Allelopathy in canola: potential for weed management

Canola (Brassica napus L) is a member of the family Brassicaceae, and is one of the leading crops in the world for the production of vegetable oil for human consumption, animal nutrition, and, more recently, biodiesel (Yasumoto et al. 2010). It has received great attention in Australian agriculture due to its price, substitution for other vegetable oils, increased demand due to population growth, demographic changes, economic growth, changing consumer preferences, domestic and foreign trade, and food policies. Moreover, as a cover and/or break crop it has played a significant role in agriculture due to its wide variety of benefits to the overall farming system. In Australia, canola is third-largest broad-acre crop (after wheat and barley), and according to the AOF (Australian Oilseeds Federation), production for 2011-12 is estimatedat 2.44 million tones from 1.81 million ha (www.australianoilseeds.com). There are many factors responsible for low yields in canola crops, among them, inevitably,the large number of weed species that occur and the difficulty of economic control.In addition, Australian farmers have moved away from aggressive tillage practice because of the extreme risk of soil erosion, damage to soil structure, and reduction in soil carbon. Consequently, current crop rotations and seeding techniques are highly dependent on herbicides. Repetitious use of herbicides has selected for resistant weed biotypes ' herbicide resistance has evolved in 25 weed species in Australia, and a number of weed species have evolved resistance to several herbicide modes of action. Foremost among them is annual ryegrass, and some of its populations have evolved resistance to all the selective-mode-of-action herbicide groups (Storrie et al. 2009). In recent years, the increasing cost of herbicides and ecological and human health concerns, have renewed interest in exploiting non-chemical alternatives including allelopathy and crop competitiveness (Holethi et al. 2008).