Weed Control, Rice Safety, and Mechanism of the Novel Paddy Field Herbicide Glyamifop

Abstract

Glyamifop (R&D code: FG001), (R)-(2-(4-(6-chlorobenzoxazol-2-oxy) phenoxy) propionyl) glycine ethyl ester is a newly developed aryloxyphenoxypropionate (HRAC Group 1) herbicide for weed control in paddy fields. This work determined the effect of Glyamifop on weeds and its safety for rice in the glasshouse. Glyamifop controlled the common gramineous weeds in paddy fields at 100 g a.i. ha⁻¹: the fresh weight inhibition rates of Echinochloa crus-galli, Leptochloa chinensis, Setaria viridis, Eragrostis japonica, Digitaria sanguinalis and Panicum bisulcatum were all above 90%. It has almost no inhibitory effect on broad-leaved and cyperaceae weeds, such as Eclipta prostrata and Cyperus iria. Glyamifop inhibited cyhalofop-butyl-resistant L. chinensis, penoxsulam-resistant E. crus-galli and quinclorac-resistant E. crusgalli var. zelayensis by 100%, 99.98% and 96.37%, respectively, at 100 g a.i. ha⁻¹, based on the fresh weight. The selectivity index of Glyamifop foliage treatment in the rice varieties japonica ‘Huaidao 5’, indica ‘Xiangliangyou 900’ and glutinous ‘Zhennuo 29’ was 5.93, 6.81 and 4.91, respectively; therefore, Glyamifop is safe for the 3 different rice varieties. Fresh weight rice inhibition rates were 7.18%, 2.99% and 7.93% at the 2.5-, 3.5- and 5.5-leaf stage, respectively, and the selectivity index was 5.18, 6.04 and 7.93, respectively, indicating that Glyamifop was safe for rice at these leaf stages. L. chinensis ACCase activity decreased with increasing Glyamifop concentration, and the inhibitory effect was similar to that of cyhalofop acid; this confirmed that Glyamifop is an ACCase inhibitor. In conclusion, Glyamifop has potential for the management of gramineous weeds as it has good activity against weeds that are resistant to common herbicides in paddy fields.

1. Introduction

China accounts for approximately 20% of the world’s rice planting area and is one of the world’s largest rice planting and production countries [1]. The increase in China’s rice grain production in recent years is considered an important guarantee for food security [2]. However, weeds account for approximately 15 million hectares, or 45%, of the rice planting area in China [1]. Therefore, approximately 10 million tons of rice (~15%) are lost every year. The yield of paddy rice without weed control is generally reduced by 5%~15%, and 15%~30% in serious cases [3,4]. Chemical weed control is vitally important in global agriculture modernization [5]. There are approximately 26 herbicide sites of action, including acetohydroxyacid synthase (AHAS, HRAC Group 2), acetyl-coenzyme A carboxylase (ACCase, HRAC Group 1), protoporphyrinogen IX oxidase (PPO, HRAC Group 14) and 4-hydroxyphenylpyruvate dioxygenase (HPPD)-inhibiting herbicides [6]. Although herbicides have greatly reduced the harm caused by weeds, they have caused a herbicide resistance problem in weeds [7]. In addition, the side effects caused by the wide and irrational use of herbicides threaten the environment and human health. Herbicide resistance in weeds has increased due to more frequent use of chemical herbicides in China and globally [8,9,10,11,12,13]. Therefore, there is an urgent demand for the discovery of new herbicides that can control resistant weeds and slow herbicide resistance evolution [14].

