Final Report Prepared for the Prairie Adaptation Research Collaborative

Daniel J. Archambault, Xiaomei Li, Darren Robinson, John T. O’Donovan, Kurt K. Klein

EXECUTIVE SUMMARY

The dynamics of competition between crops and weeds are affected by environmental conditions, and have been shown to change with CO2 enrichment. Differential responses of C3 and C4 plants to elevated CO2 and temperature may cause shifts in their competitive interactions. There is a need to evaluate the effects of elevated CO2 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 CO2 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 CO2 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 CO2 levels on wild oats and green foxtail grown in competition with barley,
3. develop a CO2 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 CO2,
4. study the interactive effects of elevated CO2 and temperature on herbicide efficacy in wild oats,
5. conduct an economic analysis to provide preliminary monetary values of the effectsof elevated CO2 and temperature on weed/crop competition and herbicide efficacy.

We screened several herbicide/weed combinations and selected crops for effects of elevated CO2 using both greenhouse-based and growth-chamber based gas exposure systems. We found that responses of weeds and crops to increasing CO2 levels were species-specific. Herbicide efficacy can be negatively affected by elevated CO2 and effects were dependent on the mode of action of herbicides, on weed species and on competition. While double-ambient CO2 caused a decrease of 57% in efficacy of the herbicide Fusion applied to wild oats (C3), no effects of elevated CO2 were found when the herbicide was applied to green foxtail (C4). CO2-related 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 CO2 levels approximately 160 ppm above ambient. Based on these findings, an experiment was designed to study CO2/temperature interactions on growth of wild oats and herbicide efficacy using either ambient levels of CO2 or ambient + 160 ppm and daytime temperature of either 23,26 or 29oC. Daytime temperatures above 23oC decreased growth both in control and herbicide-treated plants. Increasing daytime temperature from 23 to 29oC caused decreased efficacy in the herbicides Fusion and Liberty but not in Assert 300. Decreases in efficacy were greatest at ambient CO2 for Fusion and greatest at ambient + 160 ppm CO2 in Liberty. While analysis of variance did not detect a significant interaction between CO2 and temperature, both elevated CO2 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 CO2-induced decreases in herbicide efficacy can be partially or totally overcome by increases in crop yields caused by elevated CO2. Nonetheless,the results also suggest that weed control will be crucial in realizing potential increases in economic yield of crops as atmospheric CO2 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 CO2 levels and herbicide efficacies. The changes in biomass caused by increased levels of CO2 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 CO2 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.

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 CO2.
2. Effects of elevated CO2 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 CO2. According to the current rate of change in atmospheric concentrations of CO2, this corresponds  to approximately 50 years from present.
4. Elevated temperature tended to decrease herbicide efficacy and the effects of temperature and CO2 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 CO2. Nonetheless, the results also suggest that weed control will be crucial in realizing potential increases in economic yield of crops as atmospheric CO2 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 CO2 and temperature on crop yields and herbicide efficacy are required to diminish the uncertainties in the economic analysis.