Impacts on Weeds - Climate Resilient Agriculture for Ensuring Food Security - page 196

2003 ). Additional research on how rising CO 2
can affect both poison ivy growth and toxicity is
currently ongoing. No information is available on
how CO 2 could alter the toxicity of secondary
compounds associated with mortality in weedy
species.

tering of insect populations and changes in their
potential range. Although this could increase
both the biological control of some weeds, it
could also increase the incidence of specifi c crop
pests, with subsequent indirect effects on crop-
weed competition. Overall, synchrony between
development and reproduction of biocontrol
agents and their selected targets is unlikely to be
maintained in periods of rapid climatic change or
climatic extremes. Whether this will result in a
positive or negative benefi t remains unclear.
A standard means of controlling weed popula-
tions and the one most widely used in developing
countries is mechanical removal. Tillage (by ani-
mal or mechanical means) is regarded as a global
method of weed control in agronomic systems.
Elevated CO 2 could lead to further belowground
carbon storage with subsequent increases in the
growth of roots or rhizomes, particularly in
perennial weeds. Consequently, mechanical till-
age may lead to additional plant propagation in a
higher CO 2 environment, with increased asexual
reproduction from belowground structures and
negative effects on weed control (e.g., Canada
thistle) (Ziska et al. 2004 ).
Overall, there are strong empirical reasons for
expecting climate and/or rising CO 2 to alter weed
management. Adaptation strategies are available,
but the cost of implementing such strategies (e.g.,
new herbicides, higher chemical concentrations,
new biocontrol agents) is unclear. Herbicide use
is controlled by individual state regulations. If an
increase in CO 2 and temperatures allow invasive
weed species to expand their geographical loca-
tions, new herbicides may be needed to combat
them. Often it takes a period of time to receive
state approval of a new chemical or a chemical
that has not been previously used.

10.10 Weed Management

Clearly, any direct or indirect impacts from a
changing climate will have a signifi cant effect on
chemical weed management. Changes in temper-
ature, wind speed, soil moisture, and atmospheric
humidity can infl uence the effectiveness of appli-
cations. For example, drought can result in thicker
cuticle development or increased leaf pubescence,
with subsequent reductions in herbicide entry into
the leaf. These same variables can also interfere
with crop growth and recovery following herbi-
cide application. Overall, herbicides are most
effective when applied to plants that are rapidly
growing and metabolizing, i.e., those free from
environmental stress. But does rising CO 2 per se
alter chemical management? There are an increas-
ing number of studies (Ziska et al. 1999 , 2004 ;
Ziska and Teasdale 2000 ) that demonstrate a
decline in chemical effi cacy with rising CO 2 . The
basis for this reduction is unclear. Recent work
with Canada thistle grown in monoculture under
fi eld conditions suggested a greater root-to-shoot
ratio and subsequent dilution effect of glyphosate
when grown at elevated CO 2 (Ziska et al. 2004 ).
However, it is not clear if this is a ubiquitous
response. In any case, if CO 2 does reduce effi cacy,
then additional work is needed to determine her-
bicide specifi city, concentration, and application
rates as possible means of adaptation.
Biological control of pests by natural or
manipulated means is likely to be affected by
increasing atmospheric CO 2 and climatic change.
Climate as well as CO 2 could alter the effi cacy of
weed biocontrol agents by potentially altering the
development, morphology, and reproduction of
the target pest. Direct effects of CO 2 would also
be related to changes in the ratio of C/N and alter-
ations in the feeding habits and growth rate of
herbivores. As pointed out by Patterson ( 1995 ),
warming could also result in increased overwin-

10.11 Mitigation

10.11.1 Selective Allelopathy

and Self-Supporting Weed
Management

Studies involving allelopathic crop residue
mulches (maize, sorghum, wheat, barley, or rye)