Diclofop-methyl was the first aryloxyphenoxypropionate herbicide developed in the 1970s, and there are currently over 10 active ingredients available, including fenoxaprop-P-ethyl, clodinafop-propargyl, haloxyfop-etotyl and metamifop [15]. The sales of these varieties are always in the top 10 of the herbicide market, and their sales and market scale are second only to sulfonylurea herbicides and HPPD herbicides. These compounds have high biological activity against gramineae weeds; therefore, only a few of them can be used in rice, wheat and other gramineae crops. Currently, only two aryloxyphenoxypropionate compounds are successfully used for post-emergence weed control in paddy fields [16]. The first, cyhalofop-ethyl, is an aromatic compound developed and produced by Dow AgroSciences in 1987 for weed control in paddy fields [17]. Aryloxyphenoxypropionate herbicides provide effective weed control in paddies; however, they have no efficacy on problematic grass weeds, including Digitaria sanguinalis and Echinochloa crus-galli. The second, Metamifop herbicide, provides effective control of many grass weeds, but it has poor efficacy against many other gramineous weeds, including Paspalum paspaloides and Setaria glauca [18,19,20]. Glyamifop (R&D code: FG001), (R)-(2-(4-(6-chlorobenzoxazol-2-oxy) phenoxy) propionyl) glycine ethyl ester is a new aryloxyphenoxypropionate herbicide developed by Jiangsu Zhongqi Technology Co., Ltd. (Nanjing, China) in 2019 (Chinese Patent Publication No. CN105801513B); its molecular formula is C₂₀H₁₉ClN₂O₆ (Figure 1). Because Glyamifop has only been produced for less than three years, it has not been used commercially in large areas in China as there is little research on it. Only a few scientific papers have reported its physical and chemical properties and residue analysis methods [21], but its efficacy in controlling weeds in the field and its safety to crops are still unknown.

Therefore, the objectives of this study were to (1) determine the efficacy of Glyamifop against common weeds in paddy fields, and against cyhalofop-butyl-resistant Leptochloa chinensis, as well as penoxsulam- and quinclorac-resistant Echinochloa weeds, in greenhouse conditions; (2) identify the safety of Glyamifop among different varieties and leaf stages of rice; and (3) clarify the preliminary mechanism of action of Glyamifop and its selectivity to rice.

2. Materials and Methods

2.1. Plant and Chemical Materials

The tested weeds are common autumn weeds in China, which were collected from farmland from 2017 to 2020. The collected weed seeds were uniformly registered, placed in a cool place for air drying, sorted and bagged, and stored in a 4 °C refrigerator. The special weed populations are multiplied every year to maintain viability. The resistant weed populations used in the experiment are from the previous research findings of the laboratory (

Table 1. Collection sites of tested seed populations.

Weed Seed	Collection Site			Time
	Province	County	Village
Echinochloa crus-galli var. mitis	Jiangsu	Gulou	Yinzhuang	September 2019
E. crus-galli	Jiangsu	Huaiyin	Lizhuang	November 2017
E. glabrescens	Jiangsu	Jiangdu	Xiannv	October 2019
E. crusgalli var. zelayensis	Jiangsu	Luhe	Xiaoying	September 2019
E. caudata	Jiangsu	Wujin	Dongliutang	October 2019
E. phyllopogon	Jiangsu	Gaoyou	Guzhuang	October 2019
Leptochloa chinensis	Jiangsu	Luhe	Xiaoying	October 2019
Setaria viridis	Jiangsu	Xuanwu	Xiaolingwei	October 2019
Eragrostis japonica	Jiangsu	Huaiyin	Jiegou	October 2019
Digitaria sanguinalis	Jiangsu	Jiangdu	Qili	September 2019
Panicum bisulcatum	Anhui	Wuwei	Sanxi	November 2019
Eclipta prostrata	Jiangsu	Jiangdu	Qili	November 2017
Cyperus iria	Jiangsu	Ganyu	Songzhuang	September 2019

and

Table 2. Collection location and resistant seed details.

