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Evaluation of rice varieties for allelopathic effects on Australian rice weeds - linking laboratory to field

Alexa N. Seal1, Jim Pratley1 and Terry Haig2

1 EH Graham Centre for Agricultural Innovation,
Charles Sturt University Locked Bag 588, Wagga Wagga, NSW, Australia 2678
Email and
School of Science and Technology, Charles Sturt University. Email


Australia’s high rice yields are jeopardised by the presence of weeds, many of which are native to Australia. This paper reviews the rice allelopathy research which has been conducted in Australia on Australian rice weeds over the past six years. Both laboratory and field research have been undertaken to confirm that allelopathic interactions exist in the system, to identify cultivars with high allelopathic potential, and to substantiate the impact of these cultivars in a field situation.

Results from laboratory studies using the Equal Compartment Agar Method (ECAM) bioassay on barnyard grass (Echinochloa crus-galli), lance-leaved water plantain (Alisma lanceolatum), starfruit (Damasonium minus), arrowhead (Sagittaria montevidensis) and S. graminea are discussed. The specificity of allelopathy is also explored when comparing certain cultivars with high potential against more than one member of the Alismataceae family. Several key cultivars identified as having either relatively strong or relatively weak allelopathic activity in the bioassays were also examined in the field against one of the most economically important rice weeds, Damasonium minus, in an attempt to elucidate the allelopathic contribution to plant interference. Correlation between laboratory and field performance is also reviewed.

This research does not imply that allelopathic potential in crops will negate the necessity to apply synthetic herbicides, but rather that allelopathy could be a valuable component in an integrated weed management program.

Media Summary

Several rice varieties with the ability to suppress the growth of rice weeds have been discovered, and could play a valuable role in an integrated weed management program.

Key Words

Alismataceae weeds, rice allelopathy, weed suppression


Herbicide resistance is becoming a problem in rice based farming systems and poses a major threat to the sustainability of the rice industry. Already, high levels of bensulfuron resistance have been detected in Australian weed populations of the Alismataceae family (Broster et al. 2001) due to the high dependency on a very limited number of herbicides available for effective control against all rice weeds.

Consequently, alternatives to conventional synthetic herbicide application have become the focus of much research in Australia and worldwide. The potential use of allelopathy as part of a weed control program is one option which has been gaining attention. Dilday et al. (1991, 1994, 1998) first detected the presence of allelopathy in rice more than 15 years ago in a field trial. This research has sparked global interest, and there are currently several international groups looking at using allelopathy in rice to control rice weeds such as red stem, duck salad, E. crus-galli (barnyard grass), Trianthema portulacastrum and C. difformis (dirty dora) (Fujii 1992; Hassan et al. 1994, 1998; Olofsdotter et al. 1995, 1999; Olofsdotter and Navarez 1996; Marambe 1998, Chung et al. 2001, Seal et al. 2004).

Weeds such as dirty dora and barnyard grass are ubiquitous weeds of rice and have been tackled by international research groups. However, the community of Australian rice weeds contains many weeds native to Australia that are not problematic in other countries. For this reason, research into the allelopathic potential of rice against several main broadleaf weeds found in New South Wales (NSW) rice paddocks was undertaken in Australia. The following paper will review the agronomic rice allelopathy research which has been conducted in Australia on Australian rice weeds over the past six years.

Evaluation of rice varieties for allelopathic effects on Australian rice weeds

Damasonium minus (starfruit), Sagittaria montevidensis (arrowhead), Echinochloa crus-galli (barnyard grass), Sagittaria graminea, Cyperus difformis (dirty dora), Alisma plantago-aquatica (water plantain) and Alisma lanceolatum (lance-leaved water plantain) all infest Australian rice paddocks. All of these weeds, except for barnyard grass, are members of the Alismataceae, a family with global presence.

Arrowhead was selected as the original test species to determine if rice allelopathy could offer an alternative with potential as part of an integrated weed management program. To establish the presence of allelopathy in rice germplasm, rice cultivars were screened against arrowhead using a bioassay designed to eliminate competition from the system. The Equal Compartment Agar Method (ECAM) bioassay was developed by Wu et al. (2000) and modified by Seal et al. (2004). Any effect on arrowhead root growth would presumably be due to compounds released by rice roots growing in the same agar medium as the arrowhead. Results from the ECAM bioassay showed that a range of allelopathic potential existed within the rice cultivars selected in their ability to suppress the root growth of arrowhead. This agreed with several other reports on rice allelopathy (Fujii 1992; Dilday et al. 1994; Hassan et al. 1994; Olofsdotter et al. 1995). However, some of the cultivars reported to have either high or low allelopathic potential against various weeds did not result in similar potential against arrowhead. On the other hand, some cultivars allelopathic towards arrowhead were also reported to be allelopathic against other weeds. This finding sparked further research looking at the specificity of allelopathy.

To determine whether certain rice cultivars had the capability to inhibit the growth of more than one weed species, several weeds from the Alismataceae were screened, including starfruit, lance-leaved water plantain and S. graminea. Barnyard grass, which does not belong to the Alismataceae family, was also included. Recently conducted research by Seal et al. (in prep) shows that some cultivars exhibit considerable allelopathic capability against multiple weeds belonging to the same family. Figure 1 shows the average weed root growth across all five tested weed species.

