A manual of rice seed health testing - IRRI books - International Rice ...

CHAPTER 12
Weed seed contaminants
K. Moody and R. Lubigan
Most weed species have been, and
still are, inadvertently disseminated
between and within continents and
countries as contaminants in traded
seed (Horne 1953, Delouche 1988).
Weeds of Philippine origin have
been spread widely through export
of high-yielding rice varieties.
Weeds are also being disseminated
in the continuing rice trade within
South America; between South and
North America; among Africa, North
America, and Asia; and among most
other combinations of trading partners
(Delouche 1988). In California,
with the exception of Echinochloa
crus-galli (L.) P. Beauv. var crus-galli,
all the introduced species of rice
weeds have been brought into the
state in rice seed (Fuller and Barbe
1983).
Rice seed contaminated with
weed seeds may introduce new species
to a field or add to an existing
weed population and is a source of
weed perpetuation in crop fields. In
the southern USA, many weeds are
spread almost entirely in rice seed
(Smith et al 1977).
If there are 1,000 weed seeds per
kilogram of rice seed, 100,000 weed
seeds will be planted per hectare
(10/m 2 ) if the seeding rate is 100 kg/
ha. Planting contaminated seed will
be costly to a weed management
program for many years (Rao and
Moody 1990).
In marketable rice grain, mixtures
of weed seeds detract from its appearance
and lower its grade and
price. In Thailand, the price of rice
drops by about one-half when the
grain is contaminated with weed
seeds (Suwunnamek 1986).
Standardized procedures are
available for the types of inspection
and testing needed to prevent weed
seed contamination of crop seed. The
12.1 Seeds of
Synedrella nodiflora
(courtesy of
K. Moody and
R. Lubigan).
12.2 Seeds of
Cyperus compactus.
12.3 Seeds of
Cyperus iria
(courtesy of
K. Moody and
R. Lubigan).
12.4 Seeds of
Fimbristylis miliacea
(courtesy of
K. Moody and
R. Lubigan).
12.5 Seeds of
Scirpus juncoides
(courtesy of
K. Moody and
R. Lubigan).
12.6 Seeds of
Digitaria ciliaris
(courtesy of
K. Moody and
R. Lubigan).