Weed life history: identifying vulnerabilities - Ecological Management of Agricultural Weeds - page 78

grassland weeds thrive on road margins, ditch banks, fence lines, and hedge-
rows where they are relatively free from trampling and grazing by animals
and cutting by mowing machines. From there, they disperse into fields, espe-
cially during the establishment year of leys and during periods when pastures
are rested from grazing. Consequently, preventing fruiting of these weeds in
ruderal habitats adjacent to farm fields is an important part of their manage-
ment.
Human dispersal of weeds: risks and potential reductions
Dispersal in contaminated seed
Weed contamination of crop seed has been a major source of introduc-
tions at all scales from continents to individual fields.Weed seeds are still reg-
ularly transported between countries in seed shipments (Tasrif et al ., 1991;
Huelma, Moody & Mew, 1996). Probably the only effective method for pre-
venting this is inspection of large samples from every international shipment
(Tasrif et al ., 1991). This could be facilitated by computer visualization proce-
dures that identify contaminated samples.
Improved seed-cleaning techniques and seed certification programs have
greatly reduced the spread of weeds between farms in some regions, and have
led to the near elimination of some weed species (Salisbury, 1961). However,
even in developed countries, some growers still plant contaminated seed
(Tonkin, 1982; Dewey, Thill & Foote, 1985; Dewey & Whitesides, 1990), and
this is the norm in most developing countries. For example, Rao & Moody
(1990) found an average of 3800 weed seeds per kg (17 species) after rice was
processed by farmers in the Philippines,and 660 seeds per kg (15 species) after
local commercial cleaning. Use of contaminated seed guarantees that the
worst weeds will become ubiquitous throughout all the fields of a farm, and
leads to spread between farms when seed is traded. In addition to mechanical
cleaning, the propagation of weeds with crop seed can be greatly reduced by
reserving one field or part of a field for production of next year's seed, and
weeding this area intensively. This reflects the approach used to produce cer-
tified seed (Wellington,1960),but may be more cost-effective than purchasing
certified seed for many growers in both developed and developing countries.
Dispersal with manure, feed, and transported animals
Weed seeds may be moved to previously uninfested fields by applica-
tion of manure. Mt. Pleasant & Schlather (1994) found a total of 13 grass and
35 broadleaf species in manure samples taken from 26 New York dairy farms.
On most farms,the density of weed seeds in the manure was too low to signifi-