Significant differences existed between the rice cultivars in their ability to suppress weed root growth (l.s.d. = 15.7, p<0.001) using the percent control data. The average percent root growth inhibition of all five rice weeds obtained from the individual full screenings against 27 rice cultivars is shown in Figure 1. Average weed root inhibition ranged from 27.4 % (Langi) to 92.5 % (Giza 176). Two cultivars which are widely sown in Australia, Amaroo and Jarrah, are among the top most allelopathic cultivars. On average, these cultivars inhibited weed growth by 90 % when all five weeds were considered. Langi, another key cultivar used in Australian rice growing, was the least allelopathic of all cultivars on average, only inhibiting weed root growth by 27 %. Langi actually stimulated the growth of barnyard grass when compared to the control values in the individual screening experiments (Seal et al. in prep).

In Seal et al. (2004), comparisons between cultivar performance in allelopathy research from Australia, the Philippines, the United States and Egypt have been discussed. Most importantly, Giza 176, a variety which was chosen for its apparent lack of allelopathic potential against dirty dora or barnyard grass (Hassan et al. 1994), is actually the overall most allelopathic cultivar against the five rice weeds examined using the ECAM bioassay. When all five weeds were tested individually, Giza 176 consistently ranked among the top seven most allelopathic cultivars (Seal et al. in prep). Of the seven most allelopathic cultivars overall in Figure 1, all of them ranked among the top ten cultivars in the screening against the individual weeds (Seal et al. in prep). A possible explanation for the discrepancy in the observed allelopathic potential of this cultivar is the presence of competition which influenced overall plant interference in the work by Hassan et al. (1994, 1998). However, results from a field trial conducted in Yanco over the 2000/2001 growing period indicate that Giza 176 performs well in the field (Seal et al. unpublished). Clearly, the specificity of allelopathy remains to be determined. It is difficult to compare such studies when different plant parts have been used for analysis, different bioassays have been employed and there is no standardised protocol to follow. However, one established and accepted criterion is that laboratory studies need to be followed on with field trials.

Validation of bioassay results with field evidence

In addition to encouraging bioassay results, field research is necessary to validate the results and establish the potential contribution of allelopathy in plant interference under natural field conditions (Olofsdotter and Navarez 1995, 1996; Blum 1999; Inderjit and Callaway 2003). Results from such field studies are important indicators of the future potential role of allelopathy in weed control. A practical application of allelopathy in the field with an observable contribution to the overall weed control needs to be realised to warrant the necessary time and money for further research. Nevertheless, few studies have attempted to correlate laboratory and field results.

To examine the correlation of bioassay results with observations in the field and determine if performance in laboratory studies could be a suitable indicator of future performance in the field, a trial was conducted at the Yanco Agricultural Institute in NSW over the 2000/2001 growing period. Starfruit, one of the most economically important rice weeds in the Australian rice growing regions, was selected for the trial as it establishes an almost monoculture-like infestation in these regions. Seal et al. (unpublished) found that there were significant differences between the rice cultivars in their ability to interfere with starfruit growth. Starfruit dry weights were inhibited by 27.8 % (Rexmont) to 95.4 % (Tono Brea 439) compared to the no-rice control plots. Table 1 shows the most and least interfering cultivars in the field.

Table 1*: Reduction in starfruit dry matter grown in association with different rice cultivars

Starfruit Dry Weight (% Inhibition)

Most Interfering Cultivars**

Least Interfering Cultivars

Tono Brea 439




Hungarian #1




Kingmen T. C. M.


CI Selection 63


Giza 176






Woo Co Chin Yu




BG 34/8


IET 1444




* this table has been reproduced from Seal et al. (2005) with kind permission
** ‘interfering’ has been used as field observations are the result of both competitive and allelopathic interactions

The influence of rice cultivars against starfruit in the field was then compared with the aforementioned laboratory bioassay results. A relatively high correlation co-efficient of 0.84 was obtained when the raw field trial data were compared with the raw bioassay data (Seal et al. unpublished). Several of the most allelopathic cultivars in the bioassay also performed well in the field. This linear relationship between results obtained from the laboratory and from the field was also observed in an important paper by Olofsdotter et al. (1999). This step is paramount in establishing the potential of allelopathy for weed control in the field and that results from bioassays therefore have some ecological and field relevance.


The fact that several cultivars identified in the individual screenings of rice weeds were successful in substantial root growth inhibition of more than one weed is a very important result indicating that it is worthwhile to conduct the necessary work on the chemical and genetic aspects of rice allelopathy. The demonstration of a high correlation between bioassay results and actual performance in the field highlights the possibility that using allelopathy as part of an integrated weed management program is realistic.

Using allelopathic mulches or breeding crop cultivars with good overall chemical and physical interference mechanisms which give them an advantage over establishing weeds are two ways such allelopathic potential can be exploited (Putnam and Duke 1974, Rice 1995). The research outlined here does not imply that allelopathic potential in crops will negate the necessity to apply synthetic herbicides, but rather that allelopathy could be a valuable component in weed control.


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