Effectiveness of Herbicide to Control Rice Weeds in Diverse Saline Environments

: To mitigate environmental pollution and food contamination caused by inappropriate and excessive herbicide usage, most potent herbicides should be screened to control rice weeds. A research trial was executed for assessing the comparative efﬁcacy of different herbicides to control rice ﬁeld weeds and to evaluate the toxicity on rice under normal (distilled water) as well as different salinity levels (4 and 8 dS m − 1 ). The study was designed to select the most potent herbicide and its appropriate dose for weed control of rice crop in coastal areas. Fourteen herbicidal treatments were included weed free crop, Pretilachlor (0.25, 0.50, 0.375 and 0.75 kg a.i. ha − 1 ), Propanil + Thiobencarb (0.6 + 1.2, 0.9 + 1.8, 1.2 + 2.4 and 1.8 + 3.6 kg a.i. ha − 1 ), Bensulfuron + MCPA (0.03 + 0.05, 0.045 + 0.075, 0.06 + 0.1 and 0.09 + 0.15 kg a.i. ha − 1 ) and weedy check (control). The results revealed that all tested herbicides in higher than recommended doses for non-saline rice ﬁelds were effective in controlling Cyperus iria, Echinochloa colona (salt-tolerant) and Jussiaea linifolia but showed in light injury in rice plants grown in non-saline soils. These higher doses of herbicides recorded severe crop injury under saline conditions indicating their differential efﬁcacy from normal non-saline conditions. Treatments including Pretilachlor (0.375kg


Introduction
Rice (Oryza sativa L.) is the staple food for more than half of the world's population, particularly in Southeast Asia and Latin America [1,2]. It is the third most important crop in Malaysia, grown mainly in eight granaries in Peninsular Malaysia covering an area of about 205,548 ha [3]. Self-sufficiency of rice in Malaysia is about 70% and rising population along with decreasing land area are expected to boot rice import needs [4]. However, as the population of Malaysia is predicted to reach 66.4 million in 2056, it calls for more research and technological advancement to increase rice production to meet consumption within the nation [5].
Salinity is one of the most significant abiotic stresses influencing the metabolic activities and causing plant injures [6]. More than eight lack hectares of agricultural land have been estimated to be impaired by salinity worldwide [7].Rice production, especially in saline areas is becoming increasingly important because more rice areas are becoming saline due to the addition of anthropogenic contributions to global warming. Though salinity has not yet become a crucial problem in Malaysia, but it is predicted to affect productivity in 100,000 ha of rice growing areas that would be salt affected by the year 2056 [5]. Continuous intrusion of saline water will result in decreasing areas for rice production and lead to food shortages. Therefore, researchers and policy makers need to consider economic and efficient use of saline areas for rice production. Recently, saline water intrusion in coastal areas is expected to decrease rice production areas that lead to reduce total rice production and food shortages [5]. In order to ensure food and nutritional protection, extensive research is necessary to allow effective utilization of saline areas for rice production.
Globally, weeds are a serious pest of rice, which tend to decrease growth and rice yield especially if weeds invasion reaches beyond threshold levels at seedling stage. Weeds have emerged as one of the major constraints for rice production in saline areas. Weeds compete with rice for water, light, physical space, and nutrient thus reducing yields, while weed seeds contaminate harvested rice grains, and lowers grain quality and cash value of the crop [8]. Worldwide the annual rice yield loss due to weed infestation is about 15-21% and annual losses of 10 million metric ton of rice due to weed competition have been reported in China [9]. In fields without any weed control, the yield loss in rice due to weeds is estimated at 41-70%, and rice yield losses in Malaysia had been recorded to about 42% in uncontrolled fields infested with Fimbristylis miliacea [10]. The distribution and nature of the weeds of coastal area are expected to be different compared to flood plain areas. Therefore, introduction of salt tolerant rice varieties should also be followed by effective and appropriate weed control practices. The critical period of weed control is considered as a 'window' in the crop cycle to prevent unacceptable yield loss [11]. Herbicide-based weed management is becoming increasingly popular, but there is a great need to select appropriate herbicides and determine the effective minimum dose of the herbicides for controlling salt tolerant weeds in saline rice fields. Worldwide numerous reports have been published on salinity stress and weed management in rice [2]. In addition to salinity stress, the weed management of rice in coastal areas is one of the most potent challenges in all rice-producing countries such as Malaysia [12]. The composition weed species in saline areas is different from flood plain areas [13]. Salt tolerant weeds include Cynodon dactylon, Eleusine coracana, Echinochloa crus-galli, Puccinellia ciliate, Cyperus iria and Echinochloa colona [14]. Effective management of salt tolerant weeds is very critical for sustainable rice production in coastal areas. The chemical weed management is the most commonly used in Malaysia, and reliable method to control the weeds in the rice fields [15,16]. Herbicides may not be successful for the control of saline weeds by Malaysian farmers in coastal areas as per recommendation for the non-saline environment. Chemical weed control in saline environment can be different from the non-saline environment [17]. There is limited information on rice weed control under saline conditions reported so for which has necessitated conducting investigations to ensure food security under changing climate. It was hypothesized that different herbicides applied solely or in conjunction with each other might perform differently under saline environment. Therefore, the study was designed to select the most promising herbicides and their optimal doses to control rice field weeds in saline environments.

