Agriculture Reference
In-Depth Information
Dispersal in raw wool and other bulk commodities
Weed species adapted to cling to animal fur may be transported thou-
sands of kilometers in raw wool, and then dispersed with textile wastes.
Several hundred species have apparently been introduced into Britain by this
route (Dony, 1953; Salisbury, 1961), although many never became natural-
ized. Weeds probably also move in raw cotton. However, this remains to be
documented, and the problem is likely smaller than with wool since cotton
fields often have fewer weed species than sheep pastures.
The recent rapid shift of the textile industry from developed to developing
countries is probably providing new opportunities for weed introductions in
raw fiber.In addition to inspection and quarantine measures,introductions of
weeds with textile raw materials can be curtailed by heat treatment or high-
temperature composting of wastes prior to application to land.
Dispersal by machinery
Tillage machinery moves few seeds further than three meters within a
field and moves most seeds only a meter or less (Howard et al ., 1991; Rew,
Froud-Williams & Boatman, 1996; Rew & Cussans, 1997; Mayer, Albrecht &
Pfadenhauer,1998).However,the few seeds that are carried long distances can
form foci from which the weed can spread in future years. Roots, rhizomes,
and tubers of wandering perennials can catch on the shanks of tine imple-
ments,particularly once the soil is loosened by primary tillage.Although most
fragments do not move far,the few that do can spread an infestation over large
areas in a single tillage operation (Schippers et al ., 1993).
Few studies have quantitatively examined potential between-field move-
ment of seeds in soil on tires and machinery. Schmidt (1989) observed over
3900 seedlings of 124 species emerging in the soil scraped from an automo-
bile that had been used for field research in Germany. Mayer, Albrecht &
Pfadenhauer (1998) found that seeds were moved between fields on tractor
tires and a rotary tiller,but not by a plow,rotary harrow,or heavy cultivator.In
their experiment, the equipment passed through 25 m of clean soil after
encounter with the seeds.
Some idea of the potential for between-field dispersal in soil clinging to
tillage machinery can be gathered from the density of seeds in soil. Many
species commonly achieve densities of a few thousand seeds per m 2 (Jensen,
1969; Ball & Miller,1989).Assuming a plowing depth of 20 cm,1000 seeds per
m 2 is about 1 seed per 200 g of soil. This represents a fairly small risk, except
that (i) conservation tillage keeps seeds close to the surface where they are
more likely to be picked up, (ii) field edges and headlands are typically plowed
and cultivated last, and they are usually weedier than the rest of the field
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