Herbicide-resistant Palmer amaranth is one of the most problematic weeds in agronomic cropping systems in Georgia. The wide germination window of Palmer amaranth seed allows it to emerge after field corn harvest, and if left uncontrolled, can contribute significantly to the weed seed-bank causing problems in future rotational crops. One option for a lay-by (in-crop) or post-harvest burndown application in field corn for postemergence and residual control of Palmer amaranth is metribuzin. However, the current metribuzin label prohibits peanut planting for 18 months after application. Peanut tolerance to metribuzin has not been well documented. Therefore, the objective of this research was to evaluate the tolerance of peanut to metribuzin. Field studies were conducted in 2017-2019 in Ty Ty, GA to evaluate the tolerance of peanut to various rates of metribuzin. In a RCBD with four replications, metribuzin was applied preemergence (two days after planting) at 0, 70, 140, 280, 420, and 560 g ai/ha. Rainfall in the first month after planting was 13.1, 15.9, and 11.8 cm for 2017, 2018, and 2019, respectively. Data were subjected to nonlinear regression using log-logistic analysis to demonstrate a dose-response relationship. Year by treatment interactions were significant for late season injury, so data were separated by year. However, early season injury, stand loss, and yield loss data were pooled over years. There was a direct relationship between rate and the response variables. As metribuzin rate increased, injury, stand loss and yield loss increased. Generally, visual injury, stand loss, and yield loss were negligible at rates less than or equal to 140 g ai/ha. With a targeted application rate of 280 g ai/ha and an estimated half-life of 30 to 60 days, metribuzin residues should have minimal impact on peanut grown in rotation when used in lay-by or post-harvest treatments for the prevention of Palmer amaranth seed rain in field corn.

ABSTRACT Picloram (4-amino-3,5,6-trichloropicolinic acid) injury, in the form of leaf roll, is often observed in peanut fields due to short crop rotations, contaminated irrigation water, treated hay, and contaminated livestock waste. Limited data on peanut response to picloram is available. Field trials were conducted near Tifton, GA from 2015-2017 to determine the effects of picloram plus 2,4-D (2,4-dichlorophenoxyacetic acid) on peanut growth and yield. Picloram plus 2,4-D was applied to ‘GA-06G' peanut at four different timings: preemergence (PRE), 30 d after planting (DAP), 60 DAP, and 90 DAP. At each timing, three rates of picloram plus 2,4-D were applied including the following: 1/10 th X (0.18 + 0.67 kg ai/ha); 1/100 th X (0.018 + 0.067 kg ai/ha); and 1/300 th X (0.006 + 0.023 kg ai/ha). A non-treated control (NTC) or 0 rate was included for comparison. Peanut plant density was not influenced by any rate or timing of picloram plus 2,4-D. For peanut injury (leaf roll), a significant rate x timing interaction was observed (P=0.047). At 120 DAP, leaf roll was significant for the 1/10 th X rate applied at 30, 60, and 90 DAP, the 1/100 th X rate applied at 60 and 90 DAP, and for the 1/300 th X rate applied at 90 DAP. When averaged over timing, peanut height at 120 DAP was significantly reduced by the 1/10 th X and 1/100 th X rates. When averaged over rate, peanut height reductions were greatest when picloram plus 2,4-D was applied at 60 DAP. When averaged over timing, only the 1/10 th X rate caused significant yield reductions (11%). When averaged over rate, timing had no effect on yield (P=0.5403). Peanut fields unintentionally exposed to picloram plus 2,4-D rates ≤ 1/100 th X can exhibit typical injury symptoms but most likely will not experience yield losses.

ABSTRACT Recent research on the effects of time of d (TOD) when glufosinate is applied to cotton ( Gossypium hirsutum L.) and several protoporphyrinogen-inhibiting herbicides in soybean ( Glycine max L.) has growers concerned about potential TOD effects on peanut weed control. Consequently, research was conducted in 2015, 2016, and 2017 to determine if TOD influences the performance of peanut herbicides acifluorfen, bentazon, imazapic, lactofen, paraquat, and 2,4-DB. Both non- (bare-ground) and in-crop (peanut) studies were conducted. For non-crop, paraquat plus bentazon plus acifluorfen plus S -metolachlor, imazapic plus S -metolachlor plus 2,4-DB, and lactofen plus S -metolachlor plus 2,4-DB were applied to Palmer amaranth and a non-uniform mixture of annual grasses including Urochloa texana (Buckley), Dactyloctenium aegyptium (L.), Eleusine indica (L.), Digitaria spp. at 7:00, 12:00, 17:00, and 22:00 hr. For in-crop studies, two peanut weed control programs were used and herbicide programs were applied at the same TOD. Herbicides were paraquat plus acifluorfen plus bentazon plus S -metolachlor (EPOST) followed by imazapic plus S -metolachlor plus 2,4-DB, or lactofen plus S- metolachlor plus 2,4-DB (POST). For the non-crop studies, a significant interaction between TOD and herbicide program was observed for the 7 d after treatment (DAT) rating of Palmer amaranth control. Control was reduced with imazapic applied at 22:00 hr. At 14 DAT, there was no TOD effect and control was reduced with all imazapic treatments due to ALS resistance. There was no interaction between TOD and herbicide program for annual grass control. Annual grass control was unacceptable (<50%) with lactofen. For in-crop studies, there was no interaction between TOD or herbicide program. Peanut injury was lower at 7:00 hr and 22:00 hr when compared to other timings. Lactofen was more injurious to peanut than imazapic. Palmer amaranth control was not influenced by timing or herbicide program. A reduction in sicklepod control was observed at the 22:00 hr timing and with lactofen. While TOD influenced peanut injury and weed control, peanut yield was not affected.

ABSTRACT The increase in herbicide-resistant weeds over the past decade has led to the introduction of crops that are resistant to auxin herbicides. Strict application procedures are required for the use of auxin herbicides in auxin-resistant crops to minimize off-target movement. One requirement for application is the use of nozzles that will minimize drift by producing coarse droplets. Generally, an increase in droplet size can lead to a reduction in coverage and efficacy depending upon the herbicide and weed species. In studies conducted in 2015 and 2016, two of the potential required auxin nozzle types [(AIXR11002 (coarse) and TTI11002 (ultra-coarse)] were compared to a conventional flat-fan drift guard nozzles [DG11002 (medium)] for weed control in peanut herbicide systems. Nozzle type did not influence annual grass or Palmer amaranth control in non-crop tests. Results from in-crop tests indicated that annual grass control was 5% to 6% lower when herbicides were applied with the TTI nozzle when compared to the AIXR or DG nozzles. However, Palmer amaranth control and peanut yield was not influenced by coarse-droplet nozzles. Peanut growers using the coarse-droplet nozzles need to be aware of potential reduced grass control.