Efficacy of herbicides and reductant at different levels to control invasive tall reed (Phragmites karka) in oil palm plantations

Controlling Phragmites karka, a highly invasive weed, is critical for peat areas since this species is a significantand hard-to control weed in oil palm plantations. Herbicides are one of the most effective and efficient tools for controlling P. karka, and their use is increasing due to their ability to provide rapid and long-term results. However, herbicides must be applied responsibly and carefully to minimize any environmental impacts. The study included an evaluation of the effectiveness of several combinations of herbicides on P. karka in an oil palm plantation and a comparison of their efficiency and cost. Results showed that combination of 1.8 L/ha glyphosate, 0.045 L/ha topramezone and 0.3 L/ha reductant Y (G7) has a promising result in controlling P. karka (weed mortality of 97% until 8 WAA and 12% regrowth until 12 WAA) and was more cost-effective compared to estate practice, reducing cost up to 0.43% per hectare because of the use of pesticide reductant. The use of reductant technology in this treatment also showed an effectiveness in reducing application frequency. This study demonstrates that herbicides can effectively and efficiently control P. karka in oil palm plantations, especially in peat land. Reductant can be one of the solutions to reduce the use of herbicides without lessening the efficacy, affordably, and sustainably.


Introduction
The land conversion for industry, housing, and tourism, as well as the development of connecting infrastructure, has caused the availability of arable land for plantations to become increasingly limited.As a result, agricultural and plantation lands shifted to marginal lands such as peatlands [1].The conversion of peatlands into oil palm plantations is facing problems related to the productivity and sustainability of oil palm cultivation [2].Flooded conditions, high soil acidity, high organic acid content, low soil fertility, thick peat layers, which cause increased anaerobic activity, and subsoil make this land not ideal for oil palm cultivation [3].In the case of Pekan Tua, Indragiri Hilir, Riau, the oil palm productivity on thick peat is severely hampered due to a lack of management.With insufficient attention given to proper agricultural practices, the plantations can only generate 3 tons of FFB per hectare, falling significantly short of the potential maximum yield of 30 tons per hectare [4,5].The existence of plant pest organisms, including weeds, also adds to the complexity of the problem of suboptimal palm oil 1308 (2024) 012008 IOP Publishing doi:10.1088/1755-1315/1308/1/012008 2 production.The need for Crude Palm Oil (CPO) continues to increase, which is indicated by the increasing volume of CPO exports and domestic demand for CPO from year to year, reaching 25.9 million tons of CPO in 2020 [6].
Weeds are one of the main problems in cultivating oil palm on peatlands.Moreover, weeds on peatlands are also higher when compared to mineral lands [1,7].Several wild plants have high resistance to acid conditions, such as on peatlands, act as weeds in oil palm plantations, and are invasive and difficult to control [8][9][10].Weeds cause losses due to direct competition in the need for nutrients, water, sunlight, CO2, and space to grow with cultivated plants.In addition, weeds also cause indirect losses because they can act as hosts that carry pests and diseases and produce growth-inhibiting chemical (allelopathic substances), especially in reeds, sedges, and stolonic vines, which can poison cultivated plants.The presence of uncontrolled weeds can decrease FFB production by 20-80% [11,12].
Phragmites karka (Perumpung or Prumpung, local name) is an invasive halophyte plant found in flooded areas such as peatlands [13].This plant grows massively in oil palm plantations resulting from the conversion of peat and becomes a weed that interferes with oil palm cultivation.P. karka is an invasive weed with spreading rhizomes and upright stems that can reach 10 meters in height, so many seeds that it spreads very quickly, and is difficult to control.In addition, the presence of spreading rhizomes and their resistance to acid soil and alkalinity stress could support its massive growth [10,14].This weed is categorized as viscous because it is aggressive and difficult to control, causing great losses [15].
To date, P. karka is controlled by cutting or burning, but its effectiveness is still relatively low because it grows back quickly [16].This thick underground stem allows for storing food reserves and, with its extensive growth of rhizomes, allows for very rapid regrowth [17,18].This mechanical control must be done repeatedly, requiring a lot of energy and costs, and is undoubtedly less efficient when applied to large-scale plantations such as oil palm [16].Therefore, chemical control using synthetic herbicides is considered as the first choice because of its good effectiveness, more consistent results, time efficiency, and more economical when compared to mechanical control.However, the excessive use of agrochemicals, especially synthetic herbicide, without following the recommended dosage can cause problems in oil palm cultivation, including weed resistance to certain active ingredients which makes weed control more difficult and costly.Moreover, pesticide residue can accumulate and bound in air, soil, or water can pose a serious threat to human health and biological diversity [21].
However, on the other hand, literature and research discussing effective and sustainable ways to control P. karka in oil palm plantations are still limited.Therefore, this research was conducted to find an effective and sustainable method to control P. karka in oil palm plantations, especially on peatlands.Pesticide reductant technology used to reduce the pesticide dosage without lessening the efficacy and jeopardizing the environment.Therefore, plantations can significantly reduce the number of pesticides used, substantially saving cost, preventing their spray workers from overexposing excessive chemicals contained in pesticides, and also being more sustainable and environmentally friendly.

