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THE EFFECTS OF ELEVATED
CO
2
AND TEMPERATURE
ON HERBICIDE EFFICACY
AND WEED/CROP
COMPETITION
Final Report Prepared for the
Prairie Adaptation Research Collaborative
Daniel J. Archambault
1
, Xiaomei Li
1
, Darren Robinson
2
,
John T. O’Donovan
3
, Kurt K. Klein
4
Change Technologies
Alberta Research Council
Vegreville, Alberta
2
Crop and Plant Management
Alberta Research Council
Vegreville, Alberta
3
Agriculture and Agri-Food Canada
Beaverlodge, Alberta
4
University of Lethbridge
Lethbridge, Alberta
June 30
th
, 2001
1
Climate
EXECUTIVE SUMMARY
The dynamics of competition between crops and weeds are affected by environmental
conditions, and have been shown to change with CO
2
enrichment. Differential responses of C
3
and C
4
plants to elevated CO
2
and temperature may cause shifts in their competitive
interactions. There is a need to evaluate the effects of elevated CO
2
and temperature on
crop/weed competition and herbicide efficacy to develop strategies for agriculture in the face of
climate change.
The objective of this study was to evaluate the effects of elevated CO
2
and temperature on the
efficacy of commonly used herbicides and on crop/weed competition. Specifically, the
objectives were to:
1. determine the effects of elevated CO
2
on the efficacy of herbicides in controlling wild
oats, Canada thistle, redroot pigweed, green foxtail, lambsquarters, kochia and
common groundsel,
2. study herbicide efficacy at ambient and elevated CO
2
levels on wild oats and green
foxtail grown in competition with barley,
3. develop a CO
2
dose response curve that will be used to establish a timeline of change
in herbicide efficacy by taking into account current rates of change in atmospheric
CO
2
,
4. study the interactive effects of elevated CO
2
and temperature on herbicide efficacy in
wild oats,
5. conduct an economic analysis to provide preliminary monetary values of the effects
of elevated CO
2
and temperature on weed/crop competition and herbicide efficacy.
We screened several herbicide/weed combinations and selected crops for effects of elevated
CO
2
using both greenhouse-based and growth-chamber based gas exposure systems. We found
that responses of weeds and crops to increasing CO
2
levels were species-specific. Herbicide
efficacy can be negatively affected by elevated CO
2
and effects were dependent on the mode of
action of herbicides, on weed species and on competition. While double-ambient CO
2
caused a
decrease of 57% in efficacy of the herbicide Fusion applied to wild oats (C
3
), no effects of
elevated CO
2
were found when the herbicide was applied to green foxtail (C
4
). CO
2
-related
Herbicide efficacy
June 30
th
, 2001
i
reduction in efficacy of Round-up Transorb applied to Canada thistle was reversed when weeds
were grown in competition with canola. Dose response experiments showed that efficacy of
certain herbicides could be adversely affected at CO
2
levels approximately 160 ppm above
ambient. Based on these findings, an experiment was designed to study CO
2
/temperature
interactions on growth of wild oats and herbicide efficacy using either ambient levels of CO
2
or
ambient + 160 ppm and daytime temperature of either 23, 26 or 29
o
C. Daytime temperatures
above 23
o
C decreased growth both in control and herbicide-treated plants. Increasing daytime
temperature from 23 to 29
o
C caused decreased efficacy in the herbicides Fusion and Liberty but
not in Assert 300. Decreases in efficacy were greatest at ambient CO
2
for Fusion and greatest at
ambient + 160 ppm CO
2
in Liberty. While analysis of variance did not detect a significant
interaction between CO
2
and temperature, both elevated CO
2
and temperature caused
decreased efficacy of the herbicide Liberty on wild oats.
The economic analysis performed using plant growth and herbicide efficacy changes suggested
that potential monetary losses due to CO
2
-induced decreases in herbicide efficacy can be
partially or totally overcome by increases in crop yields caused by elevated CO
2
. Nonetheless,
the results also suggest that weed control will be crucial in realizing potential increases in
economic yield of crops as atmospheric CO
2
concentrations increase. Since yields were not
measured directly in this study, several assumptions were made to estimate the expected
changes in yields that may occur as a result of the changes in CO
2
levels and herbicide efficacies.
The changes in biomass caused by increased levels of CO
2
were translated into expected
changes in yields using three different case scenarios. Case one assumed that yield increases
were directly proportional to the biomass increases that occurred. Case two assumed that the
increases in yields were half of the increase in biomass. Case three assumed that yields did not
increase as biomass levels increased. Further studies on the effects of elevated CO
2
and
temperature on crop yields and herbicide efficacy are required to diminish the uncertainties in
the economic analysis. If effects of climate change on crop/weed competition and herbicide
efficacy are common, they will have a significant impact on agriculture.
Herbicide efficacy
June 30
th
, 2001
ii
It was concluded that:
1) The efficacy of herbicides either decreased, increased or did not change when
herbicides were applied to weeds grown at elevated CO
2
.
2) Effects of elevated CO
2
on herbicide efficacy may change when weeds are grown in
competition with crops.
3) Herbicide efficacy changes were only found to occur at 160 ppm above ambient
levels of CO
2
. According to the current rate of change in atmospheric concentrations
of CO
2
, this corresponds to approximately 50 years from present.
4) Elevated temperature tended to decrease herbicide efficacy and the effects of
temperature and CO
2
can be additive.
5) The economic analysis performed using plant growth and herbicide efficacy changes
suggest that potential monetary losses due to decreased herbicide efficacy can be
partially or totally overcome by increases in crop yields caused by elevated CO
2
.
Nonetheless, the results also suggest that weed control will be crucial in realizing
potential increases in economic yield of crops as atmospheric CO
2
concentrations
increase.
6) In this study, most of the data used to produce the economic analysis were
extrapolations from short-term screening experiments and several assumptions
needed to be made. Further studies on the effects of elevated CO
2
and temperature
on crop yields and herbicide efficacy are required to diminish the uncertainties in the
economic analysis.
Herbicide efficacy
June 30
th
, 2001
iii
ACKNOWLEDGMENTS
This project was supported by funds from the Prairie Adaptation Research Collaborative and
the Alberta Research Council. The authors thank Karen Sorensen, Marlene Boissoneau and
Andrea Brockelbank for their assistance on this project. Thanks to Dr. Jan J. Slaski for his
valuable input.
Herbicide efficacy
June 30
th
, 2001
iv

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