Crop Management and Observations
Soil collected from a rice field in Tanjung Karang, Malaysia (2 • 30 N 112 • 30 E) was used to fill the pots of the experiment.The soil was loamy clay in nature with sand, silt and clay contents of 18.3, 43.7 and 38% respectively. It was found to be acidic having 6.1 pH, 1.02% organic carbon and 1.56 dSm −1 EC. The soil contained total N (0.19%), available P (11.12 ppm), available K (122 ppm), Ca (620 ppm), Mg (290 ppm), S (7.63 ppm) and Zn (0.96 ppm).
Commercial salt (NaCl, Batch# 088K0089, SIGMA-ALDRICH Co., Saint Louis, MO, USA) was used in concentration of 2.54 and 5.08 gL −1 of distilled water for preparing salinity levels of 4 and 8 dSm −1 , respectively. The control treatment was supplied distilled water only. The reconfirmation of desired salinity levels was done by measuring EC with EC meter (model: Z 865/SCHOTT Instruments, Hattenbergstraße 10·55122 Mainz, Germany), and subsequently necessary adjustments were made. No significant difference among the replications of individual salinity treatments was observed measurement of EC with the EC meter.
Each pot (33 cm diameter × 23 cm depth) was filled with 10 kg soil which was thoroughly mixed with urea (4.5 g N pot −1 ) (applied as basal dose 50% as well as rest of the portion as top dressing at 30 and 60 DAT), triple supper phosphate (6.0 g P 2 O 5 pot −1 ), muriate of potash (11.4 g K 2 O pot −1 ), and gypsum (1.5 g S pot −1 ) were applied as basal dose during final pot preparation. Twenty-one days old three rice seedlings were transplanted into each pot considering one seedling as one hill. In each pot, 20 pre-soaked non-dormant weed seeds per species were sown on the same day of rice transplanting. All the weed seeds in all the pots were germinated, and these were sustained until spraying of herbicides. To impose salt stress, the salt treatments were applied during final pot preparation. The salt solutions were applied to transplanted rice into three splits with an aim to avoid osmotic shock. The EC of growth media was maintained as per treatments (4 and 8 dSm −1 ) till the initiation of panicle. The agronomic management practices were carried out as per crop requirements [19]. For monitoring of EC of growth media in each pot, salt leachates were collected on daily basis. Necessary adjustments were made to maintain salinity of the specific treatments. The EC meter (Model: EC Tester, Spectrum Technologies Inc., 3600 Thayer Court, Aurora, IL 60504, USA) was used for the determination of leachates electrical conductivity throughout the trial period. The herbicides mixture was sprayed using hand sprayer (Model: Miaomanyoga Portable 800mL Chemical Sprayer, Mainland, China) having adjustable brass nozzle and the time has mentioned in Table 1. Weed population was recorded after 15 days of treatment application ( Figure 1). There were no major changes in crop phenology due to salinity treatments because the chosen rice variety was salt-tolerant. levels of 4 and 8 dSm −1 , respectively. The control treatment was supplied distilled wat only. The reconfirmation of desired salinity levels was done by measuring EC with E meter (model: Z 865/SCHOTT Instruments, Hattenbergstraße 10·55122 Mainz, Germany and subsequently necessary adjustments were made. No significant difference among