no alternative herbicide tools for the immediate future and thus farmers need to take a more integrated approach to their weed management, combining optimum agronomic practices with reduced use of herbicides to manage their weeds. Attention to detail in management practices to minimise weed presence at drilling is, and will be, of increasing importance. Modern 21st Century practices such as GPS navigation and computer-based engineering technologies are beginning to support more integrated management practices.
