Herbicide resistant crops are not new. We have been using them since the introduction of selective herbicides. Were they not resistant, they would have been killed along with the weeds upon receipt of the application of the herbicide.

Where herbicides have been very effective against weeds, nature has fought back by having a proportion of the population able to resist the effects of the herbicide. The more effective the chemical, the more often it is used and the greater is the chance that herbicide resistance will build up to economic proportions. The consequence of this is that the range of effective selective herbicides is diminishing. We are regularly reminded by the agricultural chemical industry that there are no new herbicide chemistries in the pipeline and that any new releases will be modifications of existing modes of action.

Chemical Industry Response

What then is the approach of the agricultural chemical industry to this problem? Clearly, the development of new herbicides is long term and very expensive and the prospects of a substantial payout needs to be high. This

approach at this time is largely discounted. Of greater immediate prospect is the extension of use of existing herbicides or modifying their use pattern by changing the tolerance of crop and pasture varieties. Thus, emerging onto the international scene are crop and pasture varieties which have herbicide resistance capabilities that were not previously available.

The net result of this approach is that crops for which selective herbicides were not available, or for which there was a limited range of chemical control options, now can be grown on fields previously not appropriate because of the weed spectrum. Higher yields can now be obtained from the cleaner crops and pastures because of better weed control or by reduced crop or pasture damage from the chemical.

An obvious example of this approach is the ability to control broadleaf weeds in crops such as canola and lupins.

A particularly interesting example is the use of the more broad spectrum herbicides such as glyphosate (Roundup) and glufosinate (Basta). In the case of glyphosate resistance this has the effect of changing the chemical from a non-

selective pre-planting herbicide to a post-emergent herbicide with selectivity against a broad spectrum of weeds.

A summary of some of the options which are being developed is listed in Table 1 (Connor and Field, 1995). In this table the mechanisms of obtaining the resistance in the crop are identified and it should be noted that the process can be via traditional methods or through the new biotechnologies. To a large extent the process does not matter greatly but we need to consider the management implications.

Advantages of HR cultivars

Several advantages can be identified for the utilisation of these new cultivars and these include:

(i) increased options - the availability of an extra herbicide option is extremely valuable. In some cases it simply allows the crop to be grown and in others it provides an alternate mode of action for the management of herbicide resistance development in the weed population;

(ii) more flexibility - the increased options provide greater flexibility in terms of crop rotations and the ability to respond quickly to market opportunities;

(iii) increased safety - where safer chemicals are able to be used, the risks to personal safety are clearly reduced. There may also be advantages where the chemical involved is environmentally benign or is used in very low concentrations;

Table 1. Some of the herbicide resistant crops under development, grouped according to the techniques of development (Connor and Field, 1995).

Traditional Selection
* triazine-resistant canola

Seed Mutagenesis
* terbutryn-resistant wheat
* sulfonyl urea-resistant soybean
* imidazolinone-resistant wheat

Cell Selection
* sulfonyl urea-resistant canola
* atrazine-resistance in soybean

Genetic Engineering
* sulfonyl urea-resistance - cotton

* glufosinate (basta) resistance
- rice
- canola

* glyphosate (Roundup) resistance
- cotton
- soybeans
- maize
- wheat

* bromoxynil-resistant
- cotton
- subclover

* 2,4-D resistant cotton

(iv) reduced crop/pasture damage - in some cases herbicides do inflict damage to the crop or pasture in addition to the weeds. By having the resistance character in the productive plant the chances of yield-depressing damage may be averted. This is exemplified by the use of bromoxynil in subterranean clover where damage to the pasture can be significant (Dear et al. 1996), depending on the timing of the application;

(v) simplicity - in many cases the production system can be simplified - one application can replace two applications or two herbicides. We need to be aware, however, that in nature, simple means unstable and we need to think of the system in holistic and not marginal terms.

