In 2023, cotton (Gossypium hirsutum L.) and peanut (Arachis hypogaea L.) were harvested on approximately 2.86 and 0.64 million hectares in the U.S. (USDA-NASS, 2024). These two uniquely different crops share common ground, as cotton is considered to be an acceptable crop to include in rotation with peanut to reduce soil pathogens and nematodes (Johnson et al., 1998). But, the differing weed management strategies for these two crops make management implications inherently difficult when they are produced within very close proximity (Johnson III et al., 2001; Johnson et al., 2012).

The rapid adoption of herbicide-tolerant technology in crops such as cotton and soybean [Glycine max (L.) Merr.] has made neighbouring crops such as peanut, more likely to be subjected to off-target movement (physical drift and/or volatility) and/or spray tank contamination of non-registered herbicides (Lassiter et al., 2007; Leon et al., 2014; Prostko et al., 2011). In the U.S., it has been reported that 86% of the cotton hectares are planted using stacked gene cultivars (insect and herbicide-resistant) and 95% of soybean hectares are planted using herbicide-resistant cultivars (USDA-NASS, 2023).

This expansion of herbicide-resistant crops has led to an increase in the use of both glyphosate and dicamba for weed control. Glyphosate is a broad spectrum, systemic herbicide that provides postemergence (POST) control of numerous annual and perennial broadleaf and grass weeds (Shaner, 2014). Dicamba is a systemic synthetic auxin herbicide that is used for the control of annual and perennial broadleaf weeds (Shaner, 2014).

Numerous studies have evaluated peanut response to glyphosate or dicamba applied separately (Blanchett et al., 2015; Grey and Prostko, 2010; Johnson et al., 2012; Lassiter et al., 2007; Leon et al., 2014; Prostko et al., 2011). Single exposure events of glyphosate plus dicamba at rates ranging between 1/512X and 1X have also been evaluated (Daramola et al. 2023; Grichar et al., 2021). Collectively, the above research suggests that, although peanut injury symptoms from either dicamba or glyphosate are vastly different, yield response to these herbicides is similar. Generally, peanut yield loss from accidental exposure to either glyphosate or dicamba is a function of rate and peanut stage of growth.

Depending upon the herbicide label, two to four applications of dicamba are permitted on tolerant cotton and soybean cultivars including preplant, preemergence (PRE), and POST applications with POST application cut-off dates of 30-Jun. (soybean) and 30-Jul. (cotton) (Anonymous, 2022a; 2022b; 2023). In the dicamba-tolerant cotton and soybean production systems, dicamba is typically applied in a tank-mixture with glyphosate to increase the spectrum of weed control (Cahoon et al., 2015; Striegel et al., 2021). Thus, ample opportunities exist for unintentional multiple off-target exposure events of glyphosate plus dicamba to occur on near-by planted peanut. Since 2017, off-target events of glyphosate plus dicamba have been observed in commercial peanut fields across the peanut belt. However, no research has described the potential negative effects of multiple exposure events of low rates of glyphosate plus dicamba on peanut.

Therefore, the objective of this study was to determine the impact of multiple exposure events and low rates of glyphosate plus dicamba on peanut yield and grade across multiple locations in the southeast and southwest production regions of the U.S.

During 2019 and 2020, seven land-grant universities conducted field trials, for a total of 15 site-years, investigating peanut yield/grade response to low-use rates and multiple exposures of glyphosate plus dicamba (Table 1). Six treatments were arranged in a randomized complete block design with a two (timing) by three (rate) factorial with three to four replications. Timings occurred at 30 + 60 days after planting (DAP) or 30 + 60 + 90 DAP. Research has shown that off-target movement of herbicides can range somewhere between 1/10X and 1/100X (Al-Khatib and Peterson 1999; Bailey and Kapusta, 1993; Snipes et al., 1991; Snipes et al., 1992). It has also been reported that spray particle drift from ground sprayers is equivalent to 1% to 8% of the total spray volume (Maybank et al., 1978). Thus, if dicamba is applied at 560 g ae/ha (1X), 1% drift would be equivalent to a 1/100X rate. Therefore, glyphosate plus dicamba rates were 25.2 g ae/ha + 11.2 g ae/ha (1/50X) and 12.6 g ae/ha + 5.6 g ae/ha (1/100X), respectively. A non-treated control (NTC) was also included. Growth stage of peanut at 30, 60, and 90 DAP were beginning bloom (R1), beginning peg to beginning pod (R2 - R3), and full pod to full seed (R4 - R6), respectively (Boote 1982).

Peanut were planted in freshly prepared seedbeds with four-row plots for each treatment. The inner two rows of each plot were treated which left a two-row border on each side of the plot to minimize any off-target movement to adjacent plots. All herbicide treatments were applied using a CO₂-pressurized backpack sprayer to deliver 140 L/ha or 187 L/ha in TX (South). Herbicide application techniques varied by location but were consistent with local practices and implemented to minimize off-target movement.

Data collected included pod yield (all locations), market grade (AL, GA, OK, and TX), and pod malformations (GA and NC). Peanut yield data were obtained using commercial harvesting equipment with moistures adjusted to 10%. Peanut grades (total sound mature kernels) were obtained from local Federal/State Inspection Services using a 500 g pod sample collected from each harvested plot (USDA-AMS, 2019). Additionally, peanut pod malformation data were collected in GA and NC and were obtained by visually inspecting 100 random pods from each replication/treatment. Malformed pod shapes included any pods that were abnormal in size and/or had a “parrot-beak” shape (Figure 1).

Data were subjected to ANOVA using PROC GLIMMIX in SAS, version 9.4 (SAS Institute, Cary, NC). Peanut yield, grade, and pod abnormalities were set as the response variables with year and replication within year included in the model as random factors. Yield data was grouped by region and pooled across years. All P-values for tests of differences between least-square means were compared and separated using the Tukey-Kramer method (P<0.10).

Based upon these results, off-target movement of glyphosate plus dicamba at rates ≤1/100X is unlikely to result in significant peanut yield and grade losses. However, glyphosate plus dicamba drift may result in abnormal pod development which could be a major concern for peanut producers in the Virginia-Carolina region where peanut is grown primarily for the in-shell market. Off-target movement of glyphosate plus dicamba, and all other pesticides, is undesirable and multiple mitigation practices need to be implemented to minimize this potential problem.