The prevalence of herbicide-resistant weeds in U.S. corn (Zea mays), soybean (Glycine max), and cotton (Gossypium hirsutum) production systems has left farmers with a limited number of weed control options (Norsworthy et al., 2012). To combat herbicide-resistant weeds, agrochemical companies have developed soybean and cotton that are resistant to 2,4-dichlorophenoxyacetic acid (2,4-D) and dicamba (Behrens et al., 2007; Wright et al., 2010). Recent deregulation of dicamba and 2,4-D resistant crop cultivars in the United States allows farmers to use these technologies to control herbicide-resistant weeds (U.S. Department of Agriculture, 2014, 2015). Previous research has shown that dicamba and 2,4-D are effective on waterhemp (Amaranthus tuberculatus), horseweed (Conyza canadensis), palmer amaranth (Amaranthus palmeri), and giant ragweed (Ambrosia trifida), which are some of the most problematic weed species encountered in U.S. crop production systems (Craigmyle et al., 2013; Johnson et al., 2010; Kruger et al., 2010; Robinson et al., 2012; Shergill et al., 2017; Spaunhorst et al., 2014; Van Wychen, 2017). It is likely that the adoption of 2,4-D and dicamba resistant crops will result in an increased number of applications of 2,4-D and dicamba with or without glyphosate in the near future which could, in turn, result in increased off-target movement of 2,4-D or dicamba with or without glyphosate to neighboring plant species, including homeowners’ annual gardens or to wholesale greenhouses and nurseries.
The ability of 2,4-D and dicamba to move off-target and cause damage to nearby sensitive plants can be influenced by a variety of factors, such as wind speed, nozzle type, boom height, and herbicide formulation (Alves et al., 2017; Egan and Mortensen, 2012; Holterman et al., 1997; Nordby and Skuterud, 1974; Sosnoskie et al., 2015; Wang and Rautmann, 2008). Several studies have shown that higher wind speeds can result in greater off-target movement (Alves et al., 2017; Nordby and Skuterud, 1974; Wang and Rautmann, 2008; Wolf et al., 1993). For example, Alves et al. (2017) found that when wind speeds were increased from 1 to 5 m·s−1, greater downwind detection of dicamba occurred. Wolf et al. (1993) reported that 1.8% to 16.0% of the applied 2,4-D moved off-target with wind speeds of 9 to 30 km·h−1. New formulations of 2,4-D choline and dicamba set restrictions on their labels pertaining to maximum wind speeds at which these products may be applied, restricting applications to wind speeds less than 10 mph (BASF Corp., 2017; Dow AgroSciences, 2017).
Herbicides such as 2,4-D and dicamba are also susceptible to off-target movement through secondary drift, which includes volatilization. Several environmental factors such as temperature, wind speed, addition of glyphosate, or relative humidity can influence volatility (Behrens and Lueschen, 1979; Bish et al., 2019; Egan and Mortensen, 2012; Sosnoskie et al., 2015). However, two of the most important factors that influence volatility are the vapor pressure and formulation of a herbicide. Both 2,4-D and dicamba have relatively high vapor pressure and are susceptible to volatilization (Shaner, 2014).
Few studies have examined the effects of off-target movement of 2,4-D and dicamba on a variety of garden annual species. Hatterman-Valenti and Mayland (2005) applied sublethal rates of 2,4-D, dicamba, and 2,4-D + dicamba + mecoprop at 5%, 10%, and 20% of the labeled rates on various annual flowers including impatiens (Impatiens walleriana), zonal geranium (Pelargonium ×hortorum), and african marigold (Tagetes erecta) during early flowering stages. Impatiens and zonal geranium were some of the most tolerant species tested, with less than 10% and 16% visual injury, respectively, across all rates and herbicides. These authors also found that sublethal rates of dicamba caused an increase in flowering on impatiens, whereas sublethal rates of 2,4-D did not. However, some annual flower species tested were more sensitive to sublethal rates of 2,4-D and dicamba including floss flower (Ageratum houstonianum) and sweet alyssum (Lobularia maritima). Another study conducted by Hatterman-Valenti et al. (1995) found that impatiens and zonal geranium were more tolerant to sublethal rates of triclopyr and 2,4-D but that african marigold, petunia, and wax begonia were more sensitive to these herbicide treatments. Reduced flowering was also observed with increasing rates of 2,4-D and triclopyr on petunia and african marigold but not on impatiens (Hatterman-Valenti et al., 1995).
Collectively, these results show that select garden annuals are variable in their response to herbicide treatments containing 2,4-D or dicamba. However, few studies exist that have investigated the effect of 2,4-D or dicamba combined with glyphosate on garden annuals. Because 2,4-D and dicamba are selective herbicides that only control of broadleaf weeds, adding glyphosate to 2,4-D or dicamba will provide broad spectrum control of broadleaf and grass weeds. Glyphosate will be a common tank-mix component in these 2,4-D-or dicamba-resistant cropping systems. Therefore, the likelihood of drift from a combination of these herbicides will increase. Often, homeowners or commercial greenhouses and nurseries will be located near or around row crop production areas, and therefore drift from these herbicides may affect sensitive plants, including garden annuals. For example, Bradley (2017) reported that in 2017, 40 residential properties, which included garden annuals, trees, and shrubs, were injured by off-target movement of dicamba in Missouri. In addition, 11 commercial gardens were reported to have injury symptoms from dicamba drift in 2017. In June 2018, there were four reported cases of annual flowers experiencing injury from dicamba off-target movement between Arkansas and Missouri (Bradley, 2018). Because 2017 and 2018 were the first years that the dicamba-resistant crop technology was commercially available with an approved herbicide to apply over top of these crops, it is likely that at least some portion of these incidents were a result of off-target movement of dicamba from nearby dicamba-resistant cotton or soybean fields. In the future, if greater adaption of 2,4-D-resistant crops occurs in U.S. agriculture, there is also the potential for an increased number of off-target movement incidents with 2,4-D. Therefore, the objective of this research was to determine the sensitivity of common garden annuals to sublethal rates of 2,4-D and dicamba with and without glyphosate.
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