Concerns

Whilst the emergence of these cultivars is attractive in many cases, there is a number of aspects that should not be ignored:

(i) further development of resistance - unless the principles of the management of herbicide resistance in weeds continue to be applied, there are no guarantees that persistent use of a herbicide will not result in herbicide resistance developing in the weed population. Given the investment in these cultivars it would seem important to control more strictly the rotational schemes of crops and herbicides;

(ii) self-sown crop seed - these HR crops will often germinate at the following season break where seed has been carried over from the previous harvest. Where they have resistance to herbicides, then clearly they will not be controlled by the application of that herbicide. Thus, in the case of glyphosate resistance, application of Roundup to provide a weed-free seedbed will be somewhat less successful where self-sown crop plants are present;

(iii) resistant escapes - in some cases, the opportunity exists for the resistant genes to escape. In the case of canola, for example, the ability exists through cross pollination to transfer the resistance character to related species (for example, wild radish, wild turnip) ultimately defeating the purpose for which the HR cultivar was developed;

(iv) abandonment of IWM - as previously described, there will always be the temptation to forget the application of the principles of integrated weed management because of the increased simplicity of the system. Such temptation needs to be avoided in order that we preserve the lives of both the cultivars and the herbicide;

impact on conservation farming - this is particularly important when considering the impact of glyphosate-resistant crops. Roundup is an essential part of the conservation farming system and we need to preserve its role. Resistance buildup by weeds to this herbicide threatens our modern, soil-conserving practices of reduced tillage and we do not want to revert to the traditional ways of intensive cultivation. Resistance to glyphosate is now with us (Pratley et al. 1996) and we need to proceed with caution.

Case Studies

(a) Table 2 looks at a possible scenario whereby triazine-resistant canola is introduced into a rotation in a ley farming situation. What is demonstrated is the regular and frequent use of simazine in the system, the most serious consequence of which would be the establishment of resistance to simazine by vulpia. What then would our options be for vulpia control?

Table 2. A possible scenario with the introduction of simazine-resistant canola into the rotation. The serious implication is the potential buildup of herbicide resistance in vulpia.

ROTATION

pasture ← simazine

wheat

canola ← simazine

wheat

lupins ← simazine

pasture

pasture ← simazine

pasture

(b) Table 3 shows the use of Roundup herbicide in a ley-farming rotation where glyphosate-resistant wheat and lupins have been included. Such a change in a reduced tillage system may result in up to 11 applications of Roundup relative to 4 or 5 under the current regime.

Table 3. A possible scenario for the introduction of glyphosate-resistant wheat and lupins into the rotation. The number of applications of glyphosate may increase by more than a factor of 2.

ROTATION

Pasture - Roundup spraytopping

G-Wheat - Roundup presowing

Roundup post sowing

(x2?)

Canola - Roundup presowing

G-Wheat - Roundup presowing

Roundup post sowing

(x2?)

G-Lupins - Roundup presowing

Roundup post sowing

(x2?)

Pasture

Whereas the present system allows escapes from the application of Roundup pre-planting to be controlled during sowing or a post-emergent herbicide, the proposed change would provide consistent selection pressure to the weed population and fast-track glyphosate resistant weeds.

Conclusion

The availability of herbicide-resistant cultivars is likely. These will provide more options and create new uses for old chemicals. It is important when embracing the new scheme that the future implications are considered and addressed. We need to preserve whatever options we have.

References

1. Connor, A.J. and Field, R.J. (1995). Herbicide-resistant crops: a new approach to an old problem or a radical new tool. In “Herbicide-Resistant Crops and Pastures in Australian Farming Systems”, pp.53-72, Dept.Primary Industries and Energy, Bureau of Resource Sciences, Canberra.

2. Dear, B.S., Sandral, G.A., Pratley, J.E. and Coombes, N.E. (1996). Effect of time of application of MCPA and bromoxynil in relation to flowering on subterranean clover seed yield and quality. Australian Journal of Experimental Agriculture 36:177-84.

3. Pratley, J., Baines, P., Eberbach, P., Incerti, M. and Broster, J. (1996). Glyphosate resistance in annual ryegrass. Proceedings NSW Grasslands Conference, Wagga Wagga (in press).

4. Proceedings 25th Riverina Outlook Conference 1996 - Page 54