PhD

Floristic diversity, autecology and competitive behaviour of weed flora in the
wetland rice ecosystem were studied by conducting a field survey and two field
experiments. The field survey was conducted during the first and second crop seasons of
2003 in the rice fields of 30 panchayaths in Thiruvananthapuram district having more
than 50 hectares of net sown area, covering all the four thaluks of the district. Weed
flora was surveyed from both cultivated and fallow fields. The design adopted for
conducting the floristic survey was Stratified Multistage Random Sampling and the
methodology was Census Quadrate Method.
The field experiments were conducted during the first crop seasons of
2003 and 2004 at College of Agriculture, Vellayani. The design adopted was
Strip plot. The treatments included four levels of nutrient management and
three levels of crop spacing and the number of replications was five. Part III of
the investigation was an autecological study wherein the growth and
development pattern of two of the major weed flora observed in the
experimental area was studied by keeping an area of 1.2 X 4.2 m2 on the same
side of all the plots in Part II.
Perusal of the results of the field survey indicated that substantial diversity of
weed flora existed in the wetland rice ecosystem in Thiruvananthapuram district of
Kerala. In the cultivated rice fields, there were 46 weed specii during the first crop season
and 49 species during the second crop season. In the rice fallows during the first crop
season there were 43 specii while during the second crop season there were 54 specii.
In the cultivated fields during both seasons, the most prominent species
among the different classes of weeds (grasses, sedges and broad leaved weeds)
were Isachne miliacea Roth ex Roem et Schult, Cyperus iria L. and
Monochoria vaginalis (Burm.f.) Presl. Ex Kunth respectively. Isachne miliacea
Roth ex Roem et Schult was the dominant grass weed in the fallows as well.
However the dominant sedge weed in the fallows was Cyperus distans L.f.
during the first crop season while it was Fimbristylis miliacea. during the second
crop season. Monochoria vaginalis (Burm.f.) Presl. Ex Kunth and Ludwigia
perennis topped the list of broad leaved weeds in the rice fallows during first
and second crop seasons respectively.
Results of the field experiment showed that weed growth in the rice field
was influenced significantly by the sources and levels of nutrients as well as by
crop spacing. During both the years at 20 and 40 DAT the weed density and dry
weight were the lowest when the rice crop was treated with NPK @ 90:45:45
kg ha-1 applied with 25 per cent N as organic (N2) while at 60 DAT enhanced
nutrient level (NPK @ 112.5:56:25:56.25 kg ha-1) applied with 25 per cent N
as organic (N4) recorded the lowest value. During both the years, at all growth
stages the weed density and dry weight were the lowest under the closer
spacing of 15 X 15 cm (P1) The grain and straw yield were maximum under
NPK @ 112.5:56:25:56.25 kg ha-1 applied with 25 per cent N as organic in
combination with widest spacing (n4p3). Dead heart occurrence was minimum
under the widest crop spacing
The net income and Benefit : Cost ratio were the highest when rice crop
was treated with 125 per cent of the recommended NPK with 25 per cent
organic substitution in combination with widest spacing (n4p3)
Autecological studies showed that the growth and development of Isachne
miliacea Roth ex Roem et Schult. was significantly influenced by the
management practices for the associated rice crop. The higher dose of NPK
applied as chemical fertilizers only (N3) along with widest spacing (P3)
stimulated all the vegetative parameters (shoot / root length and plant spread)
of the weed. During both the years plants growing in rice fields treated with
recommended dose of NPK with 25 per cent organic substitution (N2) and rice
crop planted at wider spacing (P3) recorded delayed flowering but produced
more seeds per panicle
The root and shoot length of Cyperus iria L growing in rice fields treated with
100 per cent recommended NPK as chemical fertilizers (N1) with minimum spacing (P1)
recorded the maximum value during the both years. Delayed flowering of the sedge was
noticed under N2. The number of seeds produced per plant was significantly high under
N4 and P3. The general trend was that addition of organic manures in rice delayed
flowering but increased seed production in the associated sedge weed
From the studies it may be concluded that by altering nutrient management and
adjusting the plant population, the competitive ability and productivity of rice crop can be
improved substantially. Though, closer spacing of 15 X 15 cm significantly reduced the
density and dry weight of weeds, the wider spacing of 20 X 20 cm in conjunction with
enhanced fertilizer dose of 112.5:56:25:56.25 kg NPK ha-1 applied with 25 per cent N as
organic manures significantly enhanced the rice grain and straw yield. The treatment also
resulted in significant enhancement in net income and B:C ratio in medium duration
variety Aiswarya in first crop season.