th replications of individual salinity treatments was observed measurement of EC with th EC meter. Each pot (33 cm diameter × 23 cm depth) was filled with 10 kg soil which was tho oughly mixed with urea (4.5 g N pot −1 ) (applied as basal dose 50% as well as rest of th portion as top dressing at 30 and 60 DAT), triple supper phosphate (6.0 g P2O5 pot −1 ), m riate of potash (11.4 g K2O pot −1 ), and gypsum (1.5 g S pot −1 ) were applied as basal do during final pot preparation. Twenty-one days old three rice seedlings were transplante into each pot considering one seedling as one hill. In each pot, 20 pre-soaked non-dorma weed seeds per species were sown on the same day of rice transplanting. All the wee seeds in all the pots were germinated, and these were sustained until spraying of herb cides. To impose salt stress, the salt treatments were applied during final pot preparatio The salt solutions were applied to transplanted rice into three splits with an aim to avo osmotic shock. The EC of growth media was maintained as per treatments (4 and 8 dSm till the initiation of panicle. The agronomic management practices were carried out as p crop requirements [19]. For monitoring of EC of growth media in each pot, salt leachat were collected on daily basis. Necessary adjustments were made to maintain salinity the specific treatments. The EC meter (Model: EC Tester, Spectrum Technologies Inc., 36 Thayer Court, Aurora, IL 60504, USA) was used for the determination of leachates elect cal conductivity throughout the trial period. The herbicides mixture was sprayed usin hand sprayer (Model: Miaomanyoga Portable 800mL Chemical Sprayer, Mainlan China) having adjustable brass nozzle and the time has mentioned in Table 1. Weed po ulation was recorded after 15 days of treatment application ( Figure 1). There were no m jor changes in crop phenology due to salinity treatments because the chosen rice varie was salt-tolerant.    T1  T2  T3  T4  T5  T6  T7  T8  T9 T10 T11 T12 T13   Weed control data rating and plant phytotoxicity were visually evaluated after herbicide application at early tillering stage of rice. The visual rating scale of 1-5 [20] was used in this study (Table 2). Plant height: Plant height (cm) of the rice plants were measured from the ground level to the tip of the longest leaf by using measuring scale.
Tillers: The total numbers of tiller hill −1 , productive tiller hill −1 was counted at maturity of the rice plants at 90 days after transplanting (DAT).
Chlorophyll: ChlSPAD values were measured by using a chlorophyll meter (ChlSPAD-502, Minolta Camera Co., Osaka, Japan) from fully expanded young leaves at 60 days after transplanting and 90 days after transplanting.
Gain yield: The crop was harvested when 90% of grains attained golden yellow color, while the grain yields were recorded after post-harvest processing. The grain yield adjustment was done based on grain moisture of 12%.
Weed dry matter: Weed dry matter was recorded at 60 DAT after drying in an oven at 80 • C for 72 h, and it converted from g pot −1 to gm −2 .The weed control efficiency (WCE) was calculated based on weed dry matter using the following equation [21].
where, DWC = Weeds dry weight in weedy check plots (Weedy average check plots), DWT = Weeds dry weight in herbicidal treated plots.

