Georgia is the nation’s top producer of pine tree (Pinus spp.) timber volume harvested annually and has ~4.49 million ha of pine production which are separated between natural and planted stands (Georgia Forestry Commission, 2024). Herbicides play a crucial role in pine tree timber production as they have effectively changed the way pine silviculture is performed (Bullock, 2011; Lauer and Quicke, 2022; Minogue, 2021). To promote successful pine production and reduce or eliminate herbaceous or woody competition, herbicides such as imazapyr are used to release pines by controlling various types of grass, herbaceous broadleaf weeds, and other trees (Anonymous, 2011).

Imazapyr is an acetolactate synthase (ALS) inhibiting herbicide that is a member of the imidazolinone family (Shaner 2014). Imazapyr’s herbicidal activity is broad-spectrum, offering control of weeds from a wide window of application timings from residual activity to postemergence (POST) applications (Douglass et al., 2016; Gianelli et al., 2014). The average half-life of imazapyr is 25 to 142 d which can be influenced by soil type and environmental conditions with weed control that can last from three months up to two years depending on the rate of application (Shaner, 2014). Due to imazapyr’s high water solubility, long persistence and weak absorption in some soils, the potential for leachability to sensitive areas is a concern (Gianelli et al., 2014). However, imazapyr generally remains within the top 50 cm of soil (Shaner, 2014). The herbicide is primarily degraded by soil microbial organisms under aerobic conditions with photodegradation being another potential pathway for breakdown (Shaner and O’Connor, 1991).

With the wide spectrum of weed control, efficacy at relatively low doses, and ability to persist in the soil profile, imidazolinone herbicide carryover to rotational crops can be of concern (Loux et al., 1989; Loux and Reese, 1993; Shaw and Wixson, 1991). The persistence of these herbicides is affected by many factors including soil moisture, soil pH, organic matter, and soil type (Loux et al., 1989; Loux and Reese, 1993; Shaw and Wixson, 1991). Yields of certain crops including field corn (Zea mays L.), grain sorghum [Sorghum bicolor (L.) Moench], cotton (Gossypium hirsutum L.) and cucumber (Cucumis sativus L.) have been reduced when these crops were planted the year following application of imidazolinone herbicides (Alister and Kogan, 2004; Goetz et al., 1990; Loux and Reese, 1993; York et al., 2000).

Similar to pine tree production, Georgia is also the nation's top producer of peanuts harvesting 637,247 ha and accounting for approximately 53% of the nation’s crop in 2023 (USDA-NASS, 2024). Peanut production in Georgia has a farm gate value of $791 million with production concentrated in the Coastal Plain region (Anonymous, 2024). With both pine trees and peanuts common to the region, the proximity in production is often very close. For example, Mitchell County (133,000 ha) is one of the top peanut-producing counties in Georgia with 18,031 ha while also maintaining 34,668 ha of pine forest (USDA-CropScape, 2023). This gives a ratio of 1.92 ha of pine trees to peanut further demonstrating the close proximity to which pine trees and peanut could be grown or managed within Georgia.

With both pine tree and peanut production relying on the use of herbicides to manage weeds in conjunction with their close proximity in the Coastal Plain region, the potential for crop damage to occur from herbicide drift is enhanced. In fact, University of Georgia Extension has documented this event occurring with imazapyr applied to pine trees moving off-target to peanut fields. Forestry herbicides can be applied using ground application methods or aerially via the use of helicopters (Bullock, 2011). Depending on the pines' age class, imazapyr's herbicide application to pine trees could occur before peanut planting or during the growing season. Due to the variability of application and timings of this forestry herbicide, implications of off-target herbicide drift into surrounding crop fields pose a significant risk.

A paucity of information about the response of peanut to imazapyr exists. Therefore, the objective of this research was to determine the effects of imazapyr, applied preemergence (PRE) or POST, on peanut growth and yield.

Field trials were conducted from 2020 through 2022 at the Ponder Research Farm in Ty Ty, Georgia (31.507654̊ N, -83.658395̊ W) to determine the effects of direct imazapyr applications to peanut (three site-years). Soil 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 (application timings) X five (herbicide rates) factorial arrangement having three to four replications. Imazapyr (Arsenal® Powerline™ 2SL, BASF Corp, Research Triangle Park, NC) application timings consisted of PRE 1 day after planting (DAP), 30 DAP, and 60 DAP with herbicide rates consisting of 0, 4.2 (1/100X), 42 (1/10X), 84 (1/5X), and 420 g ai/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 stage at 30 and 60 DAP was beginning bloom (R1) and beginning pod to full pod (R3 – R4), respectively (Boote, 1982). Plots were maintained weed-free throughout the season using an herbicide program recommended by the University of Georgia Extension (Prostko et al., 2022) and hand-weeding. 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 estimates of peanut injury (stunting and chlorosis), plant height and width, and pod yield. Peanut density was obtained by counting the number of emerged plants/1-row m. Visual estimates of crop injury (stunting and chlorosis) were obtained 65 DAP using a scale of 0 to 100 (0=no injury; 100=plant death). Plant height (cm) and width (cm) data were collected at 110 DAP by measuring 5 plants/plot. Heights were measured from individual plants from the soil line to the top of the terminal and plant width measurements were recorded from measurements of the lateral branches from the twin-row. Peanut yield data were obtained using commercial harvesting equipment. Yields were adjusted to 10% moisture. A complete summary of planting/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 and chlorosis), plant height/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 was not affected by any rate of imazapyr when applied PRE or POST. However, rates greater than 4.2 g ai/ha applied PRE reduced peanut yields at least 30%. Assuming normal temperature, rainfall patterns, and supplemental irrigation programs, these data suggest that peanut could be planted following imazapyr after approximately three field half-lives, or ~ 426 d have occurred in coarse-textured soils. Peanut subjected to imazapyr applied POST at rates ≤ 84 g ai/ha (1/5X) during the growing season did not result in significant yield losses. Thus, it is unlikely that off-target or drift rates will cause peanut yield losses. Maybank et al. (1978) reported that particle drift from ground applications was 1% to 8% and 20% to 35% from aerial applications depending upon nozzle type and wind speed. This would equate to a range of 1/100X to 1/3X rates of imazapyr. In peanut fields without supplemental irrigation or with inadequate fertility, greater yield losses could be observed than reported herein.