PhD

The use of herbicides has become an indispensable part of agriculture,
particularly in rice cultivation, where weeds are the most harmful of the biotic
constraints to production. A usual practice among rice farmers in the Kole lands of
Kerala is the tank mixing of herbicides, for a broader spectrum of weed control,
enhanced weed control efficiency and reduction in the cost of application. However,
mixing of herbicides is done without any knowledge of the probable synergistic or
antagonistic interactions on weed flora. Herbicides are also mixed with fertilizers like
urea under the common belief that herbicidal efficiency is enhanced by this practice.
Application of herbicides may affect various plant pathogens and microbial
bioagents which occur naturally in the rice ecosystem. Information of the effect on
main disease causing as well as beneficial microorganisms would help in selecting
appropriate herbicides.
Hence the major objectives of the research programme entitled “Bio-efficacy
of tank mixed herbicides and urea in wet seeded rice” were to study the synergistic or
antagonistic effect of tank mixing of commonly used herbicides, and to evaluate the
efficiency of these herbicides on mixing with urea, on major weeds of rice. The effect
of these herbicides on beneficial and pathogenic microorganisms were also
investigated.
Two field experiments were conducted at Alappad padasekharam in the Kole
lands of Thrissur from October to January 2019-20 and 2020-21. The soil of
experimental field was clayey in texture, high in organic carbon (1.1-1.3%), low
available nitrogen (180-188 kg/ha), and medium phosphorus (20-21.5 kg/ha) and
potash (152-159 kg/ha), with a soil pH of 4.5-4.7. The feasibility of mixing herbicides
was evaluated in experiment I (Bio-efficacy of tank mixed herbicide combinations in
wet seeded rice). There were fourteen treatments and they included tank mixtures and
sequential applications of five herbicides viz., fenoxaprop-p-ethyl (0.06 kg/ha),
(chlorimuron-ethyl + metsulfuron-ethyl) (0.004 kg/ha), carfentrazone ethyl (0.02
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kg/ha), bispyribac-sodium (0.025 kg/ha), and (cyhalofop-butyl + penoxsulam) (0.15
kg/ha). These were compared with two broad spectrum herbicides, bispyribac-sodium
(0.025 kg/ha) and (cyhalofop-butyl + penoxsulam) (0.15 kg/ha), as well hand weeded
and unweeded controls. In experiment II (Bio-efficacy of tank mixed herbicides and
urea in wet seeded rice), there were twelve treatments. Six herbicides, viz., cyhalofop
butyl (0.08 kg/ha), (cyhalofop butyl + penoxsulam) (0.15 kg/ha), bispyribac-sodium
(0.025 kg/ha), fenoxaprop-p-ethyl (0.06 kg/ha), carfentrazone ethyl (0.02 kg/ha) and
(chlorimuron ethyl + metsulfuron methyl) (0.004 kg/ha) were applied with and
without urea (1%) mixing. Hand weeded and unweeded controls were also included
for comparison. Randomized Block Design with three replications was adopted for
both experiments. Wet seeding of rice at the seed rate of 100 kg/ha was done.
Nitrogen, phosphorus and potassium @ 90:35:45 kg/ha were supplied through urea,
factomphos and muriate of potash (KAU, 2016).
In experiment III (In vitro evaluation of herbicides on beneficial and
pathogenic microorganisms) treatments included the effect of herbicides mentioned in
experiment II which were evaluated under in vitro conditions with recommended,
higher and lower doses on beneficial microbial bioagents (Trichoderma viride and
Pseudomonas fluorescens) and pathogenic microorganisms (Rhizoctonia solani,
Pyricularia oryzae and Xanthomonas oryzae pv. oryzae). Factorial Completely
Randomized Block Design was adopted with 18 treatments and three replications
each.
Tank mixing of herbicides was found to be more effective than their sequential
application for most of the combinations. Tank mixed combination of (cyhalofopbutyl + penoxsulam) with (chlorimuron-ethyl + metsulfuron-methyl) showed
synergistic effect and had lowest weed density (32, 16 and 20 nos./m2
), lowest weed
dry matter production (14.68, 50.50 and 88.95 kg/ha), and highest weed control
efficiency (91, 88, and 82%) at 15, 30 and 60 days after application respectively. The
number of tillers per sq. m (411), number of panicles per sq. m (288), number of
grains per panicle (103), percentage of filled grain (91.6) at harvest, nutrient uptake
by rice at 60 DAS (105.06, 14.32 and 74.28 kg/ha of N, P and K respectively), grain
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yield (3.97 t/ha), net returns (Rs.71,406/ha) and benefit: cost ratio (2.25) were highest
in the same treatment. The next best treatment was fenoxaprop-p-ethyl + bispyribac
sodium.
On tank mixing of herbicides with urea, bispyribac-sodium and urea interacted
synergistically, resulting in highest weed control efficiency (82.5 and 78% at 15 and
30 days after application respectively), highest grain yield (5.03 t/ha), and profitability
(Rs. 93,509/ha) over the other treatments. Mixing of (cyhalofop-butyl + penoxsulam)
with urea could also be recommended for good results (grain yield of 4.94 t/ha and net
returns of Rs.87,463/ha).
In vitro evaluation of herbicides revealed that bispyribac-sodium and
(cyhalofop-butyl + penoxsulam) had greatest inhibitory effect on pathogenic
microorganisms Rhizoctonia solani and Pyricularia oryzae, but showed no effect on
Xanthomonas oryzae pv. oryzae, and were less harmful to biocontrol agents
Trichoderma viride and Pseudomonas fluorescens at recommended doses.
Tank mix combination of (cyhalofop-butyl + penoxsulam) with (chlorimuronethyl + metsulfuron-methyl) was the best treatment with compatibility, efficient weed
control, high grain yield and net returns. Tank mixing of bispyribac-sodium with urea
1% had synergistic effect resulting in highest weed control efficiency, production and
profitability over the other treatments. Mixing of (cyhalofop-butyl + penoxsulam)
with urea 1% could also be recommended for good results. Bispyribac-sodium and
(cyhalofop-butyl + penoxsulam) were the best herbicides on in vitro evaluation in
their effect on pathogenic microorganisms Rhizoctonia solani and Pyricularia oryzae,
and at recommended doses were compatible with biocontrol agents Trichoderma
viride and Pseudomonas flourescens.