Materials and Methods
The research was conducted during 2018-2019 in oil palm plantations located in Central Kalimantan.The soil type is peatland with low pH.The research began with the trial site determination and weed analysis to identify the needed target weeds and herbicide treatments needed.Analysis of weed vegetation showed that the predominant weed type was P. karka.After the area to be plotted and determined, locations are marked where the demo plot will be carried out according to the treatment applied.
The study was conducted using a Randomized Block Design (RBD) with the types and concentrations of herbicides according to table 1. G1 is the estate practice (SOP) to control P. karka using two times applications.This research was conducted by comparing several herbicides with varying concentrations and reducing concentrations using Reductants (G2-G6).Then, the frequency of spraying was modified to single sprayed and reduced concentration of herbicides (G7-G9) compared with estate practice (G1).The layout treatment unit within a group is determined so that the distribution of target weeds is relatively even.
Each treatment was applied by spraying the solution evenly over the entire surface of the target weeds using a spray tank with a capacity of 14 liters.Applications are made when the target weed coverage reaches more than 75%.The application technique follows estate practices.The application is made in the morning and in a suitable condition.Observations were made up to 12 WAA (weeks after application).The intervals of observation i.e 0, 4, 8, 12 WAA in all treatments by visually observing weed mortality and regrowth percentage at each plot.Percentage levels of phytotoxicity are based on standards set by the estate in table 2. The data obtained were then analyzed using the analysis of variance method (ANOVA) with a 95% confidence level.If there is a significant difference between the treatments, a further test is carried out using Duncan's Multiple Range Test with a significance level of 5%.
The cost efficiency achieved from several treatment combinations compared to the control determined based on equation (1) below: Where c0 and c1 stands for the SOP (estate) practice and each treatment cost for P.karka control respectively.

Result and Discussion
3.1 Mortality rate of Phragmites karka with chemical treatment at several concentration combinations of the active ingredients of the herbicide and Reductant P. karka usually lives in flooded areas such as peatlands and is difficult to control due to its rhizomes and ability to produce large numbers of seed banks [16,17,21].The results of controlling P. karka up to 8 WAA (weeks after application) using several active ingredients of herbicides with varying concentrations and combined with reductants are presented in table 3 and figure 1 below.

