PhD

The investigation entitled "Bioefficacy and soil health impact of flucetosulfuron in wet seeded rice (Oryza sativa L.) was carried out at College of
Agriculture, Vellayani, Thiruvananthapuram during the period of 2015-2018. The
objectives were to evaluate the bioefficacy of flucetosulfuron in wet seeded rice; to
assess its impact on soil health by determining the microbial count, earthworm
population, enzyme status and herbicide residue in soil through bioassay using
indicator plant as well as to assess the weed seed bank. It was also envisaged to
study the in vitro sensitivity of biofertilizer organisms and biocontrol agents to
flucetosulfuron.
Field experiment was conducted during the I and II crop seasons of 2016-
’17, in a farmer’s field in Kalliyoor Panchayat, Nemom block, Thiruvananthapuram
district, Kerala, with 12 treatments replicated thrice in Randomized Block Design.
The treatments were: T1 : Flucetosulfuron @ 20 g ha-1 at 2-3 DAS, T2 :
Flucetosulfuron @ 25 g ha-1 at 2-3 DAS, T3 : Flucetosulfuron @ 30 g ha-1 at 2-3
DAS, T4 : Flucetosulfuron @ 20 g ha-1 at 10-12 DAS, T5 : Flucetosulfuron @ 25 g
ha-1 at 10-12 DAS, T6 : Flucetosulfuron @ 30 g ha-1 at 10-12 DAS, T7 :
Flucetosulfuron @ 20 g ha-1 at 18-20 DAS , T8 : Flucetosulfuron @ 25 g ha-1 at 18-
20 DAS , T9 : Flucetosulfuron @ 30 g ha-1 at18-20 DAS, T10: Bispyribac sodium
@ 25 g ha-1 at 15 DAS, T11: Hand weeding at 20 and 40 DAS and T12 : Weedy
check. The short duration variety Kanchana was used for the trial.
The weed flora in the experimental field comprised of four species of
grasses, six species of broad leaved weeds and three species of sedges. Based on
density and dry weight, sedges were the predominant group in the experimental
field followed by broad leaved weeds and grasses. Application of flucetosulfuron
@
20,
25
and
30
g
ha-1 at 10-12 and 18-20 DAS (T4, T5, T6, T7, T8 and T9) recorded lower total weed
density, total weed dry weight and nutrient removal by weeds and higher total weed
control efficiency and were on par with the hand weeding treatment (T11). Yield
reduction due to weeds was found to be 52.33 and 55.61 per cent during the I and
II crop seasons respectively.
The highest grain yield was recorded by the application of flucetosulfuron
@ 25 g ha-1 at 10-12 DAS (T5) and flucetosulfuron @ 20 g ha-1 at 10-12 DAS (T4)
during the first and second crop seasons respectively. The same trend was observed
in the case of dry matter production also. Pooled analysis of the grain yield for the
two seasons indicated the superiority of T5 which recorded the highest grain yield
of 8.33 t ha-1. This treatment was on par with the other levels of flucetosulfuron viz.,
20 and 30 g ha-1 at 10-12 DAS (T4 and T6) and hand weeding treatment (T11). Higher
yield attributes, harvest index and nutrient uptake were also recorded for these
treatments. However, net income and B:C ratio were significantly lower for hand
weeding treatment.
Herbicide application did not cause any significant inhibitory effect on the
earthworm population and spider count. Application of herbicides increased the
microbial population in herbicide treated plots. Flucetosulfuron, at the tested doses
(20, 25 and 30 g ha-1) and time of application (2-3, 10-12 and 18-20 DAS), did not
show any adverse impact on the activity of soil enzymes (dehydrogenase, acid
phosphatase and urease) and soil organic carbon status.
To identify the most suitable indicator plant for assessing flucetosulfuron
residue in soil, four separate pot culture experiments were conducted using
barnyard millet, cucumber, sunflower and maize. Sunflower was identified as the
most sensitive indicator plant and shoot length of sunflower was adjudged as the
best parameter to assess the flucetosulfuron residue in soil. Bioassay after each field
experiment using sunflower revealed that there was no residual toxicity of
flucetosulfuron in the post experiment soil.
Pot culture experiments were carried out before and after each field
experiment, to study the weed seed bank dynamics. Results revealed that non-
herbicidal plots recorded higher weed count compared to herbicide applied plots.
In all the herbicide treated plots, effective reduction in weed seed bank could be
obtained irrespective of the dose of the herbicide. Regarding the time of application,
application of flucetosulfuron at 10-12 and 18-20 DAS caused significant depletion
of weed seed bank during both the seasons compared to its application at 2-3 DAS.
In vitro sensitivity of biofertilizer organisms viz., Azospirillum lipoferum,
Azotobacter chroococcum, Bacillus megaterium and Frateuria aurantia to
flucetosulfuron indicated that growth of none of the organisms were significantly
inhibited by the tested doses of flucetosulfuron. Similarly, in vitro sensitivity to
biocontrol agent Trichoderma viride revealed that, at all the tested concentrations
of flucetosulfuron, it showed a colony diameter of 9 mm uniformly with zero per
cent inhibition indicating the compatibility of the herbicide. The compatibility
studies with Pseudomonas fluorescens also revealed the safety of the herbicide.
The present study revealed that application of flucetosulfuron @ 20, 25 and
30 g ha-1 at 10-12 DAS was very effective in controlling the weeds and recorded
significantly higher grain yield and monetary benefits in wet seeded rice.
Application of flucetosulfuron @ 20, 25 and 30 g ha-1 at 2-3/10-12/18-20 DAS had
no adverse impact on soil health and was found compatible with the tested
biofertilizer organisms (Azospirillum lipoferum, Azotobacter chroococcum,
Bacillus megaterium and Frateuria aurantia) and biocontrol agents (Trichoderma
viride and Pseudomonas fluorescens).