HERBICIDES in Asian rice - IRRI books - International Rice ...

demand for herbicide combinations capable of controlling annual as well as perennial
weeds with one application. Most of these mixtures are based on sulfonyl urea types,
such as bensulfuron and pyrazosulfuron. A single application of a herbicide mixture
seems to be effective and economical, but relatively early application, within 10 d
after transplanting, does not give satisfactory control of E. kuroguwai and Sagittaria
triforia, which need a long period for emergence and often escape the residual effect
of the herbicides. These species have become the most troublesome weeds in transplanted
rice in Korea (Kim 1993a).
Changes in weed flora caused by cultural practices
About 92 weed species belonging to 27 families are found in Korean ricefields; 30
species are considered to be common weeds (Kim 1981). Changes in weed flora over
the last 3 decades may be useful in predicting weed flora changes in other countries as
their rice culture changes (Kim 1993a).
About 10 species, including both annuals and perennials, were the major weeds
in transplanted rice for half a century. The relative importance of different species has
been affected by recent changes in rice cultural practices and weed control measures
used by farmers, in particular by the type and amount of herbicide applied. The heavy
applications of herbicides in rice since 1980 have stabilized weed populations, with
perennial weeds dominating.
In 1965, before herbicide use had become prevalent, 17 species were identified
as common (Table 6). Echinochloa crus-galli was the most troublesome, followed by
M. vaginalis, which was also a serious weed in rice seedbeds. Other species were
minor weeds. In 1971, Rotala indica was recognized as the dominant species, followed
by Eleocharis acicularis, M. vaginalis, C. difformis, E. crus-gulli, Lindernia
procurnbens, Potarnogeton distinctus, Aneilema japonica, E. kuroguwai, S. juncoides,
and Persicaria hydropiper. These species constituted 86% of all ricefield weeds; 81%
are annuals. In 1981, the most important weed was again M. vaginalis, followed by
Sagittaria pygrnaea, S. trifolia, P distinctus, Cyperus serotinus, R. indica, A. japonica,
L. procumbens, E. kuroguwai, and Ludwigia prostrata. These species constituted 87%
of all ricefield weeds; 54% are perennials. In 1990, S. trifolia had become the dominant
weed species, followed by S. pygmaea, E. crus-galli, P. distinctus, C. serotinus,
and M. vaginalis. About 60% of these species are perennials.
Similarity coefficients of change in the level of dominance of different weed
species over time were 37-39 from 1971 to 1981, but 62 from 1981 to 1991 (Table 7).
This confirms a major change in weed flora between 1971 and 1981, but indicates
little change between 1981 and 1991, except that E. kuroguwai and E. crus-galli became
the dominant species.
Eleocharis kuroguwai and S. trifolia are regarded as the most difficult weeds to
control. They require a relatively higher cumulative temperature (417 °C for E.
kuroguwai and 297 °C for S. trifolia ) and can easily escape the effects of herbicide
applied early, within 5-10 d after transplanting.
Ecological forces influencing weed competition and herbicide resistance 137