Existing herbicide tolerant canola varieties in Australia arn't perfect.

Robert Norton has reviewed conventional canola and herbide practices in 2003. Some limitations to these existing options mentioned in Robert's comments have been highlighted by GMO Pundit. The point of highlighting conventional variety limitations is to show that alternative herbicide tolerant GM canola varieties, such as glyphosate tolerant canola, offer croppers some additional options for improving their financial bottom line.

Herbicide Traits in Conventional Canola (pages 4-5, Norton 2003)
Perhaps the major weakness in conventional canola production systems is the limited opportunity for broadleaf weed control. Prior to 1993, there were no postemergence herbicides for the control of weeds such as wild radish (Raphanus raphanistrum), muskweed (Myagrum perfoliatum) and charlock (Sinapis arvensis) in canola crops (Sutherland 1999). There are several group
A herbicides registered for grass weed control, and a group I herbicide (clopryralid) for control of various Asteraceous weeds. In addition, robust weed control has been routinely provided to canola and other crops by the pre-emergence use of the group D herbicide, trifluralin.
Despite these control options, the pressure of particular weeds and need for lower costs have led to the inclusion of various herbicide tolerance traits into canola and varieties with these traits have been widely adopted in Australia (Preston 2003). Conventional canola varieties are susceptible to particular triazine (group C), sulfonyl urea and imidazolinone (group B) and most other broadleaf herbicides used in grain production. The presence of residues of herbicides is now a major factor determining the position of crops within a rotation. For example, the plant-backs from the prior crop can vary from nine months (simazine) to 34 months (imazethapyr), which restricts crop selection following the use of these herbicides. As well, there is widespread weed resistance to group A and group B herbicides through grain producing areas (Heap 2002).
Appendix 1 describes commonly used canola herbicides and their resistances management groups.

TT Canola
Triazine-tolerant (TT) canola varieties were the first herbicide tolerant (HT) crops in Australia with the release of the cultivar Siren in 1993 (Colton and Potter 1999). Triazine herbicides are widely used in pulse crops and for winter cleaning of pastures prior to the cropping phase
(Gill and Holmes 1997). The adoption of the TT canola variety Karoo contributed to much of the production increase in Western Australia, where area sown went from 95,000 hectares to 950,000 hectares between 1996 and 1999. By 1999, Karoo comprised about 90% of the area
sown in Western Australia (Carmody et al. 2001). TT cultivars make up less than one per cent of the area grown in North America but about 55% of the area sown in Australia (Figure 4).
TT canola cultivars are able to tolerate high dose rates of triazine (group C) type herbicides. This tolerance is inherited in the cell cytoplasm, which means the genes for resistance are not carried in pollen, but remain in the maternal parent. TT varieties suffer from an inefficient photosynthetic system, which leads to lower vigour, reduced growth, yield and seed oil content (Arntzen et al. 1982). Robertson et al. (2002) used a simulation modelling approach which
identified a 26% lower seed yield for TT canola compared to conventional canola types in weed free situations. Despite this penalty, TT cultivars have enabled growers to use robust in-crop herbicides to control weeds that were intractable in conventional varieties. This technology has been reliable and relatively cheap, which has contributed to its widespread adoption in Australia.
IT Canola
In 2000, canola varieties (Clearfield®) with tolerance to imidiazolinone (group B) herbicides were released in Australia. This trait was induced by using mutagenisis, a traditional plant breeding technique and the varieties do not carry the inherent yield penalty of TT canola. The herbicide used with IT canola (On-Duty®, imazapic/imazapyr) has good activity on a wide spectrum of weeds although it does have residual action which mayrestrict crop selection. For example, while peas, beans and chickpeas can beplanted in the year after IT canola, conventional canolacannot be sown for 34 months after using On-Duty®.In Canada, IT canola varieties comprise about 17% of the area sown (Buzza 2001), whereas, it is less than five per cent in Australia. Resistance to group B herbicides has been identified in 15 Australian weed species, including populations of wild radish and annual ryegrass (Lolium rigidum) (Heap 2002). This pre-existing resistance would be likely to restrict the widespread use of IT canola in Australia. Carmody and Hashem (2001) found that the best IT lines produced higher gross margins than the best TT lines in Western Australia, although they recognised that appropriate herbicide rotation was integral to ensuring the technology remained useful to growers.

Useful discussions of the general topic can be found in Herbicide-resistant Crops and Pastures, Ed G. D. McClean and G. Evans, Bureau of Resource Sciences, Canberra, 1995.