Agriculture Reference
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weed seedling emergence by consuming seed tissues directly, and by burying
seeds in “caches”at depths from which seedlings were unable to reach the soil
surface (Zhang, 1993; Hartke, Drummond & Liebman 1998). When seeds of
Echinochloa crus-galli and Brassica kaber were placed for one-week periods in
trays (73 seeds per tray) covered with wire mesh that excluded small mammals
and birds but not insects, 40% to 95% of the seeds were attacked or removed
(Zhang, 1993).Andersson (1998) reported that seeds of Bilderdykia (Polygonum)
convolvulus , Chenopodium album , Matricaria perforata , Polygonum lapathifolium ,
and Thlaspi arvense were removed at rates of 20% to 90% by unidentified organ-
isms in Swedish pastures and oat fields.
Natural populations of soil-borne fungi can reduce the germinability of
buried weed seeds. In a Nebraska field experiment with weedy genotypes of
Sorghum bicolor , Fellows & Roeth (1992) compared the germination of seeds
that were treated or not treated with fungicides (carboxin and thiram) before
burial for four to five months during late autumn and winter. In the first year
of the experiment, 17% of the treated seeds versus 0.5% of the untreated seeds
germinated after recovery from the soil.In a second year,which was drier,40%
of the treated seeds versus 17% of the untreated seeds germinated.The effects
of soil-borne organisms on seed viability during warmer months of the year
may be at least as great, but experimental studies are needed to quantify them
(Kremer, 1993).
Resident soil-borne fungi may also inflict substantial damage on popula-
tions of emerged weeds. Lindquist et al . (1995 a ) measured survival of Abutilon
theophrasti seedlings that emerged in a Minnesota soybean field at different
times during two growing seasons and found that maximal survival rates
after emergence were only 12% to 21%. They attributed much of the observed
mortality to attack of the seedlings by an unidentified Verticillium species .
Similarly,in an experiment in which A.theophrasti was grown at a fixed density
and allowed to produce seed, Hartzler (1996) noted a 20-fold difference in
seedling recruitment between two Iowa locations that differed in the degree
of Verticillium infestation. Results of a multiyear simulation model that
included temporally variable Verticillium infection indicated that attack of A.
theophrasti by the fungus increased net financial return from a maize-soybean
rotation up to 19% and decreased the number of years herbicide was necessary
to control the weed up to 35% (Lindquist et al ., 1995 b ).
Resident insects can defoliate weeds and reduce their biomass production
substantially, although such damage may be insufficient to limit crop yield
loss. In California sugar beet fields, Norris (1997) observed that a sawfly,
Schizocerella pilicornis , and a weevil, Hypurus bertrandi , often reduced leaf area
and biomass of the weed Portulaca oleracea by 70% to 80%. In competition
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