Trial Weeds	Collection Site			Resistance to Herbicide	Reported
	Province	County	Village
Echinochloa phyllopogon (HSQX-R)	Heilongjiang	Qingxian	Qingyuanzhen	Penoxsulam	[8]
E. glabrescens (JHHZ-R)	Jiangsu	Hongze	Hepingnongchang	Penoxsulam	[22]
E. crusgalli var. zelayensis (JNNX-R)	Jiangsu	Xuanwu	Xiaolingwei	Quinclorac	[23]
Leptochloa chinensis (JHQP-R)	Jiangsu	Huaiyin	Lizhuang	Cyhalofop-butyl	[3]
E. crus-galli (AXYZ-R)	Anhui	Xuanzhou	Xingyang	Penoxsulam	[9]

). Glyamifop standard (>99%) and oil dispersion (8%) were from Jiangsu Zhongqi Technology Co., Ltd. (Nanjing, China). Standard-grade cyhalofop (free acid) used for the ACCase assay was supplied by Dr Ehrenstorfer GmbH (Augsburg, Germany), and NaH¹⁴CO₃ was from PerkinElmer (Waltham, MA, USA). All other chemicals were purchased from Solarbio Science & Technology Co., Beijing, China.

2.2. Sensitivity to Glyamifop

The whole plant bioassay method involved filling an 8-cm × 7-cm × 7-cm (0.392 L) plastic pot (containing holes at the bottom) with 2:1 w/w sandy soil and nutrient matrix, and then evenly mixing the nutrient soil (pH = 6.3, organic matter content 1.1%). Twenty weed seeds were evenly sown in each pot. A layer of fine soil was sprinkled on the surface of tested weeds to cover the seeds after planting. The plastic bowl was placed into a plastic tray, and water was poured into the tray so that the soil could absorb water from the bottom. Seedlings were grown in an outdoor natural light culture frame. The incubation temperature was 30 ± 5 °C in the daytime and 25 ± 5 °C in the evening. The seedlings were thinned to 10 plants per bot when the plant reached the 2~3-leaf stage. Seedlings were sprayed with Glyamifop using a laboratory sprayer (machine model: 3WP-2000, Nanjing Research Institute for Agricultural Mechanization, Nanjing, National Ministry of Agriculture of China) equipped with a flat-fan nozzle delivering 280 L ha⁻¹ at 230 kPa. Glyamifop doses were set at 6.25, 12.5, 25, 50, 100 and 200 g a.i ha⁻¹, according to the preliminary experiment with water used as the control. Treated plants were returned to the greenhouse with a 30/25 °C day/night temperature in 12 h light period. The light intensity was 8000 lx, and the relative humidity was kept constantly around 75%. Plants were sub-irrigated with water when needed. The aboveground tissue was collected at 21 days after treatment (DAT), and fresh weight was measured. There were three replicates for each dose, and the whole experiment was repeated.

2.3. Rice Safety

Japonica rice ‘Huai Dao 5′ was purchased from the market, while indica rice ‘Xiang Liang You 900′ and glutinous rice ‘Zhen Nuo 29′ were purchased from the Jiangsu Academy of Agricultural Science, China. The whole plant bioassay method was used to fill the 10-cm × 10-cm × 8.5-cm plastic pot (with holes at the bottom) using the same soil mix as stated in the previous paragraph, and 15 rice seeds were evenly sown per basin. The safety experiment of different rice varieties involved 8 plants per pot when the rice grew to the 3-leaf stage. The safety experiment of rice involved planting D. sanguinalis and rice seeds at the same time at different growth stages. The seedlings were thinned down to 8 plants per pot when D. sanguinalis and rice seeds reached the 1.5-, 2.5-, 3.5- and 5.5-leaf stage. The other conditions were the same as described in the previous paragraph. The aboveground fresh weight was measured 21 days after herbicide treatment. The Glyamifop concentrations causing 90% growth rate inhibition (GR₉₀) of D. sanguinalis and 10% growth rate inhibition (GR₁₀) of rice were calculated and the selectivity index (Z) between rice and D. sanguinalis determined according to Equation (1). Z < 2 means that it is unsafe for rice; 2 < Z < 4, indicates that it is relatively safe for rice, while Z > 4 indicates that it is safe for rice [24].

