Special Example 3 Efficacy of Herbicides under Elevated Temperature and CO

Abstract

The dynamics of competition between weed and crop plants are affected by environmental conditions and have been shown to change with CO 2 enrichment (Patterson and Flint 1980). Differential responses of C 3 plants (carbon from CO 2 initially fixed into 3-carbon compounds; e.g., Hordeum vulgare , Avena fatua , Amaranthus retroflexus ) and C 4 plants (carbon from CO 2 initially fixed into 4-carbon compound; e.g., Setaria viridis ) to elevated CO 2 and temperature may cause shifts in their competitive interactions. These changes have particular significance given that most of the world’s crop species are C 3 plants, many of the major weed species are C 4 plants, and that C 3 plants are expected to benefit more from elevated CO 2 than C 4 plants. While this might suggest that crops will gain a competitive advantage over most weeds, other factors, such as changes in herbicide efficacy, may come into play and limit this advantage and decrease potential yield increases in crops. While many studies have examined the effects of environmental change on crop and weed interactions, relatively few have included the effects of environmental change on herbicide efficacy. Environmental factors such as temperature, precipitation, wind, and relative humidity influence the efficacy of herbicides (Hatzios and Penner 1982; Muzik 1976). Elevated temperatures and metabolic activity tend to increase uptake, translocation, and efficacy of many herbicides (Patterson et al. 1999), while moisture deficit, especially when severely depressing growth, tends to decrease efficacy of postemergence herbicides, which generally perform best when plants are actively growing. High concentrations of starch in leaves, which commonly occurs in C 3 plants grown under CO 2 enrichment (Wong 1990), might interfere with herbicide activity (Patterson et al. 1999). Although the effects of elevated atmospheric CO 2 concentrations on weed‐crop‐herbicide interactions are not well known, Ziska et al. (1999) showed that elevated CO 2 levels diminished the efficacy of the widely used herbicide glyphosate. If these effects are common and widespread, they will have a significant impact on agriculture. There is a need to evaluate the effects of elevated CO 2 and temperature (including their interactive effects) on herbicide efficacy to develop strategies for the agricultural