Figure 1. Phragmites karka percentage of mortality in several treatments
The results showed herbicide application following estate practice (G1) gave control results in the form of mortality, which was not significantly different from all treatments (G2-G9) up to 8 WAA.All treatments showed to have succeeded in controlling P. karka properly and were acceptable based on the standard level of weed phytotoxicity, according to the European Weed Research Council (EWRC).Based on the percentage value of weed mortality, the treatments that gave the best control results were G4 and G5 (for the twice spraying) and G9 (for the single spraying).In the twice spraying method, G4 and G5 resulted in 100% mortality up to 8 MSA, while G1 (estate practice) resulted in 98.3% mortality up to 8 WAA.G1 used herbicides with active ingredients of glyphosate and metsulfuron-methyl in the first application, followed by the application of paraquat and triclopyr in subsequent applications.Meanwhile, treatments G4 and G5 used glyphosate and metsulfuron-methyl for the first application and continued with paraquat and metsulfuron-methyl for the second application.Reductants are used in conjunction with reduced herbicide concentrations in both applications.A comparison was made between the G4 and G5 treatment by the dosage of glyphosate used.
The results of this study similar with Cowbrough et al. [16].Applying glyphosate provides good control of P. karka up to 4 WAA and continues up to 14 WAA with a control of 97%.Glyphosate is quickly absorbed and translocated throughout plant tissues, deteriorating all plant tissues from leaves, and stems to rhizomes.Meanwhile, application with a mixture of the active ingredient triclopyr gave good initial results and gradually decreased by the time.These findings are consistent with this research.The weed mortality of G1 treatment decreased until the end of observation (14 WAA) and became less than optimal.In addition, the growth of the rhizome and the formation of a vast canopy produced by P. karka might be a reason for more variation in the control effectiveness in each treatment.To date, there are only three active ingredients of herbicides are known to specifically mention that they can be used to control P. karka on their product labels, i.e., glyphosate, imazapyr, and sodium chlorate [10,16].Imazapyr application alone or in combination with glyphosate is the most effective treatment with a more extended control period [19].The use of herbicides with other systemic herbicides needs further investigation through single applications and synergistic mixtures of several active ingredients to obtain more effective and efficient P. karka control results.Herbicides considered adequate for controlling P. karka are systemic herbicides.Systemic herbicides can be absorbed and translocated through plant tissues to rhizome tissues [14].Other systemic and broad-spectrum herbicides have been tested.They can control P. karka, including quizalofop-p-ethyl, flazasulfuron, glufosinateammonium, and a combination of glyphosate and imazapyr [16,19,20].
The use of metsulfuron-methyl, paraquat, glufosinate-ammonium and topramezone to control P. karka has never been studied before.Seeing the effectiveness of the control of the treatment with these three active ingredients which is better than the estate practice (G1), indicates the potential for the use of these four active ingredients for use in controlling P. karka in the oil palm plantations.Although G9 showed the best performance with 97.7% of weed mortality, this treatment was considerable cannot be applied in the estate because it cost more than estate practice (G1).
G7 was chosen to be the best treatment because besides providing the same good weed death, this treatment also provides cost savings for the company.Furthermore, G7 with the application of a reductant and a combination of the active ingredient glyphosate and topramezone can reduce the intensity of herbicide application to a single application compared to estate practice, which required two applications regularly.The use of reductant herbicides can reduce herbicide use without lessening the effectiveness of the herbicides.This can be explained because reductant formulations act as active ingredients carrier and allow the herbicide's active ingredients to reach the target cells in weeds more easily.Reductant is also can form complexes with herbicidal active ingredients, thereby increasing the phytotoxicity.Besides effectively controlling P. karka on peatland in oil palm plantations, the G7 is also more time efficient, easy to apply, and can cut operational costs, especially paying for sprayers.

Regrowth rate of P. karka with herbicide application at several concentration combinations of the active ingredients of the herbicide and Reductant.
The regrowth of P. karka was observed from 8 WAA to 12 WAA.Application of systemic herbicides allows longer weed control so that regrowth can be suppressed.The results of the P. karka regrowth analysis of variance in table 4 showed significant differences between all treatments.The effectiveness of herbicides began to decrease at 8 WAA which was marked by initial regrowth.At 8 WAA, the highest regrowth value occurred in treatment G4 (9.7%), and the lowest regrowth value occurred in treatment G2 (1%).These two treatments were significantly different from G1 (4.3%) as the standard used by the plantations.Up to 12 WAA, G1 showed the highest regrowth compared to all treatments, with a regrowth percentage of up to 25%.Meanwhile, the G2 showed an effective control, indicated by a regrowth value of 1%.Most research findings suggest that two times herbicide application (re-spray) are more effective at controlling weeds than single sprayed.However, this evidence only exists when initial spraying is effective to control weeds [22].An interesting result was found in this study, where the treatment with two times application (G1 to G6) had a trend of significant increase in weed regrowth (8 WAA to 12 WAA) compared to the single application treatments (G7 to G9).Whereas, treatment with two times application is the standard method used by plantation or estate practice (SOP).This re-spraying method produces P. karka control that does not last long, with a regrowth outbreak reaching 25% until 12 WAA.Meanwhile, applying herbicides with a single application can withstand the regrowth of P. karka not exceeding 12% until 12 WAA.
Haarmaan [22] stated in his study that a greater number of weed regrowth exists when the first application fails to control the weeds effectively.These failures are caused by a number of reasons.All of which are a result of significant quantities of active ingredients failing to reach the target site: reduced absorption and translocation; resistance; inadequate spray coverage; and the herbicide usage.In this research, the significant regrowth is likely due to the less effective control in controlling P. karka.The type of active ingredient, dosage, and intensity of application of the given herbicide is not sufficient to kill all weed tissue down to the rhizome, so new shoots may appear from the rhizome that has not killed completely [17].Its resistance to stress conditions and ability to produce large reserves of seed banks in the soil might also contribute to massive P. karka regrowth at each treatment plot [14,20].
Based on regrowth value, G2 was the best treatment with regrowth of 1% to 12 WAA (two times application system) and G7 was the best treatment in a single application with regrowth of 12% to 12 WAA.The use of topramezone, glufosinate-ammonium, glyphosate, and pesticide reductant (reductant Y) in G7 treatment provide complete weed control and suppress the weed regrowth although it's done by single application.Topramezone, classified as a selective systemic herbicide, exhibits excellent absorption through leaves, roots, and shoots, allowing it to efficiently translocate to the vital growth areas of susceptible weeds.Its mechanism of action involves the inhibition of carotenoid biosynthesis as an HPPD inhibitor, effectively managing the emergence of weeds [23].Meanwhile, glufosinateammonium works by inhibiting an enzyme glutamine synthetase that is central to plant metabolism.Its unique mode of action makes it to be used in rotation with other herbicides to mitigate weed resistance, especially of those difficult-to-control Glyphosate-resistant weeds.Resistant weeds itself will certainly increase the weed regrowth [24].
Beside the synergistic work of the active ingredients, the effective control of P. karka in single application is likely because of the use of reductant Y. Reductant Y featured by inorganic mineral-based and renewable surfactant.Besides acting as a carrier of active ingredients to reach the target site effectively, reductant Y can reduce the surface tension of spray droplets which helps increase leaf coverage.This results in greater leaf area in contact with herbicide, leading to greater uptake of herbicide by weeds and the possibility of the active ingredient reaching the target site.Reductant Y's surfactant feature also ensures the active ingredients adhere better to the surface of the weeds, reducing the chance of the herbicide being leached to the environment [22].

