Competitiveness of Bensulfuron-Methyl-Susceptible and -Resistant Biotypes of Ammannia auriculata Willd. in Direct-Seeded Rice

Abstract

Ammannia auriculata Willd. (eared redstem) has become one of the most troublesome weeds in paddy rice in China. Resistance to bensulfuron-methyl (BSM) has spread extensively in this species. Greenhouse and field experiments were conducted to determine how the eared redstem biotype and density affect competition against rice. In the greenhouse experiment, five treatments were tested: a BSM-susceptible biotype at low density (58 plants m⁻², SL), a BSM-susceptible biotype at high density (288 plants m⁻², SH), a BSM-resistant biotype at low density (RL), a BSM-resistant biotype at high density (RH), and a control without eared redstem (CK). Eared redstem grew slowly until 15 days after sowing (DAS); however, growth accelerated after 20 DAS, and the eared redstem plants were taller than the rice from 55 DAS on. The SH and RH treatments were associated with greater intraspecific competition: eared redstem plants in the SH and RH treatments had fewer branches, fewer capsules, and less shoot dry weight per individual plant relative to the SL and RL treatments. The SH and RH treatments also caused greater reductions in the rice yield. The dry weight of rice at 141 DAS was reduced by 73% in the SL treatment, 98% in the SH treatment, 51% in the RL treatment, and 82% in the RH treatment, all relative to the CK. At 141 DAS, BSM-resistant plants were smaller than BSM-susceptible plants, suggesting a fitness cost of herbicide resistance in the absence of a herbicide. In the field study, eared redstem densities as low as 5 plants m⁻² reduced the rice yield by 22%. A density of 50 eared redstem plants m⁻² reduced the rice yield by 70%. Overall, these findings demonstrate that eared redstem is a highly aggressive weed species and threatens the rice yield even at a low density. However, the findings also demonstrate that BSM-resistant populations are less competitive. There is an urgent need to develop integrated management programs for this aggressive weed, which may include tactics to shift competitive dynamics in favor of rice. Additionally, this research provides the theoretical basis for the damage level, resistance risk evaluation, and management strategy of eared redstem in rice crop** systems.

1. Introduction

Rice (Oryza sativa L.) is one of the most important crops in the world. In China, rice is grown in every province except Qinghai, covering an area of 30.1 million hectares and yielding 211.9 million tons [1]. Traditional methods of paddy farming in China include direct seeding, mechanical transplanting, throwing transplanting, or manual transplanting of rice seedlings. Direct seeding has rapidly increased in popularity because this method can reduce labor costs and increase sustainability relative to transplanting methods [2]. Direct seeding can improve the economic well-being of rice farmers in China as well as South Asia and Southeast Asia [3]. A disadvantage of direct seeding, relative to transplanting, is an increase in weed pressure [4]. Unlike traditional transplanted fields, direct-seeded rice fields are not flooded. Therefore, farmers who practice direct seeding cannot rely on standing water to inhibit weed germination and growth. Many farmers instead rely on chemical control. Bensulfuron-methyl (BSM), a herbicide in the sulfonylurea family of acetolactate synthase (ALS) inhibitors, is widely used for the control of broadleaf and sedge weeds in rice [5,6].

Although herbicides can provide effective weed control, over-reliance on chemical approaches has led to the evolution of herbicide resistance in 267 weed species worldwide [7]. Resistance to ALS inhibitors is the most common type of herbicide resistance, accounting for one-third of reported cases [7]. Weeds, such as Ammannia auriculata Willd. (eared redstem) [8], Echinochloa crus-galli (L.) P. Beauv. (barnyardgrass) [9], Sagittaria trifolia L. (arrowhead) [10], and Cyperus difformis L. (small-flowered nutsedge) [11], have developed resistance to commonly used ALS inhibitors. In China, herbicide resistance has been reported for 30 weed species and 47 herbicides with 11 modes of action [12]. Ten of these herbicide-resistant species (eight monocotyledons and two dicotyledons) are resistant to multiple herbicides. At least ten herbicide-resistant weed species occur in paddy rice fields in China [9]. As chemical control options become less effective, feeding China’s 1.412 billion people [1] will increasingly require integrated weed management programs based on ecological principles. Such programs promote long-term sustainability and limit selection for herbicide resistance.