Statistical Analysis
Analysis of Variance (ANOVA) procedure was used to perform statistical analyses of recorded all experimental data while significant differences among treatment means were determined using Least Significant Difference (LSD) test at the probability level of 5% with the help of statistical software package of "SAS version 9.0".

Crop Phenology
The phonology of rice crops throughout the entire lifetime was good enough to maintain plant growth, because the selected variety was salt-tolerant.

Visual Weed Control Rating and Crop Injury
Results presented that most of the herbicidal treatments controlled weeds ( Table 2). The Pretilachlor (0.25 kg a.i. ha −1 ), Bensulfuron + MCPA (0.03 + 0.05 kg a.i. ha −1 ), and Propanil + Thiobencarb (1.2 + 2.4 kg a.i. ha −1 ) showed fair control of all selected weeds under saline conditions. The results suggest that weeds can be effectively managed with minimal levels of herbicides in the fields affected by salt. Pretilachlorine (0.375 kg a.i. ha −1 ), Propanil (0.9 + 1.8 kg a.i. ha −1 ) and Bensulfuron + MCPA (0.045 + 0.075 kg a.i. ha −1 ) are clearly appropriate for successful control of weeds under the saline conditions. Crop toxicity increased with increased herbicide concentrations and increased salinity ( Table 2). For Pretilachlor, a higher herbicidal dose caused slight injury under saline condition, but recommended (0.5 kg a.i. ha −1 ) and lower than recommended doses did not show any injury to rice plants. The higher dose of Propanil + Thiobencarb was a mild injury in non-saline conditions, and significant phytotoxicity and complete killing ofrice plant were observed at 4 and 8 dS m −1 salinity levels.In case of Bensulfuron + MCPA, plants showed severe phytotoxicity as well and ultimately the plants were died under saline conditions but no phytotoxic effects were found under non-saline conditions with those herbicidal treatments. The recommended and lower thanrecommendedrates ofall herbicides did not lead to rice damage symptoms under non-salinesoil, and the recommended rate presented light injure during salinity conditions.Anwar et al. [16] also noticed light injure like leaf chlorosis along with growth stunting of rice with herbicides like Pretilachlor + Sefener (0.5 + 0.6 kg ai ha −1 ) and Bentazon + MCPA (0.1 + 0.6 kg ai ha −1 ) at 7 to 14 days after application. The result of the present findings corroborated with the findings of Yew et al. [22], who inferred that low doses of herbicides performed better under saline conditions.
Our findings revealed that the effectiveness of weed controls was considerably better if herbicides were applied more than prescribed, but they showed a high level of phytotoxicityfor rice causing death ofrice plants particularly in severe salt conditions. Therefore, higher dose cannot be acceptable under saline condition. The above results are in agreement with those findings of [21,[23][24][25].  At control salinity level, the highest WCE (83.56%) was observed for Propanil + Thiobencarb (1.8 + 3.6 kg a.i. ha −1 ) (Table 3). However, similar WCE was also observed in pots treated by Pretilachlor @ 0.75 kg a.i. ha −1 , Bensulfuron + MCPA (0.09 + 0.15 kg a.i. ha −1 ), Pretilachlor @ 0.05 kg a.i. ha −1 and Bensulfuron + MCPA (0.045 + 0.075 kg a.i. ha −1 ). The minimum WCE (60.75%) was recorded for Pretilachlor (0.25 kg a.i. ha −1 ), while other herbicidal treatments remained at par to each other under optimal conditions. At 4 dS m −1 , the highest value (87.46%) of WCE was obtained for Pretilachlor (0.75 kg a.i. ha −1 ) but other treatments gave non-significant differences. The lowest WCE (69.49%) was given by Propanil + Thiobencarb (0.6 + 1.2 kg a.i. ha −1 ).The treatments which showed lower efficiency in controlling weeds might be due to less killing effects or more emergences of new weeds (individuals) at later stages.
Our findings revealed that the effectiveness of weed controls was considerably better if herbicides were applied more than prescribed, but they showed a high level of phytotoxicityfor rice causing death ofrice plants particularly in severe salt conditions. Therefore, higher dose cannot be acceptable under saline condition. The above results are in agreement with those findings of [21,[23][24][25].