Z = GR_{10 (rice)}/GR_{90 (D. sanguinalis)}

(1)

2.4. Glyamifop Mechanism of Action

The ACCase activity of Leptochloa chinensis after Glyamifop treatment was measured, and cyhalofop acid (an ACCase inhibitor) used as a positive control. L. chinensis leaf samples were separated from individual plants prior to treatment with Glyamifop and cyhalofop acid at the 3- to 4-leaf stage. ACCase was extracted from these leaves using a slightly modified previous protocol [25]. Briefly frozen shoot tissue (3.5 g) was homogenized with a mortar and pestle in 10 mL extraction buffer containing 100 mM Tris (pH 8.0), 1 mM EDTA, 10% (v/v) glycerol, 2 mM isoascorbic acid, 1 mM PMSF, 0.5% PVP-40, 0.5% insoluble PVP and 20 mM DTT. The homogenate was centrifuged at 27,000× g for 15 min. The supernatant was brought to 40% (NH₄)₂SO₄ saturation by dropwise addition of saturated (NH₄)₂SO₄ and stirred for 30 min. The solution was centrifuged at 27,000× g for 30 min. The pellet was resuspended in 2.5 mL elution buffer (50 mM Tricine pH 8.0, 2.5 mM MgCl₂, 50 mM KCl and 1 mM DTT) and desalted using a Sephadex G-25 column pre-equilibrated with elution buffer. The eluate was stored at −20 °C until use. Enzyme sensitivity assays were performed using Glyamifop and cyhalofop acid at 0.001, 0.01, 0.1, 1, 10, 100 and 1000 μM with DMSO used as a solvent control. An untreated (no-herbicide) control was included for comparison.

ACCase activity was based on the rate of ¹⁴C incorporation from NaH¹⁴CO₃ into an acid- and heat-stable product. Enzyme activity was expressed as the percentage of the untreated control. The Glyamifop and cyhalofop acid concentrations causing 50% inhibition of enzyme activity (IC₅₀) were calculated using linear regression analysis [26,27]. Enzyme activity was assayed twice with four replications per herbicide concentration.

2.5. Selective Mechanism of Glyamifop on Rice and Weeds

ACCase extraction and activity assays were described in Section 2.4. ACCase activity was determined by comparing the ACCase activity of Leptochloa chinensis and rice treated with Glyamifop (0.01, 0.1, 1, 10, 100 and 1000 μM) with that of rice without any chemicals. The Glyamifop concentrations causing 50% inhibition of L. chinensis and rice ACCase activity (IC₅₀) were calculated.

2.6. Statistical Analysis

All dose-response data from two experiments were analyzed by ANOVA (SPSS 21, SPSS Inc. (Chicago, IL, USA)). There was no significant trial-by-treatment interaction from each experiment (p > 0.05). Therefore, the data were pooled and fitted to a four-parameter non-linear logistic-regression model (Equation (1)) using SigmaPlot 10.0 (Systat Software, Inc., CA, USA). The effective herbicide dose causing 10%, 50% and 90% fresh weight inhibition (GR₁₀, GR₅₀, GR₉₀) was computed by Equation (2):

Y = c + (d − c)/[1 + (x/g)^b]

(2)

where Y is the aboveground fresh weight expressed as a percentage of the nontreated control, x is the herbicide rate, b is the slope of the curve, c is the lower limit, d is the upper limit and g is the herbicide rate at the point of inflection halfway between the upper- and lower limit. The same regression analyses were used for the enzymatic experiment to calculate 50% inhibition of ACCase activity (IC₅₀).

3. Results

3.1. Efficacy of Glyamifop on Sensitive- and Resistant Weeds in a Paddy Field

The Glyamifop spray has high biological activity on gramineous weeds in the 2~3-leaf stages, especially on annual gramineous weeds, including Echinochloa crus-galli, Leptochloa chinensis, Digitaria sanguinalis and Setaria viridis, but not E. phyllopogon (

Table 3. Effect of Glyamifop on common weeds in the paddy field.

