Triclopyr 2-[(3,5,6-trichloro-2-pyridinyl)oxy] acetic acid, is an auxin-type herbicide that is a member of the pyridine carboxylic acid family (Johnson et al.,1995; Shaner, 2014). Triclopyr is commercially available in several formulations including a triethylamine salt (TEA), butoxy ethyl ester, pridinyloxyacetic acid, and choline salt and difference in plant response to these formulations have been observed (Dias et al., 2017).

Triclopyr-TEA was first introduced in 1979 for the control of woody plants and broadleaf weeds for non-crop areas and forestry applications (Douglass et al., 2016; US-EPA, 1998). Triclopyr is readily absorbed through foliage and transported through phloem tissues to growing points where it accumulates, but low percentages of triclopyr can accumulate in the roots of certain species, such as horsenettle (Solanum carolinense L.) and honey mesquite (Prosopis juliflora var. glanulosa Torr.) (Bovey and Mayeux, 1980; Shaner, 2014). Triclopyr and other auxin-type herbicides control plants by inhibiting protein synthesis in meristematic regions of plants, along with mimicking indoleacetic acid (IAA) stimulating ethylene evolution which can lead to characteristic epinasty symptoms (twisting and bending of petioles, leaves, and stems) associated with this class of chemistry (Mithila et al., 2011; Shaner, 2014).

Triclopyr is used for both residual activity and postemergence (POST) control of woody plants and broadleaf weeds across a multitude of landscapes including but not limited to forests, rice, home lawns, pastures, rangelands, and rights-of-ways, contributing to a wide window and range of application timings (Sperry et al., 2020; US-EPA, 1998). The average half-life for triclopyr is 10 to 46 d which is influenced by soil type and environmental conditions (Shaner, 2014). The primary breakdown of triclopyr is through microbial degradation in the soil where it is to some extent persistent and mobile; however, photodegradation occurs within two-six hours in water (Shaner, 2014; US-EPA, 1998). Other herbicides in this chemical family such as picloram can persist at phytotoxic levels in certain soils for up to five years depending on the application rate and soil type (Lym and Messersmith, 1988).

To promote successful pine (Pinus spp.) production and reduce or eliminate herbaceous or woody competition, herbicides such as triclopyr-TEA release pines by effectively controlling various types of herbaceous broadleaf weeds and other tree species (Pusino et al.,1994). Forestry herbicides such as triclopyr have a role in helping Georgia’s pine tree production, as forestry is a leading agricultural commodity in Georgia with an economic impact of $44 billion dollars (Georgia Forestry Commission, 2024). Forestry herbicides have effectively changed the way pine silviculture is performed, and Georgia is a direct beneficiary of that as the nation’s leading producer of pine timber volume harvested annually which accounts for 4.49 million ha in production (Bullock, 2011; Georgia Forestry Commission, 2024; Lauer and Quicke, 2022; Minogue, 2021; USDA-CropScape, 2023).

In 2024, Georgia growers harvested 342,105 ha of peanuts (Arachis hypogaea L.) accounting for 50% of the nation’s crop (USDA-NASS, 2025). One of the top peanut producing counties in southwest Georgia (Decatur) produces ~17,140 ha of peanut and maintains ~64,200 ha of pine forest resulting in a ratio of 3.75 ha of pine forest to peanut (USDA-CropScape, 2023). This pine/peanut relationship is typical in many counties in Georgia which increases the risk for off-target movement events from pine forests to peanut fields.

While peanut herbicides are applied by mostly ground application methods, forestry herbicides are applied using ground application methods or aerially via the use of helicopters, which leads to greater risks of off-target movement of herbicides (Bullock, 2011). During a typical planting and growing season, there is a wide range of times when peanut fields could be subjected to off-target movement of triclopyr due to the surrounding pine stand’s age. Therefore, a peanut field prior to planting or up to harvest is at risk of off-target movement due to the variability of timing and aerial application method of triclopyr on neighboring pine forests.

Previous research has been conducted on peanut to determine the effects of other auxin-like herbicides including 2,4-D, dicamba, and picloram (Blanchett et al., 2017; Carter and Prostko, 2020; Leon et al., 2014; Prostko et al., 2011). However, no information has been published about the potential effects of triclopyr on peanut. The effects of triclopyr on rhizoma perennial peanut (RPP) (Arachis glabrata Benth.) has been documented in previous research (Martin et al., 2021; Valencia et al., 1999). Results from these studies suggest that the tolerance of RPP to triclopyr is a function of rate and cultivar.

