MSc

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.