Weed Species	Weeds	Dose (g a.i. ha−1)						GR50 (SE) g a.i. ha−1	GR90 (SE) g a.i. ha−1	r2 (Coefficient)
		Inhibition Rate of Fresh Weight (%)
		6.25	12.5	25	50	100	200
Gramineae	Echinochloa crus-galli var. mitis	23.98	47.31	73.72	92.06	99.98	100.00	12.91 (1.31)	41.95 (3.97)	0.9974
	E. crus-galli	11.96	24.87	61.71	91.45	98.76	100.00	18.14 (7.5)	49.28 (7.94)	0.9939
	E. glabrescens	44.18	71.76	89.03	98.57	100.00	100.00	7.40 (1.09)	23.62 (2.43)	0.9964
	E. crusgalli var. zelayensis	14.49	25.98	48.13	82.27	94.33	100.00	21.44 (3.58)	78.37 (20.70)	0.9907
	E. caudata	20.47	42.76	57.69	82.39	96.48	100.00	16.45 (2.96)	66.18 (13.81)	0.9918
	E. phyllopogon	11.73	18.95	34.07	53.98	82.26	91.45	36.01 (6.11)	189.64 (39.39)	0.9910
	Leptochloa chinensis	46.89	73.33	96.55	100.00	100.00	100.00	7.03 (1.88)	17.93 (7.15)	0.9888
	Digitaria sanguinalis	18.98	37.54	52.87	79.81	93.67	100.00	18.81 (2.69)	84.30 (19.11)	0.9934
	Panicum bisulcatum	31.94	58.23	65.34	77.99	90.26	94.45	11.78 (2.83)	109.23 (48.14)	0.9908
	Eragrostis japonica	17.36	25.00	42.29	64.33	99.65	100.00	41.70 (6.55)	94.46 (22.11)	0.9884
	Setaria viridis	34.82	49.84	68.22	89.14	97.44	100.00	11.48 (2.71)	52.22 (12.05)	0.9908
Broad-leaved and cyperaceae	Eclipta prostrata	9.26	14.01	14.18	18.19	34.67	49.67	318.69 (65.23)	10889.62 (101.21)	0.9484
	Cyperus iria	0.80	6.62	9.12	16.02	33.43	39.5	251.82 (78.09)	2220.59 (96.07)	0.9742

Abbreviations: GR₅₀, effective herbicide rate that causes a 50% growth rate reduction in fresh shoot biomass from the nontreated control; GR₉₀, effective herbicide rate that causes a 90% growth rate reduction in fresh shoot biomass from the nontreated control; SE, standard error.

). The GR₉₀ was 17.94~80.41 g a.i. ha⁻¹. Glyamifop has some activity against Eragrostis japonica (GR₉₀ = 94.46 g a.i. ha⁻¹) and Panicum bisulcatum (GR₉₀ = 109.23 g a.i. ha⁻¹). It has almost no activity on the broad-leaved weed Eclipta prostrata and the Cyperaceae weed Cyperus iria. Glyamifop has high biological activity against cyhalofop-butyl-resistant L. chinensis, penoxsulam-resistant E. crus-galli and quinclorac-resistant E. crus-galli var. zelayensis, with fresh weight inhibition rates of 100%, 99.98% and 96.37%, respectively, at 100 g a.i. ha⁻¹ (

Table 4. Effect of Glyamifop on common herbicide-resistant gramineae weeds in the paddy field.

Population	Dose (g a.i. ha−1)						GR50 (SE) g a.i. ha−1	GR90 (SE) g a.i. ha−1	r2 (Coefficient)
	Inhibition Rate of Fresh Weight (%)
	6.25	12.5	25	50	100	200
Echinochloa crus-galli (AXYZ-R)	6.42	11.24	26.54	45.72	89.02	100.00	56.07 (5.36)	127.55 (15.23)	0.9851
E. crusgalli var. zelayensis (JNNX-R)	4.28	10.36	26.72	52.17	91.48	100	37.66 (3.25)	99.25 (10.67)	0.9903
E. glabrescens (JHHZ-R)	13.92	16.85	27.46	48.62	95.26	100.00	50.43 (6.87)	94.67 (12.97)	0.9815
E. phyllopogon (HSQX-R)	5.72	12.38	13.15	41.63	72.41	82.61	57.05 (6.09)	309.93 (50.48)	0.9922
Leptochloa chinensis (JHQP-R)	21.48	36.09	57.39	87.51	100.00	100.00	16.78 (2.86)	67.75 (7.71)	0.9908

