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.

Laboratory and greenhouse studies were conducted to determine the absorption, translocation, dose response, efficacy, and effects on biomass of the herbicide diclosulam on bristly starbur (Acanthospermum hispidum DC.) In laboratory experiments, 14C-diclosulam absorption and translocation was evaluated in bristly starbur. Greenhouse studies determined bristly starbur growth response to postemergence applied diclosulam at 0, 0.2, 0.4, 0.8, 1.6, 3.3, and 6.5 g ai/ha. Bristly starbur seed were collected from two Georgia locations, hand cleaned, and grown in the greenhouse. Bristly starbur plants were then foliar treated with diclosulam when plants were in the 4 to 6 leaf stage of growth. 14C-diclosulam was applied to a single non-treated, most fully developed bristly starbur adaxial leaf, after the rest of the plant was foliar treated with 0.8 g ai/ha of diclosulam. At 24 and 48 hours after treatment, 14C-diclosulam was translocated acropetally in bristly starbur to the plant apex, with little to no movement to the lower plant parts including lower leaves, stem and roots. For the dose response studies, bristly starbur growth reduction dry weight (GR50) and efficacy (I50) for diclosulam ranged from 0.33 to 1.08 g ai/ha, respectively. Based on these data, bristly starbur susceptibility to diclosulam was due to its translocation to apical growing points within 24 to 48 hours where acetoacetate synthase (ALS) inhibition occurred resulting in eventual plant death. While bristly starbur is controlled with diclosulam rates below the standard use rate of other weeds, the standard field rates in combination with other herbicide mechanisms of action will continue to be recommended to peanut growers to reduce the potential for bristly starbur ALS resistance selection.