Understanding weed–crop interactions is a prerequisite for ecological weed management. Weed infestations reduce crop yield and quality while increasing expenses for farmers [13]. They often cause more economic damage than other crop pests, such as insects or fungi [14]. Crop yield losses are often driven by weed–crop competition for resources such as nutrients and light [15]. Factors influencing weed–crop competition include weed density, weed species, and weed biotype. Crop yield losses generally increase with increasing weed density, but this trend is not necessarily linear [15]. Some weed species are highly damaging in a given crop** system and environment, while others have little impact. Even within weed species, different biotypes can exhibit different competitiveness and cause different rates of crop yield loss [15]. This last factor is receiving increasing attention given the rapid proliferation of herbicide-resistant biotypes. The evolution of herbicide resistance sometimes incurs a fitness cost, i.e., resistant biotypes may achieve less growth and reproduction than susceptible biotypes when herbicides are not sprayed [16]. For example, glyphosate-susceptible lines of Echinochloa colona (L.) Link produced more biomass and more spikes than glyphosate-resistant lines in the absence of glyphosate [17].

Eared redstem is an annual herb in the Lythraceae family. This species occurs in tropical and subtropical regions worldwide, including parts of southeastern North America, Central and South America, Africa, Asia (including China), and Australia [18,19]. Plant height is often 15 to 40 cm but may reach 100 cm, exceeding the height of rice [18,20]. A single-eared redstem plant can produce approximately 50,000 seeds [20]. This species has become the third most dominant weed in main paddy fields in China, after barnyardgrass and Leptochloa chinensis (L.) Nees (Chinese sprangletop) and is increasingly harmful in direct-seeded rice in Zhejiang, Jiangsu, and Shanghai [20]. Ammannia spp. were reported to cause 39% rice yield loss at approximately 100 weeds m⁻² [21]. Eared redstem was estimated to cause a theoretical rice yield reduction of 58% at a density of 463 plants m⁻² [22]. Herbicide resistance is widespread in this genus and species. In California rice, BSM resistance was confirmed in Ammannia populations four years after this herbicide was introduced [23]. Approximately 97% of eared redstem plants collected from the Ningshao and Hangjiahu plains within the Yangtze River Delta region of China were resistant to BSM [24].

This study evaluates how eared redstem density and biotype (BSM-resistant vs. BSM-susceptible) affect competitiveness against rice. We addressed this question through complementary greenhouse and field experiments.

2. Materials and Methods

2.1. Greenhouse Study

A greenhouse experiment was conducted at Huajiachi Campus, Zhejiang University (elevation 23 m, 30°16′ N, 120°12′ E) in Hangzhou, China. The temperature range in the greenhouse was 20 to 30 °C, humidity ranged from 50 to 90%, and 14 hr of natural light was provided. The soil used was a silty loam with a pH of 6.6, 1.35% organic matter content, and 0.14% total nitrogen. Compound fertilizer (N + P₂O₅ + K₂O ≥ 45%, 15-15-15, Jiangsu Zhongdong Fertilizer Co., Ltd., Changzhou, China) was applied at 525 kg ha⁻¹ before rice sowing.

The experiment was arranged in a completely randomized design with five treatments: low density of BSM-susceptible eared redstem (SL), low density of BSM-resistant eared redstem (RL), high density of BSM-susceptible eared redstem (SH), high density of BSM-resistant eared redstem (RH), and a control without eared redstem (CK). There were three replicates for treatment. The plot area was 0.87 m². Seeds of rice (cultivar ** systems because it produces large quantities of seeds, exhibits rapid growth, and has a strong competitive ability. From a weed management perspective, it is important to avoid a rapid increase in the frequency of resistant biotypes in subsequent generations. In the future, multiple management practices need to be incorporated into an IWM system to effectively manage current BSM-resistant eared redstem populations in direct-seeded rice crop** systems. For example, the use of rape straw mulch is one promising option for controlling herbicide-resistant weeds and decreasing herbicide use in rice systems [35]. Other options for eared redstem control could include tillage or the flooding of rice fields [36], although the weed control benefits of these practices should be balanced against other considerations such as soil health. It is possible to take advantage of fitness costs in herbicide-resistant weeds to improve integrated weed management programs [33].

5. Conclusions

We performed greenhouse and field experiments to measure the competitive effects of BSM-susceptible and BSM-resistant eared redstem populations on rice. Our results demonstrate that even low densities of eared redstem pose a substantial threat to rice yield. The economic threshold for the control of eared redstem might not exceed 1–2 plants m⁻². Rice yield losses increased with increasing densities of eared redstem in both experiments. Our research also demonstrates that the evolution of BSM resistance may incur a fitness cost in eared redstem. We found that BSM-resistant eared redstem is less competitive against rice. To manage this resistant biotype, it may, therefore, be helpful to increase rice competitiveness through cultural approaches such as cultivar selection or increased planting density. Our study helps to develop more effective weed management strategies with less impact on the environment.