Weed life history: identifying vulnerabilities - Ecological Management of Agricultural Weeds - page 81

(Marshall, 1989; Wilson & Aebischer, 1995), and (iii) tractor tires can pick up
surface-lying seeds from along the field border as they leave. Consequently,
seed movement in soil is probably the source of many new weed infestations,
particularly of nearby fields. Nevertheless, the risk will usually be small until
the weed becomes dense in the potential source field. Movement of vegetative
propagules with soil on farm machinery is probably the major method of
spread for some species that do not produce viable seeds (e.g., Panicum repens -
Wilcut et al ., 1988).
Several studies on movement of weed seeds by combine harvesters indicate
that most seeds are deposited within 10 m of the source but that some are dis-
persed as far as 50 m or more in the direction of travel (Ballaré et al ., 1987 a ;
McCanny & Cavers, 1988; Howard et al ., 1991; Ghersa et al ., 1993; Rew, Froud-
Williams & Boatman, 1996). Thus, combines can rapidly spread weeds
throughout a field (Ballaré et al ., 1987 b ), with significant potential effects on
crop yield (Maxwell & Ghersa, 1992).
The spread of weeds between fields by combines is probably also frequent,
and prudence indicates that a combine should be cleaned before it is moved
into a new field.McCanny & Cavers (1988) found that more seeds lodged in the
central divider assembly of the maize header than elsewhere on the combine,
and that this could be effectively cleaned by vacuuming.Data on the spread of
weeds between fields by combines are badly needed. An interesting study
could be made by cleaning trapped seeds out of combines each night as a
custom combining operation works its way north through the midwestern
USA during grain harvest. Comparison of the weed species removed from the
combine after harvesting a field with the flora of the next field on the schedule
would indicate the likelihood of long-distance spread of species by this route.
Dispersal in irrigation water
Seeds of most weed species can survive several months of immersion
in fresh water (Comes, Bruns & Kelley, 1978), and most will float, particularly
if chaff is retained on the seed, or if pieces of inflorescence fall onto the water.
Consequently, many species of weed seeds disperse in irrigation water.Wilson
(1980) found 77 species of weed seeds in samples of irrigation water in
Nebraska, and Kelley & Bruns (1975) observed 77, 84, and 137 species in
samples taken in eastern Washington in three years. In both studies, the
density of seeds deposited was not sufficiently high to warrant concern, but
the potential for introduction of new weed species to fields by irrigation water
was substantial. Consequently, Kelley & Bruns (1975) recommended that irri-
gation water be screened to remove seeds. Both studies found that seed
density in water increased as water traveled down canals with weedy banks.In