ChlSPAD Values
The leaf ChlSPAD values of rice under various levels of salinity were significantly influenced by the herbicidal treatments (Table 6). At 45 DAT, the highest ChlSPAD value ((37.1)) was observed in the weed-free treatment followed by Pretilachlor (0.375 kg a.i. ha −1 ) under non-saline conditions, while the lowest ChlSPAD values (26.5) was found in Pretilachlor (0.75 kg a.i. ha −1 ) followed by Propanil + Thiobencarb (1.2 + 2.4 kg a.i. ha −1 ) (29.5) at the 8 dS m −1 salinity level. However, the differences among the treatments were statistically insignificant except in lethal dose. There were indications that the chlorophyll content was increased at 90 DAT which suggested that low chlorophyll content at 45 DAT might be due to the effect of herbicide which gradually recovered at maturity. At the maturity stage (90 DAT), higher ChlSPAD values was observed in the treatment of pretilachlor (0.375 kg a.i. ha −1 ) in comparison with control (weed free) treatment under non-saline conditions, while the non-significant least ChlSPAD value(33.3) were recorded in the weedy treatment which was similar with all other herbicide treatments under saline conditions. Weeds generally compete with rice for water, light, physical space and nutrients. The rice plants under weedy conditions resulted in reduced growth and photosynthesis capacity. Reduction in chlorophyll content also results in lower photosynthesis capacity. These findings are in corroboration with those of [20,30], who observed less chlorophyll content under weedy environments and rice plants treated with various herbicides.

Grain Yield
Significant differences due to various herbicide treatments were examined for grain yield under various saline conditions ( Table 7).The greatest grain yield (13.82 g hill −1 ) was found in weed-free treatments that was comparable to Pretilachlor (0.375 kg a.i. ha −1 ) (13.14 g hill −1 ), while the lowest grain yield (3.05 g hill −1 ) was noted in the weedy treatment under non-saline conditions. A similar trend was also observed for salinity levels of 4 and 8 dS m −1 . Treatments of Propanil + Thiobencarb (1.8 kg + 3.6 a.i. ha −1 ) and Bensulfuron + MCPA (0.09 + 0.15 kg a.i. ha −1 ) did not produce grain yields under saline conditions as the rice plants were completely died due to the combinedphytotoxic effects of herbicides and salinity stress. The results indicated that Pretilachlor (0.375 kg a.i. ha −1 ), Propanil + Thiobencarb (0.9 + 1.8 kg a.i. ha −1 ) and Bensulfuron + MCPA (0.06 + 0.1 kg a.i. ha −1 ) were superior in terms of yielding higher grain yield compared to other herbicidal treatments. Lower grain yield is attributed to decreased yield components, higher sterility percentage and higher weed biomass (data not presented here). The results are in harmony with the findings of many others [16,26,29] who also found that different herbicides affected yield components of rice which resulted in varying grain yields.

Conclusions
Herbicides at recommended doses for non-saline conditions are not appropriate to rice crop under saline environment due to differential physiological responses of the crop and weeds although the weeds are controlled properly. Even lower doses of herbicides under saline condition can control weeds effectively since the weeds growth is discouraged by salinity, and the rice crop grows healthy. To mitigate environmental pollution and possible food contamination through inappropriate herbicides usages, the application of potent herbicide with its appropriate doses is essential for better outcomes in rice production and environmental protection. Pretilachlor (0.375 kg a.i. ha −1 ), Propanil + Thiobencarb (0.9 +1.8 kg a.i. ha −1 ), Bensulfuron + MCPA (0.06 + 0.1 kg a.i. ha −1 ) and pretilachlor (0.50 kg a.i. ha −1 ) performed better and seems promising herbicidal treatments in saline condition of Tanjun Kerang area of Malaysia. However, the rice farmers should conduct more trials to have recommendation for general adoption.