Management of salvinia (Salvinia molesta D. S. Mitch.) in rice ecosystem

Abstract

Salvinia molesta, the invasive aquatic fern common in wetlands of Kerala, has attained the status of a troublesome weed in cultivated rice fields in many parts of the State. Biological control using the weevil Cyrtobagous salviniae is feasible for stagnant water bodies as it takes a long time to manage the weed. Hence, a study was undertaken to check the efficacy of a few new generation rice herbicides and eco-friendly chemical measures to control salvinia infestation in paddy fields. The experiment was conducted from September 2021 to August 2022 at the College of Agriculture, Vellanikkara, Thrissur. A few selected treatments were tested for field efficacy at farmer’s rice field in Aikarnad panchayath in Ernakulam district, where the weed infestation was severe. Salvinia was cultured in trapezoidal wide mouthed cement tanks of 0.30 m2 area and 30 L capacity. There were 15 treatments, laid out in CRD design with three replications. The treatments were lime and gypsum @ 0.10 kg/m2, carfentrazone-ethyl @ 0.02 and 0.04 kg/ha, (cyhalofop-butyl + penoxsulam) and (florpyrauxifen-benzyl + cyhalofop-butyl) @ 0.15 kg/ha, (butachlor + penoxsulam) @ 0.82 kg/ha, 2,4-D amine @ 1 kg/ha, acetic acid, salicylic acid, NaCl and copper sulphate each @ 5% w/v, glufosinate ammonium @ 0.3 and 0.5 kg/ha, and an untreated check. The percentage of control, as well as phytotoxicity symptoms, were observed periodically and scoring was done. Regrowth of salvinia (if any) was recorded at 10, 20, 30, and 40 days after treatment and fresh biomass were weighed at 40 days. Water samples were collected from tanks 10 days after treatment application and pH, EC, carbonate, bicarbonate, calcium, sulphate, nitrate, chloride, total hardness, biological oxygen demand, chemical oxygen demand, dissolved oxygen, alkalinity and acidity were analysed. Sediment samples were collected and observations on pH, EC, organic carbon, available N, P, K, Ca, and S were estimated. The data were subjected to statistical analysis using the statistical software GRAPES. Almost 95 percent control of salvinia was observed when sprayed with herbicides glufosinate ammonium, (florpyrauxifen-benzyl + cyhalofop-butyl), (cyhalofop butyl + penoxsulam) and (butachlor + penoxsulam). The selective herbicide, carfentrazone-ethyl was slow in action and a lower dose of 0.02 kg/ha was enough to get satisfactory control by 40 days of spraying. Among the various chemicals tested, salicylic acid, copper sulphate and acetic acid were effective and statistically at par (90 percent control). However, a few new sprouts of salvinia were observed in tanks treated with salicylic acid, copper sulphate and acetic acid 40 days after spraying. 2,4 -D amine (17% control) and lime (32% control) were not effective against salvinia and some regrowth was also observed. NaCl and gypsum were ineffective and gypsum was even found to stimulate the growth of salvinia. Sodium chloride spray @ 5% did not affect salvinia though there was a conspicuous hike in EC of water to 2.48 dS m-1 compared to 0.29 dS m-1 in untreated check, indicating its tolerance to a slightly saline situation. The pH of the water in treated tanks ranged from 6.66 -9.16 and the highest value was recorded with the lime application. Bicarbonate, carbonate, and alkalinity showed the same trend, the highest value was observed in the treatment with lime. Changes in pH and EC of water were prominent with lime, gypsum and copper sulphate application. However, these changes did not produce any direct impact on the growth suppression of salvinia. As expected, significantly high calcium content was observed with gypsum (CaSO4.2H2O) application (112 mg L-1) which is above the acceptable limit. The sulphate content and total hardness were below the permissible limit for all treatments. Chloride content was within the safe limit (< 4 meq L-1) as per FAO standards for all other treatments except for NaCl. Chemical oxygen demand was above the desirable limit of 1 mg/L and in glufosinate ammonium, carfentrazone-ethyl, (cyhalofop-butyl + penoxsulam), NaCl, and gypsum application. The dissolved oxygen level was within the desirable limit for all other treatments except for lime and copper sulphate. Biological Oxygen Demand (BOD) was high in treatments where undecomposed salvinia mat sank to the bottom of the tank as a result of spraying of carfentrazone ethyl, (butachlor + penoxsulam), (cyhalofop butyl + penoxsulam), salicylic acid and acetic acid. Various treatments did not alter the pH of sediments and the values ranged from 5.8 to 6.7, which was close to the initial pH of 6.5. Electrical conductivity increased in all treatments compared to a value of 0.05 dS m-1 observed in untreated check. In general, organic carbon, available nitrogen, and potassium content of the sediment were higher in all the treatments that showed more than 85 per cent control of salvinia. The treatments which showed efficiency under tank study gave good control under field situation also, though the efficiency was lower. The herbicides (butachlor + penoxsulam), glufosinate ammonium, and (cyhalofop butyl + penoxsulam) registered 70, 60 and 50 percent control respectively. Acetic acid spray caused severe phytotoxicity to rice, though it could effectively control the weed and hence cannot be recommended in cropped situation. Under field condition, there was no significant change in sediment quality due to various treatments. From the present study, it can be concluded that the premix herbicides (butachlor + penoxsulam), (cyhalofop butyl + penoxsulam) and (florpyrauxifen – benzyl + cyhalofop – butyl) can be used for quick control of salvinia in a standing rice crop. Being a broad-spectrum nonselective herbicide, glufosinate ammonium can be recommended for preplant application only. From an ecological point of view detailed studies on long term effect of these herbicides/chemicals on the aquatic organism and water quality parameters have to be undertaken, before arriving at a conclusive recommendation.