The objective of this research was to determine the effects of triclopyr applied PRE or POST on peanut growth and yield.

Field trials were conducted at the Ponder Research Farm in Ty Ty, Georgia (31.507654̊ N, 83.658395̊ W) from 2020-2022 (three site-years) to determine the effects of direct triclopyr applications to peanut. Soil type was a Tifton sand with 94% sand, 0% silt, 6% clay, 0.91% organic matter, and a pH of 6.0. Conventional tillage practices were used and peanut (cv. Georgia-06G) (Branch, 2007) 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). Peanuts were planted in twin rows spaced 23 cm apart on a 91 cm center. Plots were 1.8 m (two sets of twin rows) wide and 7.6 m in length.

Treatments were arranged in a randomized complete block design with a three (timing) by five (rate) factorial arrangement with three to four replications. Triclopyr-TEA (Garlon^® 3A, Corteva Agriscience, Indianapolis, IN) timings were PRE 1 day after planting (DAP), 30 DAP, and 60 DAP with herbicide rates of 0, 8.4 (1/100X), 84 (1/10X), 168 (1/5X), and 840 g ae/ha (1X, manufacturers suggested use rate). Herbicide treatments were applied using a CO₂-pressurized backpack sprayer calibrated to deliver 140 L/ha at 5.3 km/hr. Peanut growth stages at 30 DAP or 60 DAP were R1 (beginning bloom) and R3-R4 (beginning pod to full pod), respectively (Boote, 1982). Plots were maintained weed-free throughout the season using a herbicide program recommended by the University of Georgia Extension and hand-weeding (Prostko, 2024). Supplemental overhead irrigation was applied as needed to maintain optimum peanut yields (Porter, 2022). Production, and pest management practices other than specific treatments were held constant over the entire experiment to optimize peanut growth and development (Monfort, 2022).

Data collected included peanut density (stand) at 17 to 30 DAP, visual subjective estimates of peanut injury (stunting and epinasty), plant height/width, and yield. Peanut density was obtained by counting the number of emerged plants per 1-row m. Visual estimates of peanut injury were obtained at 65 DAP using a scale of 0 to 100 (0=no injury; 100=plant death). Plant height and width data were collected at 110 DAP by measuring five plants/plot at random. Plant height measurements were recorded from the soil line to the top of the highest point of the main terminal, and plant width measurements were recorded from lateral branch to lateral branch within a set of twin-rows. Peanut yield data were obtained using commercial harvesting equipment. Yields were adjusted to 10% moisture. A complete summary of planting dates, harvesting dates, and rainfall totals can be found in Table 1.

Data were subjected to ANOVA using PROC GLIMMIX in SAS, version 9.4 (SAS Institute, Cary, NC). Peanut density, injury (stunting, epinasty), plant height and width, and yield were set as the response variables with year and replication within year included in the model as random factors. All data were pooled over years. All P-values for tests of differences between least-square means were compared and separated using the Tukey-Kramer method (P<0.05).

Peanut density (stand) was only reduced by 840 g ae/ha of triclopyr applied PRE resulting in a 52% reduction in stand and ultimately a 21% reduction in yield. Assuming normal temperature, rainfall patterns, and supplemental irrigation programs, this research suggests peanut could be planted following triclopyr at 840 g ae/ha after approximately three field half-lives (~138 d) have occurred in coarse-textured soils. Triclopyr applied POST at 30 DAP with rates ≤ 168 g ae/ha (1/5X) did not influence canopy growth or yield. Peanut stunting only reached 16% with 168 g ae/ha. Triclopyr applied POST at 60 DAP with rates ≥ 168 g ae/ha (1/5X) resulted in significant plant height reductions (17% to 46%) and yield losses (24% to 62%). Thus, it is unlikely that lower off-target or drift rates will cause peanut yield losses. Previous research reported that spray particle drift from ground applications was one to eight percent and 20 to 35% from aerial applications depending upon nozzle type and wind speed (Maybank et al., 1978). This would equate to a range of 1/100X to 1/3X rates of triclopyr. It has also been reported that vapor drift rates are equivalent to 1/1000X rate (Egan and Mortensen 2012). In peanut fields without supplemental irrigation or with inadequate fertility, greater yield losses could be observed than reported herein. It is also possible that peanut could respond differently to the other formulations of triclopyr (Dias et al., 2017) warranting further investigation.