Soybean dose-response to 2,4-D and dicamba at vegetative and reproductive growth stages.

BACKGROUND: Field experiments were conducted across multiple sites in 2012 and 2013 to describe sensitivity of soybean to 2,4-D (six doses) and dicamba (seven doses) at V3 and R1 growth stages. Further experiments were conducted under greenhouse conditions in 2017 and 2018 to compare soybean response to several dicamba herbicides across a broader range of doses than those tested in the field. RESULTS: Soybean yield loss was 6.1-fold greater from 2,4-D exposure at V3 compared to R1 and 1.4 times greater from dicamba exposure at R1 than at V3. In V3 exposures, soybean was 15.4 times more sensitive to dicamba than 2,4-D and 134.4-fold more sensitive to dicamba when exposed at R1. Plant injury and height correlations to grain yield resulted in coefficients ranging from 0.65 to 0.91. In greenhouse experiments, five dicamba products were tested at up to 19 doses and as low as 0.002 g ae ha-1 (3.6 × 10-6 % of maximum single use-rate); however, no differences were observed among formulations used in dicamba-resistant crops versus traditional formulations. A no observable effects dose was not identified due to responses observed even at the lowest doses tested, although hormesis effects were observed in plant height. CONCLUSION: These data suggest that the sensitivity of soybean to dicamba is much greater than what has previously been reported. However, as has been indicated by previous work, that injury does not always result in yield loss. © 2022 Society of Chemical Industry.

Influence of flooding period and seed burial depth on Palmer amaranth (Amaranthus palmeri) seed germination.

BACKGROUND: Flooding throughout fall and winter months is an effective practice for rice (Oryza sativa L.) straw decomposition, soil seedbank depletion, and waterfowl habitat in Mississippi. Nevertheless, limited research is available regarding the effects of fall-winter flooding and seed burial depth on Palmer amaranth (Amaranthus palmeri S. Wats.) seed germination. The objective of this study was to evaluate the effect of flooding period and seed burial depth on A. palmeri seed damage and germination in three different soil textures in Mississippi. RESULTS: Amaranthus palmeri seed damage was greater when seeds were buried in sandy loam compared to silt loam soil textures. An interaction between flooding period and seed burial depth was present for A. palmeri seed germination. Flooding periods of 1-month (at 0 and 15 cm burial depth) and 2 months (at 0 cm burial depth) provided similar A. palmeri seed germination compared to no-flooding (at 0 cm burial depth). In addition, flooding periods of 3, 4, and 5 months reduced A. palmeri seed germination by 10, 10 and 14 percentage points at 0 cm burial depth, and 36, 40, and 41 percentage points when seeds were buried at 15 cm, respectively, across all soil textures. CONCLUSION: This research demonstrates that flooding for 3, 4, and 5-months throughout fall and winter is an effective cultural practice to increase soil seedbank depletion through reduced germination potential to help manage herbicide-resistant A. palmeri populations in sandy loam, silt, and silt loam soil textures. © 2020 Society of Chemical Industry.

Quantifying the Impact of Excluding Insecticide Classes From Cotton Integrated Pest Management Programs in the U.S. Mid-South.

Current assessments from the U.S. Environmental Protection Agency suggest that some current insecticides may be lost or severely restricted in the near future. An experiment was conducted from 2014 to 2015 at two locations in Mississippi to determine the impact of losses of insecticide classes on integrated pest management of insect pests in cotton. The treatments included cotton treated with all available classes of insecticides, cotton treated with all classes except neonicotinoids, cotton treated with all classes except pyrethroids, cotton treated with all classes except carbamates and organophosphates, and an untreated control. Plots were scouted weekly and insecticide applications were made with the most efficacious and economical insecticides for each treatment when that treatment reached threshold for a particular insect pest(s). The primary insects at both locations were tobacco thrips and tarnished plant bugs. Thrips pressure was similar at both locations and generally showed that all insecticide treatments provided a similar level of protection compared with the untreated control. At the Stoneville location where tarnished plant bug pressure was greatest, cotton yields and economic returns differed between plots where all classes of insecticides were applied compared with the untreated control and where neonicotinoids were excluded. However, in Starkville where tarnished plant bug pressure was less, there were no differences among treatments. Although yield and economic returns were similar in high tarnished plant bug pressure areas when using all classes compared with managing without pyrethroids or organophosphates, a rotation among all insecticide classes should be beneficial for resistance management in Mid-South cotton production.

Neonicotinoid Insecticide Resistance in Tobacco Thrips (Thysanoptera: Thripidae) of Mississippi.

Insecticidal efficacy of neonicotinoid insecticides used against tobacco thrips, Frankliniella fusca (Hinds) (Thysanoptera: Thripidae), in cotton, Gossypium hirisutum L. (Malvales: Malvaceae), was evaluated for field populations collected in Mississippi during 2014-2016. Resistance was documented in 16 and 57% of populations to imidacloprid and thiamethoxam, respectively. Resistance levels did not vary by host plant for any neonicotinoid, but resistance levels varied between the two main agricultural areas (Delta and Hills) of Mississippi and among years for some neonicotinoids. In spite of documented resistance, neonicotinoid seed treatments are still used on cotton in the midsouthern United States due to the lack of reliable alternative management strategies. The development of alternative thrips management strategies is critical to the sustainability of cotton production in the midsouthern United States.

Spray droplet size and carrier volume effect on dicamba and glufosinate efficacy.

BACKGROUND: Pesticide applications using a specific droplet size and carrier volume could maximize herbicide efficacy while mitigating particle drift in a precise and efficient manner. The objectives of this study were to investigate the influence of spray droplet size and carrier volume on dicamba and glufosinate efficacy, and to determine the plausibility of droplet-size based site-specific weed management strategies. RESULTS: Generally, across herbicides and carrier volumes, as droplet size increased, weed control decreased. Increased carrier volume (187 L ha-1 ) buffered this droplet size effect, thus greater droplet sizes could be used to mitigate drift potential while maintaining sufficient levels of weed control. To mitigate drift potential and achieve satisfactory weed control (≥ 90% of maximum observed control), a 900 µm (Ultra Coarse) droplet size paired with 187 L ha-1 carrier volume is recommended for dicamba applications and a 605 µm (Extremely Coarse) droplet size across carrier volumes is recommended for glufosinate applications. Although general droplet size recommendations were created, optimum droplet sizes for weed control varied significantly across site-years. CONCLUSION: Convoluted interactions occur between droplet size, carrier volume, and other application parameters. Recommendations for optimizing herbicide applications based on droplet size should be based on a site-specific management approach to better account for these interactions. © 2018 Society of Chemical Industry.