Growth behavior and glyphosate resistance level in 10 biotypes of Echinochloa colona in Australia

Recently, poor control of Echinochloa colona with glyphosate has been reported in no-till agriculture systems of the northern grain region (NGR) of Australia. Two experiments were conducted using 10 biotypes of E. colona selected from the NGR of Australia to understand differences in their growth behavior and resistance pattern. Growth studies revealed that these biotypes differed in plant height (53-70 cm plant−1), tiller production (30-52 tillers plant−1), leaf production (124-186 leaves plant−1) and seed head production (37-65 seed heads plant−1). Days taken to seed heads and shoot biomass in these biotypes ranged between 40-48 d and 21-27 g plant−1, respectively. Seed production in these biotypes ranged between 5380 and 10244 seeds plant−1; lowest for biotype B17/25 and highest for biotype B17/13. Correlation studies revealed that seed number plant−1 had a positive correlation with plant height (r = 0.67), tiller number plant−1 (r = 0.89), leaf number plant−1 (r = 0.73), seed heads plant−1 (r = 0.78), seed head weight (r = 0.79), shoot biomass (r = 0.77) and root biomass (r = 0.46). The glyphosate dose-response study showed a wide range of responses in these biotypes and the glyphosate dose required to reduce 50% biomass (GR50 values) was estimated between 217 to 2159 g a.e. glyphosate ha−1. GR50 values of biotypes B17/16, B 17/34 and B17/35 were 719, 2159 and 884 g ha−1, respectively, making them 3, 10 and 4-fold resistant to glyphosate compared with the susceptible biotype B17/37. Growth behavior and seed production potential in these biotypes had no correlation with the resistance index. These results suggest that some biotypes of E. colona are highly problematic; for example, biotype B17/34 was not only highly glyphosate-resistant, but also produced a high seed number (9300 seeds plant−1). This study demonstrated that there is a possibility of great risk with the increased use of glyphosate for managing E. colona in the NGR of Australia. The results warrant integrated weed management strategies and improved stewardship guidelines are required for managing glyphosate-resistant biotypes of E. colona and to restrict further movement of resistant biotypes to other regions of Australia.


Introduction 6
Biotypes B17/12, B17/13, B17/34 and B17/49 produced a similar number of seeds (8298-124 10244 plant -1 ), with their seed production being higher than biotypes B17/25 and B17/35. 125 A linear positive correlation was found for seed number with plant height (r = 0.67), 126 tiller number plant -1 (r = 0.89), leaf number plant -1 (r = 0.73), seed heads plant -1 (r = 0.78), 127 seed head weight (r = 0.79), shoot biomass (r = 0.77) and root biomass (r = 0.46) ( Table 2). 128 Shoot biomass had a negative relation with days taken to seed head initiation (r = -0.54) 129 (Table 2). Plant height, tiller production and seed head weight also had a negative relation 130 with days taken to seed head initiation. Root biomass had a positive correlation with tiller 131 production, leaf production and shoot biomass. 132 The results of this study demonstrated that characteristics like tall nature and high 133 tillering capacity allow E. colona biotypes to produce a high leaf number that resulted in a 134 large number of seed heads and seeds. Therefore, there is a need to target tiller production in 135 E. colona to reduce seed numbers. A recent study on crop-weed interference suggested that

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The high seed number observed in biotypes B17/12, B17/13, B17/34 and B17/49 was 152 largely attributed to a high number of tillers, leaves, and seed heads. Our study also revealed 153 that seed head weight also influences seed number. The number of leaves and seed heads 154 were similar between biotypes B17/13 and B17/35; however, seed production was lower in 155 B17/35, which could be due to the lower tiller production and seed head weight in B17/35.

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The time taken to seed head initiation in the present study was similar to a study conducted in Response to glyphosate 198 Out of 10 E. colona biotypes collected from the NGR of Australia, three biotypes (B17/16, B 199 17/34 and B17/35) had greater than 80% survival following treatments with 325 to 2600 g 200 a.e. ha -1 glyphosate. The probit analysis details for each biotype along with their level of 201 significance is presented in Table 3. The dose-response study of glyphosate for these biotypes  The present study on E. colona biotypes has increased our understanding of the 237 physiological basis of differences in seed production due to variations in morphological 238 characteristics and resistance behavior. It highlighted that growth parameters such as high 239 tiller production in E. colona biotypes leads to more leaves and in turn high seed production.

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The study further demonstrated that growth behavior and seed production potential in these 241 biotypes had no correlation with the resistance index. However, this research has posed more 242 questions than it has answered. This study suggested that biotypes such as B17/34 that are 243 highly glyphosate-resistant, and also produced a high seed number (9300 seeds plant -1 ) are 244 very problematic. Therefore, systematic research on weed biology, physiology and resistance 245 mechanism is required to answer these questions for better understanding. This study also suggested that there is a need to understand the likelihood of resistance transfer from resistant Statistical analyses 298 The first experiment was conducted in a completely randomized design and the second 299 experiment was conducted in a completely randomized design with a factorial arrangement.

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In both experiments, there was no interaction between experimental runs and treatments; 301 therefore, the data of the two runs were pooled for ANOVA. All the data met assumptions of