Abbreviations: GR₅₀, effective herbicide rate that causes a 50% growth rate reduction in fresh shoot biomass from the nontreated control; GR₉₀, effective herbicide rate that causes a 90% growth rate reduction in fresh shoot biomass from the nontreated control; SE, standard error.

).

3.2. Rice Safety

3.2.1. Glyamifop Safety on Different Rice Types

There was no significant difference in plant height, leaf color and fresh weight between the treatment group and the control group, and no obvious herbicide damage (data not shown). The selectivity index of the foliage treatment with Glyamifop to japonica rice ‘Huaidao 5’, indica rice ‘Xiangliangyou 900’, glutinous rice ‘Zhennuo 29’ and Digitaria sanguinalis was 5.93, 6.81 and 4.91, indicating that Glyamifop is safe for three different types of rice at the 3-leaf stage (

Table 5. Selectivity index of Glyamifop between rice and weeds.

Rice Type	Varieties	GR10 (SE) g a.i. ha−1	r2 (Coefficient)	Selectivity Index (Z) a
ssp. japonica	HuaiDao 5	500.04 (73.76)	0.9976	5.93
ssp. indica	Xiangliangyou 900	574.42 (67.01)	0.9930	6.81
ssp. glutinous	Zhennuo 29	413.94 (40.71)	0.9896	4.91

^aDigitaria sanguinalis is selected as the comparison weed, and the GR₉₀ value is 84. Abbreviations: GR₁₀, effective herbicide rate that causes a 10% growth rate reduction in fresh shoot biomass from the nontreated control; SE, standard error.

). Japonica rice showed no obvious herbicide damage at different leaf stages under various Glyamifop doses through visual observation. There was no significant difference in the fresh weight of the three types of rice using 6.25~200 g a.i. ha⁻¹ Glyamifop compared with the water control. Overall, Glyamifop was safe for three types of rice, although Glyamifop safety was higher for indica rice than for japonica rice and glutinous rice.

3.2.2. Glyamifop Safety in Rice at Different Leaf Stages

The selectivity index between japonica rice and Digitaria sanguinalis was 3.45, indicating that Glyamifop was relatively safe for 1.5-leaf stage japonica rice. The fresh weight of japonica rice ‘Huaidao 5’ treated with 6.25~200 g a.i. ha⁻¹ Glyamifop at different leaf stages was not significantly different from the water control. The inhibition rate of fresh weight rice at the 2.5-, 3.5- and 5.5-leaf stages was 7.18%, 4.92% and 2.99%, respectively, while the selectivity index was 5.18, 6.04 and 7.93, respectively, at 200 g a.i. ha⁻¹ Glyamifop (

Table 6. Selectivity index of Glyamifop between rice and weeds.

Application Period	Huai Dao 5 a		Digitaria sanguinalis		Selectivity Index (Z)
	GR10 (SE) g a.i. ha−1	r2 (Coefficient)	GR90 (SE) g a.i. ha−1	r2 (Coefficient)
1.5-leaf stage	218.11 (23.08)	0.9964	63.07 (16.74)	0.9927	3.45
2.5-leaf stage	394.16 (36.86)	0.9976	76.13 (12.48)	0.9702	5.18
3.5-leaf stage	593.01 (71.58)	0.9924	98.26 (14.71)	0.9917	6.04
5.5-leaf stage	1184.51 (102.08)	0.9948	149.28 (29.04)	0.9938	7.93

^a This is the ssp. japonica rice variety.

). This indicated that Glyamifop was safe for rice at the 2.5-, 3.5- and 5.5-leaf stages.