Cost efficiency of controlling P. karka with several combinations of herbicide and reductant concentrations
A cost comparison of each treatment in controlling P. karka is shown in table 5.The cost structure of weed control consists of the cost of herbicide used and spray workers.Based on the cost analysis, only G7 provided control cost savings compared to estate practice (G1).A combination of Glyphosate 1.8 L/ha, Topramezone 0.045 L/ha, and Reductant Y 0.3 L/ha in G7 treatment can reduce workforce costs by up to 50% and produce total control cost efficiencies of up to 0.43% per hectare.If the total area of the plantations reaches 150,000 ha with the use of herbicides rotated two times a year, the plantation can save costs of up to IDR 420,000,000 per year.The use of pesticide reductant together with systemic herbicides can reduce the herbicide usage, in terms of dosage or application intensity.Therefore, applying a combination of glyphosate and topramezone accompanied by a reductant can be an alternative for controlling P. karka effectively while simultaneously reducing the application intensity and dose of herbicides to save costs.

Conclusion
The combination of herbicides in each treatment resulted in a response in the form of different levels of weed mortality and regrowth of P. karka that were different.G7 (Glyphosate 1.8 L/ha; Topramezone 0.045 L/ha, and Reductant Y 0.3 L/ha) was the best treatments, with mortality rates reaching 97% (4 WAA) and 97% (8 WAA) weed regrowth (not more than 12%) to 12 WAA.Besides effectively controlling P. karka, this treatment is also considered economical because it can save up control cost up to 0.43% per hectare per crop rotation.Applying a reductant technology with glyphosate, topramezone, and glufosinate-ammonium can be a promising alternative to achieve effective, sustainable, and costeffective controlling P. karka in oil palm plantations.

Recommendations
The combination of herbicides needs to be further evaluated while considering their safety profile for the environment (ecotoxicology), especially for aquatic ecosystems.The use of herbicides also needs to be applied wisely by constantly adhering to the recommended dosage and method of application to achieve effectiveness and safety.Excessive herbicide application must be avoided because it will increase control costs and produce more pesticide residues that can harm ecosystems, human health, populations of non-target organisms and decrease its effectiveness due to resistance to certain active ingredients.Using herbicide-reductant technology, non-ionic surfactants, and a combination of chemical and mechanical control can potentially control P. karka and be material for further study.

Figure 3 .Figure 4 .
Figure 3. Appearance of Phragmites karka in G4 treatment with two times application from 0 to 12 weeks after application (WAA)

Table 2 .
Percentage of weed mortality observed visually

Table 3 .
Mortality rate of Phragmites karka *WAA (Weeks after application); **The percentage value of the weed mortality rate is expressed in the mean values.Values followed by the same letter notation in one row show no significant difference (p > 0.05) based on Duncan's Multiple Range Test.

Table 4 .
Percentage of Phragmites karka regrowth *WAA (Weeks after application); **The percentage value of the weed mortality rate is expressed in the mean values.Values followed by the same letter notation in one row show no significant difference (p > 0.05) based on Duncan's Multiple Range Test.

Table 5 .
Comparison of cost efficiency in controlling P. karka *The nominal rate is given to the spray worker payment in the Central Borneo estate in 2018-2019; **Herbicide prices during 2018-2019 applied in trial site; ***Cost efficiency with a negative value indicates no cost savings compared to estate practices (SOP/G1)