MSc

Sacciolepis interrupta and Isachne miliacea are two major problem weeds
of rice in Kerala. An investigation on the quantum of crop loss incurred due to
different densities of these weeds was undertaken to study the extent of damage
inflicted on the crop which would necessitate early control of these weeds.
The observations recorded on the various crop and weed characteristics
were analysed as a 52 factorial experiment. It was found that crop characteristics
like total bio- mass of paddy at harvest, number of tillers of paddy at harvest,
number of productive tillers at harvest, grain yield and strain yield. And weed
characteristics like number of tillers of S. interrupta at 60 DAS, height of
S. interrupta at 60 DAS, number of tillers of S. interrupta at harvest of rice, dry
matter production of S. interrupta and drymatter production of 1. miliacea were
found to be affected by the weeds. The intra and interspecific competition was
also brought to light based on the analysis.
Single weed species models like that of Cousens (1985), Hakansson
(1983), the first model of Watkinson (1981), Marra and Carlson (1983), Wilson
and Cussans (1983), Wilcockson (1977) and Carlson et al. (1981) fitted well to
the yield loss - S. interrupta/ 1. miliacea density relationship whereas those
models proposed by Ngouajio et al. (1999), Kropff and Spitters (1991), Dew
(1972), Zakharenko (1968) and Chisaka (1977) fitted well only to the yield loss-
S. interrupta density relationship.
The extended version of the Cousens (1985) model by Swinton et al .
. (1994a) to a multi-species model was also fitted to the data and the same
explained the yield loss - S. interrupta + 1. miliacea densities relationship to a
considerable extent. The reduced form of the multispecies model to an equivalent
single species model as worked out by Swinton et al. (1994b) also had a good fit.
The numerical assessment of yield loss _. S. interrupta + 1. miliacea density

relationship as illustrated by Berti and Zanin (1994) revealed the extent of
damage on the crop by the weeds.
The new curvilinear models tried also explained the yield loss - weed
density relationship with the exception that the role of 1. miliacea deterring the
yield of crop could not be highlighted due to its peculiar way of growth.
The threshold weed densities worked out on a economic loss basis
revealed that even the presence of two S. interrupta plants in a square meter area
was hazardous for the crop whereas even the presence of 321. miliacea plants in
the same stipulated area was not as detrimental as S. interrupta.