Reducing agroecosystem vulnerability to weed invasion - Invasive Plants: Ecological and Agricultural Aspects - page 175

it is not carefully managed [56, 57]. Without strategic fertilizer placement,
weeds may use fertilizers such as nitrogen more effectively than the crop
[65-70]. Banding fertilizer near the crop row [71-75] or banding fertilizer
deeply to reduce access to it by shallow-rooted weed seedlings [76] favours
crop growth over weed growth.
In limited quantities fertilizer can be placed directly with crop seed.
However, too much fertilizer placed with crop seed can injure the crop and
compromise weed management. When 90 kg ha -1 of nitrogen was placed in
the seed row compared to banding near the seed row, barley canopy cover in
mid June decreased from 78% to 22%, and wild oat biomass before barley har-
vest in the fall increased from 192 to 967 kg ha -1 (dry weight) [unpublished
observations]. The combination of the relatively open plant canopy and the
excess fertilizer underutilized by the damaged crop allowed the exploitation of
light, space, and fertilizer resources by the wild oat plants. Thus while strate-
gic placement of fertilizer can often benefit the crop over weeds, fertilizer
placed too close to the seed can have the opposite effect resulting in crop injury
and enhanced competition from weeds.
Diverse crop rotations
For centuries, crop producers have employed crop rotations to improve pro-
ductivity and to manage agricultural pests. Diverse rotations employ crops
with different planting and harvest dates, growth habits, residue characteris-
tics, tillage and weed management practices [77]. Derksen et al. [78] found
that weed densities were minimized when diverse cropping systems were used
to constantly change the selection pressure on weed communities. Diverse
rotations have weed management benefits because different crop species and
their associated management practices present different challenges for weeds;
this diversity prevents unrestricted growth and reproduction for any given
weed species. Nevertheless, spatial and temporal plant species diversity in cur-
rent agricultural ecosystems has dropped precipitously in recent years [77].
Most of this reduction in diversity relates to economies of scale, specialization
and the replacement of purchased inputs for labour and management time.
Diverse seeding dates may be the most important rotational factor influenc-
ing weed population composition [13, 79-81]. In addition, crop producers can
use operational diversity by seeding crops at unconventional dates. For exam-
ple, studies conducted in western Canada have shown that spring canola can
be dormant-seeded in the fall prior to soil freezing [81, 82], or seeded much
earlier than normal in the spring [60]. This seeding date variability as well as
earlier than normal harvest date adds diversity that can have beneficial impacts
on weeds in terms of their restricted success in emergence, crop competition
and seed rain. Diverse crop rotations impose resource and management prac-
tice diversity that is difficult to completely document, but that restricts the
invasive ability of weeds.