Surveys and experiments were conducted in 1988 - 1990 to study
some aspects of the biology of itchgrass [Rottboellia coch~nchinensls
(Lour) Clayton] and its interference with rainfed rice (O~yza sntivn L.)
production. Proportion of farmers that grow rainfed rice, the most
important rice weed, the major weed management methods and the usual
period of first weeding by farmers were determined through questionnaire
and oral interviews. Soil fiom four rice-growing communities were
surveyed to determine the extent of itchgrass infestation using the
seedling emergence as an index. Relative rate of germination of freshly
harvested dehusked and undehusked itchgrass seeds as well as
germinability of seeds buried at varying depths for varying periods of
time were studied. Itchgrass was grown with rice over varying periods
to determine the critical period of its interference on growth and yield
parameters of the crop. Also replacement series experiment was used
to study the effects of itchgrass interference in the crop.
Result ofthe sunley showed that rice farmers in the study area
predominantly grow rice in rainfed upland fields. Itchgrass was identified
as the most important rice weed while the weed management strategy
used mostly was hand-pulling. The farmers carried out first weeding at 6 - 7 weeks after transplanting (WAT) or 5 - 6 weeks after sowing (WAS)
depending on the cropping method.
Ndike community had the highest level of itchgrass infestation
in the two years of survey. The seeds germinated faster when they were
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dehusked than when they were not. Germination reached its peak at 4
-5 days and 9 days after sowing (DAS) in dehusked and undehuked seeds
respectively. Longevity of itchgrass seeds varied significantly with both
depth and period of burial. It decreased with depth of burial. Critical period of itchgrass interference was 4 - 6 WAT in transplanted rice (TPR) and 3 - 6 WAS in direct seeded rice (DSR). Thus DSR required longer
itchgrass-free conditions during the early crop establishment than
transplanted rice
In both TPR and DSR, the palnt height and tillers number of the
crop decreased with increase in total plant density. Itchgrass generally
exhibited higher tillering ability than rice especially in direct seeded
rice. This however decreased Milth increasing rice population in the
mixture.
Transplanted rice was a stronger competitor than itchgrass during
both seasons. The reverse was the case in DSR. In TPR, one rice plat
and about 6.8 itchgrass plants in 1989 (or 12.3 in 1990) reduced rice
height to the same extent. In DSR, on rice palnt and 1.65 itchgrass plants
in 1989 (or 1.89 in 1990) showed similar effects. Thus effects of
intraspecific competition were more in TPR while those of interspecific
competition were more in DSR. Plant height, tillering and dry matter
yield decreased in itchgrass as relative population of rice increased in
the mixture. The seed was affected more by interspecific competition
in TPR than intraspecific competition. In DSR, the reverse was observed.
Both species showed niche differentiation in TPR but not in DSR.