No abstract

The potential widespread adoption of cotton and soybean varieties with 2,4-D and dicamba resistance traits in the southeastern US will increase the risk of accidental exposure of peanut to these herbicides because of drift or application errors. When such accidents occur, growers must decide between continuing the crop and terminating it. In order to make this decision, growers need to estimate the potential yield reduction caused by 2,4-D or dicamba. Dose-response studies were conducted under field conditions in Citra and Jay, FL in 2012 and 2013 to determine peanut injury and yield reduction after exposure to 70, 140, 280, 560, and 1120 g ae ha−1of 2,4-D or to 35, 70, 140, 280, and 560 g ae ha−1of dicamba at 21 and 42 d after planting (DAP). Only herbicide by rate interactions were significant (P < 0.04). Dicamba caused 2 to 5 times higher peanut injury and 0.5 to 2 times higher yield reductions than 2,4-D. Injury ranged from 0 to 35% when peanut plants were treated with 2,4-D and from 20 to 78% with dicamba. The maximum yield reduction was 41% with 1,120 g ha−1of 2,4-D and 65% with 560 g ha−1of dicamba. Linear regression indicated that the intercept for yield reduction was 12% for 2,4-D and 23% for dicamba, and there was a 2.5% and 7.7% increase in yield reduction per additional 100 g ha−1, respectively. Although high variability was observed for the different variables, there was a positive correlation between injury and peanut yield reduction (P < 0.0001) with Pearson's Rho values ranging from 0.45 to 0.59 for 2,4-D and from 0.27 to 0.55 for dicamba, suggesting that growers can use injury data to make rough projections of yield reduction and decide if they continue their crop, especially when injury is evident.

Tropical signalgrass is one of the dominant weeds in the Florida turfgrass industry and is potentially troublesome for the southeastern turfgrass industry. Tropical signalgrass is especially problematic for St. Augustinegrass sod producers because of lack of control options. The objectives of our research were to determine the effect of light, pH, temperature, water potential, and planting depth on tropical signalgrass germination and emergence. Tropical signalgrass germination does not require light and is optimum at pH 5 to 6, temperature 25 C, and water potentials greater than − 0.4 MPa. Tropical signalgrass shoots emerged from depths of 0 to 7 cm, with maximum germination when placed on the soil surface. Tropical signalgrass seedlings emerged in the field during the second week of March in Ft. Lonesome, FL. Weekly mean soil and ambient air temperatures at the time of emergence were 20 C. Tropical signalgrass emergence was first observed at 118 and 73 growing degree-days (GDD) (13 C base temperature), with a peak emergence period at 222 and 156 GDD for 2001 and 2002, respectively.

Experiments were conducted from 1997 through 2001 in Georgia, Florida, North Carolina, and Texas to evaluate compatibility of selected postemergence herbicides and fungicides applied in tank mixtures. Control of broadleaf signalgrass [Brachiaria platyphylla (Griseb.) Nash], goosegrass [Eleusine indica (L.) Gaertn.], large crabgrass [Digitaria sanguinalis (L.) Scop.], and Texas panicum (Panicum texanum Buckl.) by clethodim applied in tank mixtures with copper-based fungicides, fungicides containing chlorothalonil, azoxystrobin, and iprodione was reduced in 80, 69, 60, and 46% of comparisons, respectively, when compared to clethodim alone. Fluazinam, tebuconazole, and propiconazole did not reduce efficacy of clethodim. Efficacy was reduced more by fungicides when clethodim was applied in 230 L/ha spray volume compared with 94 L/ha. Efficacy of acifluorfen, bentazon, imazethapyr, and 2,4-DB applied with fungicides was also compared. Smooth pigweed (Amaranthus hybridus L.) control by 2,4-DB was reduced in at least two of three experiments when applied with chlorothalonil, copper-based fungicides, tebuconazole, azoxystrobin, and fluazinam. Iprodione did not affect efficacy of 2,4-DB. Control of smooth pigweed by imazethapyr was reduced when applied in combination with copper-based fungicides but not when applied with chlorothalonil, propiconazole, tebuconazole, fluazinam, propiconazole plus flutolanil, or propiconazole plus trifloxystrobin. Smooth pigweed control by acifluorfen was reduced in one of three experiments when applied with tebuconazole. Efficacy of acifluorfen was not affected by chlorothalonil, azoxystrobin, propiconazole, or fluazinam. Yellow nutsedge (Cyperus esculentus L.) control by bentazon was reduced by propiconazole plus chlorothalonil, propiconazole plus flutolanil, and copper-based fungicides. With the exception of fluazinam and chlorothalonil applied with 2,4-DB in one experiment, fungicides did not affect peanut injury following application of acifluorfen, clethodim, imazethapyr, or 2,4-DB.

Studies were conducted in Florida to evaluate interference of common cocklebur with peanut. Peanut yield reduction ranged from 0 to 88% for common cocklebur densities of 0–32 plants 8 m−1of peanut row, and predicted loss was similar under normal moisture conditions. When moisture levels were above normal, the impact on yield was 9–24% less than when soil moisture was normal. Common cocklebur caused peanut yield loss if allowed to interfere for more than the first 2 wk after crop emergence, and peanut had to be common cocklebur free for at least 12 wk to prevent a yield reduction. These results show common cocklebur to be more competitive with peanut than other weeds evaluated previously.

No abstract

Herbicide soil/solution distribution coefficients (Kd) are used in mathematical models to predict the movement of herbicides in soil and groundwater. Herbicides bind to various soil constituents to differing degrees. The universal soil colloid that binds most herbicides is organic matter (OM), however clay minerals (CM) and metallic hydrous oxides are more retentive for cationic, phosphoric, and arsenic acid compounds. Weakly basic herbicides bind to both organic and inorganic soil colloids. The soil organic carbon (OC) affinity coefficient (Koc) has become a common parameter for comparing herbicide binding in soil; however, because OM and OC determinations vary greatly between methods and laboratories, Koc values may vary greatly. This proposal discusses this issue and offers suggestions for obtaining the most accurate Kd, Freundlich constant (Kf), and Koc values for herbicides listed in the WSSA Herbicide Handbook and Supplement.

Research was conducted during 1988 and 1989 in Georgia and Florida to determine the densitydependent effects and critical periods of wild poinsettia interference with peanut. Peanut yield loss ranged from 0 to approximately 50% over the wild poinsettia density range from 0 to 32 plants 9 m−1peanut row. Predicted peanut yield losses across 2 yr in Georgia were 0, 4, 8, 12, 15, 26, 40, and 54% for season-long wild poinsettia interference at densities of 0, 1, 2, 3, 4, 8, 16, and 32 plants 9 m−1of row, respectively. Across 2 yr in Florida, predicted peanut yield losses were 0, 9, 14, 22, 30, 37, and 41% for wild poinsettia densities of 0, 1, 2, 4, 8, 16, and 32 plants 9 m−1row, respectively. Peanuts had to be maintained poinsettia free for 10 wk after peanut emergence to prevent yield loss. Wild poinsettia that interfered with peanut for more than 2 wk after peanut emergence reduced peanut yield.