MSc

The study entitled "Persistence of selective herbicides in rice-rice system" was the
part of 'permanent herbicide trial' of All India Coordinated Research Programme on
Weed Control conducted during the years 2001 and 2002 with the objective of
determining the persistence of butachlor, pretilachlor and 2,4-D in the rice-rice cropping
system. The effect of herbicides on soil microflora, weed growth and nutrient content
were also studied. The field experiments were conducted during 2001 to 2002 at
Agricultural Research Station, Mannuthy. Residue studies related to field experiments
were conducted at Herbicide Residue Laboratory of All India Coordinated Research
Programme on Weed Control, Thrissur Centre, located at Radio Tracer Laboratory,
College of Horticulture, Vel1anikkara. Microbiological studies and physico chemical
analysis of the soil samples were conducted at College of Horticulture, Vellanikkara.
The technical programme consisted of six treatments namely (i) hand weeding
twice (25 and 40 DAS); (ii) butachlor fb 2,4-D (with 100% NPK as inorganic fertilizer)
in the first and second crop seasons of 2001 and 2002; (iii) butachlor fb 2,4-D (with
100% NPK as inorganic fertilizer) in the first crop season of 2001 and 2002, pretilachlor
fb 2,4- D (with 100% NPK as inorganic fertilizer) in the second crop season of 2001 and
2002; (iv) butachlor fb 2,4-D (with 75% NPK as inorganic fertilizer and 25% through
FYM) in first crop of2001 and 2002, pretilachlor fb 2,4-D (with 100% NPK as inorganic
fertilizer) in second crop of 2001 and 2002; (v) butachlor fb 2,4-D (with 100% NPK as
inorganic fertilizer) in the first and second crop of 2001, pretilachlor fb 2,4- D (with 100%
NPK as inorganic fertilizer) in the first and second crop of2002 and (vi) butachlor fb 2,4-
D in the first and second crop of 2001 (with 75% NPK as inorganic fertilizer and 25%
through FYM in the first crop and 100% NPK as inorganic fertilizer in the second crop),
pretilachlor fb 2,4-D in the first and second crop of 2002 (with 75% NPK as inorganic
fertilizer and 25% through FYM in the first crop and 100% NPK as inorganic fertilizer in
the second crop).

The pre-emergence herbicides butachlor @ 1.25 kg a.i ha-1 and pretilachlor @ 0.75
kg a.i ha-1 were applied at 8 days after sowing for the control of grassy weeds. 2,4-D @
1.00 kg a.i ha" was applied at 20 days after sowing for the control of broad leaved weeds.
Persistence of the above three herbicides applied in first and second crop of 2002 was
estimated at one and 30 days after spraying and at the time of harvest.
Residues of butachlor at one day after spraying ranged from 0.331 to 0.396 ug
g-l in first crop of 2002. The highest concentration of 0.396 ug g-l was recorded in the
plot where butachlor was applied with 75 per cent NPK as inorganic fertilizer and 25 per
cent through FYM. About 95.21 to 97.66% of the applied butachlor dissipated from the
soil by 30 days after spraying. Greater dissipation was observed in the FYM applied plots
(97.66%). The application of FYM enhanced the microbial degradation which resulted in
higher dissipation of butachlor. In the second crop, only one plot received butachlor
spraying which recorded residues of 0.343 ug g-l at 1 day after spraying and 0.020 ug g-l
at 30 days after spraying.
Residues of pretilachlor at one day after spraying ranged from 0.200 to 0.215 ug g-l
in the first crop season and 0.199 to 0.250 ug i1 in the second crop season. At 30 days
after spraying residues ranged from 0.020 to 0.013 ug g" and 0.010 to 0.030 ug g-l in the
first and second crop season respectively. As in case of butachlor higher degree of
dissipation was observed in FYM applied plots (97.47 to 98.05%).
Residues of 2,4-D at one day after spraying ranged from 0.310 to 0.502 ug g-l
in first crop season and 0.395 to 0.480 ug g' in second crop season of 2002. At 30 days
after spraying the residues ranged from 0.035 to 0.010 ug g' in first crop and 0.016 to
0.005 ug g-l in second crop season. Up to 97.81 to 99.27 per cent of 2,4-D had been
dissipated from the plots which received FYM. In the other plots percent dissipation of
2,4-D from the soil was 94.89 to 97.81 per cent.

On comparing the extent dissipation of butachlor, pretilachlor and 2,4-D it was
found that 2,4-D had been dissipated to a higher magnitude than butachlor and
pretilachlor. At the time of harvest residues were not detected in soil, rice grain and
straw.
Studies on the effect of herbicides on soil bacterial population showed that the
total number of bacteria in the soil had been considerably reduced by spraying herbicides.
The extent of reduction was maximum (95.24 to 96.80%) at 7 DAS. The inhibitory effect
of pretilachlor on soil bacteria was comparitively less than that of butachlor. The
herbicides reduced the population of soil fungi. However, the magnitude of reduction was
less than that of soil bacteria. It was also observed that butachlor and pretilachlor
inhibited fungal population to the same degree (76.80%). Actinomycetal population in
the soil was also inhibited by the application of herbicide and the maximum percent
inhibition (77.00 to 85.00%) was at 15 DAS. Effect of butachlor on supression of soil
actinomycetes was found to be intermediary to bacteria and fungi. In the plot where FYM
was applied, population of bacteria, fungi and actinomycetes were higher than that of
other plots, which gave an indication that FYM could reduce the adverse effect of
herbicides on soil rnicroflora.
The data on weed count and dry matter production of different weed species
showed that Echinochloa spp. contributed very much to total weed population and total
dry weight. Build up in the population of Echinochloa spp. was observed with continuous
use of pre emergence herbicides. The plot where pretilachlor fb 2,4-D was applied with
100 per cent NPK as inorganic fertilizer recorded higher counts of Echinochloa spp.
Application of 2,4-D was effective in controlling broad leaved weeds. None of the
broad leaved weeds showed build up in the population. The plot which received butachlor
fb 2,4-D with 100 per cent NPK as inorganic fertilizer recorded highest grain and straw
yield.

From the study it could be concluded that at the present recommended rate of
application, residues of herbicides butachlor, pretilachlor and 2,4-D do not persist in
paddy soil to detectable level beyond 30 days. Residues were not detected in the grain
and straw. Application of FYM enhanced microbial degradation of herbicides and
reduced the adverse effect of herbicides on soil microflora. 2,4-D was highly efficient in
controlling broad leaved weeds in the rice-rice system. Both pretilachlor and butachlor
controlled grasses except Echinochloa spp. Butachlor had shown its superiority over
pretilachlor in the weed management of rice-rice cropping system.