Agriculture Reference
In-Depth Information
dependent on many factors (Gallandt
et al
.,
1998), including species (Cook, 1980; Louda,
1989), environment (Burnside
et al
., 1977;
Donald and Zimdahl, 1987), and burial depth
(Wilson, 1988; Thompson
et al
., 1998;
Benech-Arnold
et al
., 2000). Seedbank decline
occurs due to a host of factors including germin-
ation, predation, decay, and seed death. However,
most of these processes are not well understood.
The exception is germination processes, which
are the mechanistic factors of germination usu-
ally available for important species.
Growers should have an underlying under-
standing of why cultivating conditions for weed
seed germination, for example, is an important
weed seedbank management strategy. Identify-
ing and utilizing inputs that result in seedling
death prior to planting potato, or other rota-
tional crops, reduces weed pressure within the
crop (Jordan
et al
., 1995). Interestingly, seed per-
sistence in the soil does not correlate well with
the relative abundance of the species above
ground (Roberts and Ricketts, 1979). Weeds are
opportunistic, and even a very small number
of seeds in the soil can produce a troublesome
infestation.
An effective teaching tool is a simulation
illustrating how a single failure or selection of
a herbicide-resistant biotype can profoundly
increase weed seed in the seedbank over a single
season. These models also illustrate why man-
aging the weed seedbank can be an effective
strategy for the long-term sustainability of indi-
vidual farms.
et al
., 1998). Inputs that reduce or eliminate safe
sites, or deplete them of seed, are effective tools
to minimize weed establishment.
Weed competition
Once germinated and established, weeds quickly
begin to compete with potato plants. Weed-
potato competition ultimately determines the
yield loss of potatoes, as well as the total weed
biomass and seed production. Weeds compete
with potato for light, nutrients, water, and space,
and can reduce potato yield (VanGessel and Ren-
ner, 1990; Boydston, 2010). Competition with
weeds also affects tuber size and weight (Nelson
and Thoreson, 1981; Wall and Friesen, 1990a),
and can change the chemical composition of
potatoes. Potato yield loss occurs at harvest,
caused by the reduction in efficiency of harvest
and increased likelihood of mechanical injury.
Weeds cause physical injury by rhizome growth
into the potato tuber, as is often the case with
yellow nutsedge and quackgrass. Finally, weeds
serve as hosts for numerous pests and pathogens
(Ogg and Rogers, 1989; Eberlein
et al
., 1991;
WRIPM, 1992).
Potato is a relatively competitive crop, and
thus less susceptible to yield loss from weeds
compared to relatively non-competitive crops.
Weeds that emerge with the crop and survive
mechanical control inputs can cause significant
yield loss and ultimately contribute seed to the
weed seedbank. For example, yield loss in Russet
Burbank due to competition with weeds that
survived cultivation and hilling ranged from
7
to
43% when compared to weed-free conditions
(Liebman
et al
., 1996). Yield loss can vary by
weed species, although direct comparisons of
loss caused by individual species can be difficult.
A simple method of comparison is the
Cousens rectangular hyperbola,
Minimization of seedling
establishment
A small proportion (less than 20%) of the germi-
nable seedbank actually produces seedlings in a
given year (Roberts and Ricketts, 1979; Forcella,
1992; Gallandt
et al
., 1998). Successful estab-
lishment requires that seeds are positioned in a
site allowing them to avoid hazards and to ex-
perience conditions that promote germination,
emergence, and growth (Harper, 1977). So-
called “safe sites” are a function of seed depth,
light, alternating temperatures, water availabil-
ity, crop residues, and freedom from predators
and diseases (Gallandt
et al
., 1998). The safe site
must function well past germination (Gallandt
[
Y
L
= (
ID
)/[
1
+ (
ID
/
A
)],
where the relationship between weed density per
meter of row and percent potato yield loss can be
related (Cousens, 1988). Yield loss (
Y
L
) is based
on percent reduction of weed-free yield.
A
is the
asymptote for yield loss and, where appropriate,
is constrained to 100% (Streibig
et al
., 1989).
D
is the density per meter of crop row and
I
rep-
resents the yield loss per weed as weed density
