Picloram (4-amino-3,5,6-trichloropicolinic acid) is an auxin-type herbicide that is a member of the pyridineocarboxylic acid family. Picloram controls plants by mimicking indoleacetic acid (IAA) in the new growth of the plant and inhibiting protein synthesis (Shaner, 2014). It was first introduced in 1963 for the control of broadleaf weed species and woody brush species (Hamaker et al., 1963). Picloram is commonly mixed with 2,4-D (2,4-dichlorophenoxyacetic acid) to control broadleaf weeds because of the increased spectrum of weed control and the ability to lower use rates of these herbicides when used together (Agabakoba and Goodin, 1970). This mixture is currently formulated as Grazon® P+D (10.2% picloram and 39.6% 2,4-D, Corteva Agriscience, Wilmington, DE) and is labeled for use in grasslands, permanent pastures, and non-crop land (Anonymous, 2019). While picloram and 2,4-D have relatively low mammalian toxicity, picloram is a restricted use pesticide because of its long persistence, high water solubility with potential to contaminate surface and groundwater, and its high phytotoxicity to broadleaf plants (Lym and Messersmith, 1988; Ketchersid et al., 1995). The soil half-life of picloram has been reported to be from 1 month to 4 yr depending on application rate, soil, and climate (Hunter and Strobbe, 1972; Shaner, 2014). However, phytotoxic levels of picloram residues can remain in the soil for up to five yr depending on soil type and dose (Lym and Messersmith, 1988). The high water solubility that allows picloram to move readily through the soil profile contaminating groundwater and surface water can lead to a contamination of irrigation water (Lym and Messersmith, 1988). The extreme sensitivity of broadleaf crops to picloram would allow for irrigation water to damage non-labeled crops. The combination of picloram plus 2,4-D is used on approximately 5% of all permanent pasture and grassland in Georgia (81,000 ha) (P. McCullough and D. Hancock, The University of Georgia, personal communication, 2019).
Peanut (Arachis hypogaea L.) is a self-pollinating, herbaceous legume, native to South America. Peanut is an extremely important agricultural crop for the southeastern United States and the state of Georgia. Georgia consistently contributes half of all peanut production in the US (USDA-NASS, 2019), with a value in 2015 of $684,000,000; which made up 31% of the total row and forage crop value for the state (Wolfe and Stubbs, 2016). In 2019, peanut was planted on approximately 263,158 ha in Georgia (USDA-NASS, 2019).
Georgia peanut growers have consistently, for approximately 20 yr, reported injury due to picloram five to ten times per yr (author's observations). Previous research has been conducted to determine picloram's potential effects on peanut. In Texas, picloram at 1 ng/g caused visual injury, however, impact on yield was not documented (Ketchersid et al., 1995). In Oklahoma, subsurface applied picloram at rates ranging from 0.56 to 1.12 kg ai/ha caused complete peanut death (Banks et al., 1977). Consequently, field trials were conducted to determine the effect of several rates and timings of picloram plus 2,4-D on peanut growth and yield in Georgia.
Materials and Methods
Field trials were conducted in 2015, 2016, and 2017 at the Ponder Research Farm located near TyTy, Georgia (31.507654 N, -83.658395 W). Soil was a Fuquay sand with 96% sand, 0% silt, 4% clay, 0.57% organic matter, and a pH of 6.6. Conventional tillage practices were used and 'GA-06G' (Branch, 2007) peanut was planted using a vacuum planter calibrated to deliver 18 peanut seed/m at a depth of 5 cm (Monosem Precision Planters, 1001 Blake St., Edwardsville, KS). Peanut was planted in 2 twin rows (90 cm by 22 cm spacing) with a plot size of 7.6 m by 0.9 m. While there were no border rows between the plots, no injury was observed on adjacent plots or on non-treated checks.
Treatments were arranged in a randomized complete block design with four (application timings) by four (picloram + 2,4-D rates) in a factorial arrangement. Application timings were preemergence (PRE), 30, 60, and 90 d after planting (DAP) and rates of picloram plus 2,4-D were 0, 0.2 + 0.7, 0.02 + 0.07, and 0.006 + 0.02 kg ai/ha, which are equivalent to the 1/10th, 1/100th, and 1/300th of the labeled use rate. It is important to note that previous research has shown that peanut exposure to 2,4-D at these low rates does negatively impact peanut growth and yield (Johnson et al., 2012; Leon et al., 2014; Merchant et al., 2014). Treatments were replicated three or four times depending on field size for each yr. Treatments were applied using a CO2-pressurized backpack sprayer calibrated to a pressure of 262 kPa to deliver 140 L/ha at 4.8 km/hr. Peanut plant height, width, and stage of growth at the time of application are presented in Table 1. Plots were maintained weed-free throughout the season using a combination of herbicides (pendimethalin, diclosulam, flumioxazin, imazapic, and 2,4-DB) and hand-weeding. Peanut yield data were obtained by mechanical harvesting at maturity.
