Biology Reference
In-Depth Information
'...detrimental effects of crop residue are greater for small-seeded species than
larger-seeded species. Because seeds of most major crops are one to three orders of
magnitude larger than the weeds with which they regularly compete, residue man-
agement offers important opportunities for weed suppression.' Small-seeded weed
seedlings do not have the reserves necessary to support sustained growth and to
establish themselves in the hostile (dark, dry, physical impedance) conditions that
surface crop residues create.
In no-till systems, ungerminated weed seeds left on the soil surface have higher
mortality rates (Blackshaw 2005; Roberts and Feast 1972, 1973). This effect is
likely due, in part, to vertebrate and invertebrate seed predators, which have easier
access to weed seeds left on the soil surface and are also afforded shelter from their
own predators. Weed seed predators occur in much greater numbers in no-till fields
than in conventional-tillage or organic fields relying on tillage (Menalled et al.
2000). Some insect predators and bacteria and fungi pathogens survive best in
undisturbed plant residues that are common in direct-seeding and no-till systems
(Derksen et al. 1996). Increased soil microbial diversity can directly or indirectly
increase weed seed mortality, reduce weed emergence and growth, and increase
crop competitiveness with weeds. However, it is also conceivable that greater
microbial diversity could favour weed seed survival and seedling growth. Holmes
and Froud-Williams (2005) determined that for weeds such as wild oat, lamb's-
quarters, and Canada thistle [
Cirsium arvense
(L.) Scop.], non-avian seed predators
removed more seeds than birds. Weed seeds left on the soil surface also experience
greater mortality for at least two additional reasons: physiological aging (respira-
tion-related exhaustion of reserves) and germination at soil positions and times of
year that are not suitable for seedling emergence or survival (Mohler 2001). It is
also likely that potential allelochemicals from crop residues would be more concen-
trated and inhibitory to weed seeds germinating at or near the soil surface. Thus,
the surface microenvironment of conservation-tillage systems can be rather inhos-
pitable for weed seeds and seedlings, as illustrated in the inter-row area of a barley
(
Hordeum vulgare
L.) crop seeded on canola (
Brassica napus
L.) stubble (Fig. 8.2).
Although tillage intensity influences weed community density and composition,
weed species may not consistently respond the same way to varying levels of soil
disturbance (Blackshaw 2005). Therefore, classifying weeds into functional groups
based on response to tillage intensity is difficult. Weed adaptation to specific soil
types and environments will probably have greater influence on weed communities
than tillage. Consequently, predicting trends in abundance of our important invasive
alien weeds in various agroecoregions across the Prairies in response to increasing
adoption of no-tillage cropping systems is problematic. However, 'farmers should
not be deterred from adopting zero tillage production practices because of concerns
of increased weed control problems but rather monitor their fields, and be aware of
potential changes in weed communities and how they may be effectively managed'
(Blackshaw 2005). Thus, weed monitoring is the first step to understanding weed
community changes and devising management tactics. For example, in a no-till
system, a combination of a selective in-crop herbicide with a pre-harvest applica-
tion of glyphosate reduced a relatively high Canada thistle infestation to a very low
