Reducing agroecosystem vulnerability to weed invasion - Invasive Plants: Ecological and Agricultural Aspects - page 174

imity of fertilizer to seed can impair crop health and thus exacerbate competi-
tion from weeds (see Strategic Fertilizer Placement below).
Optimizing crop seeding rate and plant density
The most important weed management input is crop seed. Enhancing crop
competitiveness with relatively high seeding rates and plant densities is an
effective form of biological weed control (although crop seed is not common-
ly considered to be a biological weed control agent). In our opinion, the major-
ity of crop producers would do well to increase seeding rates in all crops from
25% to 100%.
Numerous studies document the weed management advantages of higher
than recommended crop density. Mohler [45] found that of 91 cases in the lit-
erature in 29 different crops, only 6 failed to show decreasing weediness with
increasing crop density. Additional studies with similar results were also report-
ed in barley [56], canola [57] and soybeans [58]. On the other hand, manage-
ment practices that compromise seedling emergence, such as the low plant den-
sities associated with dormant fall-seeded canola, will require a higher level of
weed management [59, 60]. Higher plant densities have also been reported to
reduce weed biomass and increase seed yield in perennial stands of alfalfa
( Medicago sativa L.) [61]. For wide-row crops, increasing crop densities may
be a somewhat less efficient way of rapidly occupying space and preempting
resources from weeds as opposed to simply planting in narrower rows.
Despite overwhelming evidence, most crop producers seem reluctant to
choose higher seeding rates over simply applying more herbicides; they have
yet to be convinced that high crop density is a viable weed management strat-
egy. Studies that report the economic viability and risk-benefits of higher seed-
ing rates over the long-term would help growers make more informed crop
seeding rate decisions.
There are situations where high seeding rates will not be a beneficial man-
agement strategy. If crop seed is of poor quality and littered with weed seed,
then seeding at high rates may benefit weeds more than the crop. Cleaning
seed and/or sowing certified seed reduces such problems [9]. Increasing seed
rates in crops with relatively high seed costs may not be economically viable
in some cases [62, 63]. Malt barley quality traits (high test weight, high kernel
weight and low protein content) may also be compromised at higher than nor-
mal barley densities [64]. Nevertheless, in many cropping situations, increas-
ing crop density is one of the most effective weed management strategies.
Strategic fertilizer placement
High resource availability is a habitat characteristic that encourages plant inva-
sions [1]. Fertilizer is an obvious resource that weeds use to their advantage if