Agriculture Reference
In-Depth Information
high as 56% to 68% with hard-seeded species
such as wild buckwheat, round-leaved mallow
(
Malva pusilla
Sm.), and fi eld pennycress (
Thalpsi
arvense
L.). The general ranking of the ability of
animals to kill weed seeds through ingestion is
poultry > goats = sheep > pigs > horses = cattle
(Harmon and Keim 1934; Neto et al., 1987).
However, a small percentage of viable weed seed
will likely remain after passage through all animals
and, in many situations, would be adequate to
start new weed infestations.
Studies have shown that at least 4 days are
required to eliminate weed seeds from the diges-
tive tract of many animals (Neto et al., 1987;
Willms et al., 1995). Thus, keeping livestock in a
confi ned area for a few days before moving them
to a new fi eld may limit weed seed dispersal
through animal feces. This practice may be most
useful when feeding forage acquired from outside
the local region, thus limiting the spread of intro-
duced species on a farm. The fermentation process
of producing forage silage can markedly reduce
weed seed viability (Blackshaw and Rode 1991)
and thus is one means of reducing weed spread
through animal feeds. Composting manure before
spreading on agricultural land is another effective
means of reducing weed seed dispersal. Compost
temperatures of 55 to 60 ºC are required for
several days to kill weed seeds, and compost
piles or windrows must be turned a couple of
times to ensure that viable weed seed does not
persist on the outer edges (Grundy et al., 1998;
Eghball and Lesoing 2000; Larney and Blackshaw
2003).
Irrigation water often contains weed seed that
originates from species growing along the water
corridor or in nearby fi elds. Kelley and Bruns
(1975) documented seed of 137 species in irriga-
tion water and calculated that 10,000 to 94,000
seeds ha
−1
could be distributed during one season
of irrigation. Wilson (1980) found seed of 34 weed
species in irrigation water and determined that an
irrigated fi eld could receive as many as 48,000
seeds ha
−1
in one year. Individual farmers may
consider installing fi lters or decanting tanks in
their irrigation systems to minimize weed seed
dispersal. Alternatively, farmers within an irriga-
tion district could collectively maintain a weed-
free zone adjacent to irrigation canals that would
greatly reduce weed seed contamination of irriga-
tion water.
Controlling weeds on fi eld margins and along
fence rows and roadsides is another simple but
highly effective weed preventative practice
(Zimdahl 2007). Weed control may be accom-
plished by using herbicides or by mowing weeds
before they produce viable seed. Tarping of grain
trucks greatly reduces introduction of weeds on
roadsides that may subsequently spread to neigh-
boring fi elds.
Weed prevention is multifaceted and thus
sometimes diffi cult to implement. It begins with
awareness of how weeds spread in agricultural
systems and then progresses to individual actions
that restrict weed reproduction and spread. Wheat
farmers need to be cognizant of the fact that weed
preventative measures are among the most cost-
effective means of weed control and thus should
be given a high priority in IWM systems.
Cultural control
Diverse crop rotations are the cornerstone of all
sustainable pest management and crop produc-
tion systems (Karlen et al., 1994). Monoculture
cropping facilitates an increase in weed species
that are able to effectively compete with that crop
or that are able to overcome competition through
some avoidance mechanism (Liebman and Staver
2001). Weed species with similar life cycles to that
of the crop (crop mimics) tend to be the greatest
problem. Winter annual weeds proliferate in
winter crops, and summer annual weeds domi-
nate in spring-planted crops (Moyer et al.,
1994).
Long-term rotation studies have demonstrated
that the winter annual grass downy brome quickly
becomes the dominant weed in continuous winter
wheat production in Canada (Blackshaw et al.,
2001a). However, by simply including spring
canola (
Brassica rapa
L. or
B. napus
L.) in the
rotation, downy brome was maintained at suffi -
ciently low densities that crop yield was unaf-
fected (Table 12.3) (Blackshaw 1994b). Loeppky
and Derksen (1994) similarly reported that quack-
grass [
Elytrigia repens
(L.) Nevski] populations
