Why do we need them?

Herbicide resistance in annual ryegrass (Lolium rigidum) is widespread and of great economic significance in the southern wheat-belt of Australia. The only way farmers with resistant weeds can still produce profitable crops is to practice integrated weed management. Farmers must use many non-chemical ways to suppress weeds, in addition to careful use of herbicides. One tactic is to improve the competitive ability of crops. This can be done by increasing seeding rates, by growing “healthy” crops with adequate fertiliser, or by making the varieties more competitive against the weeds.

Research at Wagga Wagga has shown that the modern semi-dwarf wheats are much less competitive against weeds than older standard wheats (Lemerle et al. 1996b). This means that modern wheats are more dependent on chemicals for reliable weed management than the older types (Lemerle et al. 1996b). The availability of selective herbicides during the last three to four decades to control weeds has allowed cereal breeders to select varieties with high harvest index that yield well in the absence of weeds but which are poor competititors.

In order to reduce farmers’ dependence on herbicides, manage resistance and reduce costs, we are examining the potential to breed competitive ability back into wheat.

Differences between crop species?

Information on the relative competitive ability of different crop species enables farmers to choose competitive species for paddocks with serious weed problems to reduce dependence on herbicides. The most strongly competitive crops are oats, triticale and cereal rye, which maintain acceptable grain yields while at the same time suppress growth and seed production of the weed. In contrast, the grain legumes - field pea and lupin - are very poorly compeittive and are therefore very dependent on chemical to achieve reasonable yields. A recent trend to wider row spacing to facilitate direct-drilling of grain legumes could decrease their compeititve ability even more. Competitive ability of canola is intermediate. In all these crops there is little difference between varieties of each species in competitive ability.

In both the wheat and barley, there is a much larger variation between varieties in competitive ability against weeds than in the other winter crop species. This implies potential to improve the competitive ability in wheat and barley.

Differences within wheat?

In 1993, 300 Australian and overseas genotypes of wheat were screened against a single density of annual ryegrass to determine differences in competitive ability (Lemerle et al. 1996b). Losses in wheat yield ranged from 0-100% and ryegrass biomass varied from 0 to 500 g/m² depending on genotype. Highly competitive varieties are generally tall, high tillering, with strong early vigour and have a prostrate habit with long, broad leaves. There is clearly potential to breed for competitive ability in wheat and the next stage in the research program was to address this and determine the best approach.

How to select competitive varieties?

There are two possible ways to select for competitive ability, either by:

(i) direct selection in the presence of weeds during the breeding process; or

(ii) indirect selection in the absence of weeds for attributes which are correlated with competitive ability.

At Wagga Wagga, both of these approaches are being evaluated for potential to improve competitive ability in the wheat breeding program. Penalties associated with competitive ability are also examined and included in an economic analysis.

Direct selection by growing genotypes in the presence of weeds in replicated small plots in field trials at many sites has a number of major disadvantages:

only possible in the later stages in the breeding program when sufficient seed is available;

lower genetic variability for morphological characters at the end of the breeding program likely to lead to only small increases in competitive ability;

expensive and slow.

In contrast, indirect selection has a number of advantages:

allows plant breeeders to visually select for competitive ability in the early stages of the breeding program;

enables introduction of new genetic material with potentially large increases in competitive ability.

We are selecting for two attributes (leaf size and early vigour), which are correlated with competitive ability. The near-isogenic lines for early vigour were produced by Dr Richard Richards (CSIRO, Canberra) by back-crossing material from China and India into local varieties.

An alternative to selection is to test for competitive ability just before a variety is released, providing farmers with information on competitive ability as part of the cultivar characteristics. This allows farmers to choose strongly competitive varieties for weedy fields. The amount of testing required to establish reliable predictions of competitive ability would depend on the impact of the genotype x environment interaction; this is currently being assessed.

Acknowledgements

The author thanks Ms Birgitte Verbeek (Technical Officer), Mr Bill Littlewood (technical assistance), and the Grains Reseach and Development Corporation of Australia for financial support.

References

1. Lemerle, D, Verbeek, B. and Coombes, N. (1995). Losses in grain yield of winter crops from Lolium rigidum competition depend on crop species, cultivar and season. Weed Research 35, 503-509.

2. Lemerle, D., Verbeek, B. and Coombes, N. (1996a). Interaction between wheat (Triticum aestivum) and diclofop to reduce the cost of annual ryegrass (Lolium rigidum) control. Weed Science (in press).

3. Lemerle, D., Verbeek, B., Cousens, R.D. and Coombes, N. (1996b). The potential for selecting wheat varieties strongly competitive against weeds. Weed Research (in press).