PhD

Field experiments were conducted at Rice Research Station,
Kayamkulam from April 1996 to May 1998 to study the effect of
integrated weed management practices for rice-rice-sesamum cropping
system of Onattukara tract. The experiment was laid out in randomised
block design with twelve treatments during the first crop season and in
split plot randomised block design with twelve treatments and twelve
residual effects during second crop season and with twenty four residual
effects during the third crop season. Rice varieties Bhagya and Dhanya
and sesamum variety Thilak were used for the study during the first, second
and third crop seasons respectively.
Results of the study revealed that grasses, broad leaved weeds and
sedges competed with the crop plants in the rice-rice-sesamum cropping
system. The herbicide treatments significantly reduced the monocot,
dicot and total weed population in the cropping system. Pendimethalin
pre-emergence application alone or integrated with hand weeding/2,4-D
post emergence significantly reduced the weed growth during the first
crop season whereas thiobencarb pre-emergence + hand weeding reduced
the weed growth in the second crop of rice. The weed count and dry
matter accumulation by weeds in the sesamum crop during the third crop
season was significantly reduced by the carry over effect of herbicide

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treatments applied both to first and second crop rice compared to the
carry over effect of herbicide treatments applied to first crop rice alone.
The herbicide treatments significantly influenced the growth and
yield attributing characters .of the first and second crop of rice. The
residual effects of previous weed control treatments on the growth and
yield of sesamum was not significant. Pendimethalin pre-emergence +
hand weeding or its integration with 2,4-D post emergence influenced
the growth characters of first crop of rice, thiobencarb pre-emergence +
hand weeding found to exert considerable influence on the growth and
yield of second crop of rice. Weedy check recorded the lowest yield
and under weed free situation the crop plants had registered maximum
growth and yield. The yield loss due to weed indicated by the weed indices
was maximum under weedy check and minimum for pendimethalin pre-
emergence + hand weeding and thiobencarb pre-emergence + hand weeding
applied to first and second crop of rice in rice-rice-sesamum cropping
system.
The weed management practices and their carry over effects had
significant influence on the uptake of nutrients by the crop and weeds
during the first, second and third crop seasons. Unweeded check exploited
the available nutrients and water resulting in better weed growth and dry
matter accumulation through out the cropping system.
The soil nutrient status was also affected by the weed management
practices studied. The N, P, K status of soil was high for weedy check




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compared to herbicide treatments and weed free treatments owing to poor
crop utilization of the nutrients.
The herbicide treatments applied both to the first and second crop
of rice and their carry over• effects on third crop of sesamum had no
significant effect on the population of soil organisms.
Economic analysis revealed that herbicide treatments were
remunerative compared to weedy check and farmers practice in the first
and second crop seasons. The residual effects of weed management
practices had not influenced the economics of sesamum crop during third
crop season.
Bioassay studies revealed no phytotoxic concentration of herbicide
residues in the soil after the rice-rice-sesamurn cropping sequence.