Agriculture Reference
In-Depth Information
is necessary since relying on herbicides as the sole strategy can simply shift
the balance in favour of invasions from alternative weed species or biotypes
with variable genetic and phenotypic traits. Here we discuss management tech-
niques that may be used to reduce agroecosystem habitat vulnerability to the
invasion and establishment of weeds in cultivated crops.
Healthy crops limit weed invasion
Most agricultural weeds have rapid growth rates [7-9] and a high relative sus-
ceptibility to the negative effects of shade [10, 11]. They have adapted over
long periods of time to the repeated cultivation of annual crops [12]. Cropping
systems impose major disruptions on natural ecosystems [13], and maintain
plant communities at very early stages of succession [14]. Crop monocultures
are commonly grown over hundreds or thousands of hectares under resource-
rich cultural conditions [15]. Indeed, annual monoculture cropping favours all
three habitat characteristics that encourage plant invasions: disturbances, low
species richness and high resource availability [1]. In addition, environments
with low climatic and herbivore resistance to specific weeds also favour the
proliferation of those species [16].
Viable weed propagules can invade and establish themselves when they
occupy a “safe site” [17]. In annual monoculture cropping, tillage and/or her-
bicides are commonly employed to “clear” the ground in order to successful-
ly establish the desired crop. This major disturbance before seeding helps pro-
vide the safe site that weeds require to establish and thrive in agroecosystems
[9, 11, 18]. After soil disturbance, given their superior colonizing abilities,
weeds can rapidly preempt growing space [16, 19] and avail themselves of
resources intended for crop growth. Therefore, when, how, and to what degree
cropping areas are disturbed can determine whether crop or weed establish-
ment is favoured.
The health status of a crop influences its susceptibility to weed invasion.
Crop health implies that crop roots and shoots are robust and will compete
with weed species to the full extent of their genetic potential. In situations
where crop health is compromised due to environmental limitations such as
temperature extremes (frost and heat stress), restricted growing-degree days,
limited available water and light [20], weed safe sites will increase. Given the
diverse community of weeds present in most agricultural soil seed banks, there
is probably at least one weed species that will be favoured by a particular envi-
ronmental “limitation” more than the crop. Plant diseases and insect pests may
also reduce crop health and limit a crops ability to deter weed invasion. It may
also be detrimental to crop health, as well as biologically inefficient, to man-
age fields for yield goals significantly higher or lower than the attainable yield
in a given agroecosystem [21]; in such fields, lower crop health and vigour
exposes niches for weeds to exploit. Lastly, crop health is dependent upon soil
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