Data collected included plant density (14 and 30 DAP), visual injury (leaf roll) approximately every 14 d throughout season, plant height (120 DAP), and yield. Plant density was determined by counting plants present per m of row. Leaf roll ratings were based on a subjective visual scale of 1-4; with 1 = none and 4 = severe. Leaf roll symptoms were considered severe when greater than 75% of peanut leaves exhibited symptomology. Data were analyzed using the PROC GLM procedure in SAS 9.4 (SAS Institute, Cary, NC) considering the factorial treatment arrangement with injury and yield as random variables and application timing and rate as the fixed variables. Data were combined over yr due to no significant yr effect in the analysis. Data were combined over rate and timing when no significant interaction was present. Means were separated using Tukey's HSD (P=0.10).
Results and Discussion
All rates of picloram plus 2,4-D did not affect peanut plants/m at the PRE-application timing (P > 0.5467). Previously it was reported that peanut plant density was not negatively affected by PRE 2,4-D applications of up to 1066 g ai/ha (Blanchett et al., 2017).
Peanut injury (leaf roll)
Data presented in Table 2 show leaf roll ratings collected 14 d after each treatment was applied for each application timing. At 14 d after application, each treatment exhibited significantly more leaf roll than the non-treated control (NTC) (Table 2). Data are also presented from leaf roll ratings at 120 DAP, which reflects the peanut plant's ability to recover throughout the season (Table 3). Data are presented sperately over rate and timing due to a significant interaction. Thus, data are presented by rate for each application timing (Table 3). At the PRE application timing, rate had no effect on peanut leaf roll and injury was minor. At the 30 DAP timing, only the 1/10th labeled rate cause significant leaf roll. At the 60 DAP timing, both the 1/10th and 1/100th rates caused significant leaf roll rate. At the 90 DAP timing, all three rates of picloram + 2,4-D caused significantly more leaf roll injury when compared to the non-treated control (0 kg/ha rate). In earlier research, picloram at rates as low as 1 ppb caused visual injury (leaf roll) symptoms (Ketchersid et al., 1995). Visual injury, such as leaf cupping and epinasty, from other auxin herbicides has been observed on peanut from dicamba at rates as low as 35 g ai/ha (Leon et al., 2014). Generally, dicamba was more injurious than 2,4-D on peanut. Only 2,4-D rates >560 g ai/ha caused significant peanut injury (Leon et al., 2014).
Peanut height and yield
There was no interaction between rate and timing, therefore data were combined over the two factors and three yr (Tables 4 and 5). At 120 DAP, the 1/10th rate and the 1/100th reduced plant height by 9 and 4%, respectively. These two rates negatively impacted peanut growth. When data were combined over rates, only the 60 DAP timing had a negative effect on plant height. This timing effect is likely due to the peanut stage of growth at the time of application. The approximate growth stages of the peanut crop were V6 (last vegetative stage), R5 (beginning seed), and R6 (full seed) at 30, 60, and 90 DAP timings, respectively (Boote, 1982). Increased injury from herbicide applications at the R5 growth stage have been reported with applications of dicamba and lactofen (Prostko et al., 2011; Dotray et al., 2012).
For peanut yield there was no significant interaction between rate and timing (Table 4). When averaged over timing, the 1/10th rate (0.18 + 0.67 kg ai/ha) yielded significantly lower than all other treatments (Table 4). Yield loss with the 1/10th X rate was 11%. Previous research indicated that peanuts exposed to picloram at 0.56 to 1.12 kg ai/ha caused complete peanut death, thus no yield data was recorded (Banks et al., 1977). Yield losses up to 29% have been reported from dicamba at rates as low as 40 g ai/ha (0.14X of normal use rate) (Prostko et al., 2011). When averaged over rates, timing had no effect on yield (Table 5). While the 60 DAP timing significantly reduced peanut plant height, it did not negatively impact yield.
Summary and Conclusions
Significant peanut yield loss was only observed for the highest rate of picloram plus 2,4-D (1/10th X rate). While peanuts appeared to be more sensitive to the 60 DAP timing, timing did not negatively impact yield. Peanut growers need to be aware of the fact that picloram is a persistent herbicide and injury can occur long after the initial application. Also, while injury symptoms may appear severe, injury does not always result in yield loss. If picloram injury occurs, peanut growers should continue to manage their peanut crop as planned with the goal of minimizing potential yield losses. Growers also need to be aware that currently no tolerance is set for picloram in peanut and should have their crop analyzed to determine if there is any residue in the harvested peanuts.
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- First and second authors: Former Graduate Research Assistant and Professor, Department of Crop & Soil Sciences, The University of Georgia, Tifton, GA 31794. [^] *Corresponding author's E-mail: email@example.com