3.3. Glyamifop Mechanism of Action

The dose-response curve showed that Leptochloa chinensis ACCase activity decreased with increasing Glyamifop and cyhalofop acid inhibitor concentration; ACCase activity slowly decreased at 0.001~0.1 μM Glyamifop or cyhalofop acid (Figure 2). L. chinensis ACCase activity significantly decreased at 1 μM for both inhibitors. The IC₅₀ of Glyamifop and cyhalofop acid-inhibiting L. chinensis ACCase activity was 2.64 μM and 2.11 μM, respectively; hence, Glyamifop and cyhalofop acid have similar inhibitory effects.

3.4. Selective Mechanism of Glyamifop on Rice and Weeds

The ACCase activity of rice gradually decreased after Glyamifop addition, while Leptochloa chinensis ACCase activity decreased more sharply, and its activity was significantly inhibited by Glyamifop (Figure 3). The IC₅₀ of Glyamifop on rice ACCase activity was 11.24 μM, which is 4.24 times that of L. chinensis. This suggests that Glyamifop has a selective mechanism of inhibition of ACCase activity between rice and L. chinensis.

4. Discussion

In recent years, the research and development of new herbicides are mostly concentrated in the field of HPPD compounds [28,29,30,31]. These compounds are mostly used in dry land crop fields, including wheat and corn. Glyamifop belongs to the group of aryloxyphenoxypropionate herbicides, and this preliminary work determined that ACCase was its target enzyme (Figure 2). There are several reports of novel ACCase inhibitors in paddy fields over the last 10 years [32]. In this study, Glyamifop controlled gramineous weeds and was safe for rice at the 3-leaf stage and above. The fresh weight inhibition rate exceeded 90% at 100 g a.i. ha⁻¹ Glyamifop for 12 weeds, including Leptochloa chinensis, Echinochloa crus-galli, E. crus-galli var. mitis, E. glabrescens, E. crus-galli var. zelayensis, Eragrostis japonica, Digitaria sanguinalis, Panicum bisulcatum, Setaria viridis, cyhalofop-butyl-resistant L. chinensis, penoxsulam-resistant E. crus-galli and quinclorac-resistant E. crus-galli var. zelayensis.

In the research on the evaluation of herbicide safety, the selectivity index is usually used to evaluate the selectivity of herbicides between crops and weeds. To ensure a high weed control effect within the dose range, it is also necessary to ensure that they do not harm crops [24]. When the selectivity index is greater than 4, it indicates that the herbicide is safe for crops [33]. The Glyamifop selectivity index was above 4 for three different rice varieties at the 3-leaf stage, indicating that Glyamifop is safe for each rice variety, although indica rice showed the best safety, followed by japonica rice and glutinous rice. Sensitivity to Glyamifop was most prevalent at the low-leaf age, and this gradually decreased with increasing rice leaf age. Therefore, it is recommended to apply Glyamifop at the third leaf stage (or later) to ensure the safety of rice.

It was confirmed that Glyamifop is an ACCase inhibitor herbicide, which inhibits its activity in weeds and leads to their death. Glyamifop inhibition of rice ACCase activity was significantly less than that of L. chinensis at increasing Glyamifop concentrations. This showed that the sensitivity of L. chinensis and rice under Glyamifop treatment was different at the target enzyme level, and this difference was one of the important selective principles of Glyamifop. However, herbicide selectivity is affected by many factors, and the selective principle of this herbicide for controlling miscellaneous weeds in rice fields requires further study.

5. Conclusions

Greenhouse studies indicated that Glyamifop has good potential as an efficient POST-herbicide for controlling grass weeds in paddy fields. Glyamifop is safe for three different rice types within the range of the experiment dosages. Glyamifop significantly inhibited ACCase activity, which was like that of cyhalofop-butyl; thus, this confirmed that Glyamifop is an ACCase inhibitor. The novel structure of this herbicide could be an ideal option for weed control, especially for multiple herbicide-resistant weed species in paddy fields.