Progress in developing weed competitive wheat

| Date: 28 Jul 2010

Progress in developing weed competitive wheat
Gurjeet Gill and Michael Zerner
School of Agriculture, Food & Wine
Waite campus, University of Adelaide


Focus on integrated weed management has become sharper since the development of widespread resistance in ryegrass and 25 other Australian weed species. Figures from the CRC for Australian Weed Management indicate that growers in southern Australia spend around $80/ha/year on herbicides. Even with this level of investment, weeds are having a major impact on crop productivity. An important but often overlooked component of IWM is the competitive ability of the crop itself.

Wheat is widely accepted as the weak link in weed management in Australian cropping systems. Research undertaken in a previous GRDC project (UA466) showed that the increase in ryegrass seed bank after wheat was significantly greater than after other cereals such as barley or oaten hay. Seed bank data collected from 57 farmer paddocks in a previous project showed >15-fold increase in wheat as compared to 7 for barley and 1 for pasture and oaten hay. In a system where herbicide resistant weeds are widespread, wheat with improved competitive ability (CA) is required to restrict increases in weed seed bank.


Results and Discussion



In a field study undertaken at Yeelana on the Eyre Peninsula in 2009, ryegrass plants surviving Boxer Gold and Sakura in wheat managed to produce more than 5000 seeds/m2 (source: Kleemann and Gill, 2009). These results clearly demonstrate the weakness of our current crop production systems and also why farmers face ongoing need to use selective herbicides year after year. This result had nothing to do with resistance to these herbicides but entirely due to poor capacity to wheat to compete and suppress surviving weeds. Obviously there is a need to develop strategies to minimise seed set by weeds that survive use of selective herbicides. Many growers are experimenting with the use of non-selective herbicides for crop-topping to prevent seed set. However, crop damage and seed set control from such treatments can vary considerably depending on wheat variety, ryegrass population and the timing of the treatment itself. Some crop species such as barley achieve seed set control quite effectively through weed suppression. If competitive ability of wheat could be enhanced to a level close to barley then it is expected to have a major influence on weed populations on Australian farms.


Research undertaken at UA in a GRDC funded project has been selecting for weed competitive ability from a population of 6000 wheat lines developed at CSIRO. Some of the breeding lines have shown similar or greater early vigour than barley until the 3 leaf stage. However, once tiller production commenced barley was able to achieve greater ground cover than all wheat lines. This raises the possibility of combining early vigour and higher crop density to obtain additive or synergistic effects on weed suppression and forms part of this proposal. In the results presented in Table 1, it can be clearly seen that some of the high vigour breeding lines can produce considerably more dry matter and NDVI (measured with Green Seeker) than cultivars Wyalkatchem and Yitpi. Previous research has shown that early vigour and DM in wheat plays an important role in competition with weeds. In another study in 2009, competitive ability of different wheat lines was assessed against oats, ryegrass and mustard (data presented for ryegrass in Table 2). Results clearly show large differences between lines in their competitive ability with ryegrass. Wyalkatchem and Janz were found to be the worst performers against ryegrass which resulted in ryegrass producing 153 seed heads/m2. Barley as expected was very competitive against ryegrass and reduced its seed head production by 67% compared to Wyalkatchem and Janz. The best breeding lines of wheat reduced seed head production of ryegrass by 50% compared to Wyalkatchem. Clearly significant progress has been made in enhancing competitive ability of wheat against weeds. The challenge now is to select lines which not only possess high weed competitive ability but also possess high yield, good disease tolerance and grain quality.


RIM analysis


RIM (Resistance and Integrated Management) is a computer based model developed to evaluate biological and economic performance of integrated weed management systems for herbicide resistant weeds. This model was developed by WAHRI (WA Herbicide Resistance Initiative) and was used to determine benefits of integrating weed competitive wheat cultivars into a wheat – barley – canola rotation. Model runs showed significant benefits of weed competitive wheat cultivars when used as a component of integrated weed management program. Additional weed suppression of 30-50% by the new wheat cultivars as compared to older cultivars (e.g. Janz) was found to cause large changes in weed population trajectories as well as increases in gross margins over a 20 year time frame. The model was run on a ryegrass population that had not yet developed resistance to trifluralin. There is no doubt, the benefits from the use of weed competitive wheat cultivars would be even greater when trifluralin resistance becomes common, as is the case now in South Australia.


Table 1. Early seedling dry matter and NDVI of selected breeding lines and two commercial cultivars at Mintaro, SA in 2009.
DM per plant (% of Yitpi)
NDVI at 56 DAS

DM= dry matter; NDVI = normalised difference vegetation index


Table 2. Annual ryegrass seed head production in selected wheat lines at Roseworthy in 2009.
Ryegrass seed heads (No. per m2)
Funding support from GRDC (UA00061 and UA00112) for this research is gratefully acknowledged.
Gurjeet Gill, 08 83037744

Michael Zerner, 08 8303 7988