Palmer amaranth's ability to evolve resistance to different herbicides has been studied extensively, but there is little information about how this weed species might be evolving other life-history traits that could potentially make it more aggressive and difficult to control. We characterized growth and morphological variation among 10 Palmer amaranth populations collected in Florida and Georgia from fields with different cropping histories, ranging from continuous short-statured crops (vegetables and peanut) to tall crops (corn and cotton) and from intensive herbicide use history to organic production. Palmer amaranth populations differed in multiple traits such as fresh and dry weight, days to flowering, plant height, and leaf and canopy shape. Differences between populations for these traits ranged from 36% up to 87%. Although glyphosate-resistant (GR) populations collected from cropping systems including GR crops exhibited higher values of the aforementioned variables than glyphosate-susceptible (GS) populations, variation in traits was not explained by glyphosate resistance or distance between populations. Cropping system components such as crop rotation and crop canopy structure better explained the differences among populations. The higher growth of GR populations compared with GS populations was likely the result of multiple selection forces present in the cropping systems in which they grow rather than a pleiotropic effect of the glyphosate resistance trait. Results suggest that Palmer amaranth can evolve life-history traits increasing its growth and reproduction potential in cropping systems, which explains its rapid spread throughout the United States. Furthermore, our findings highlight the need to consider the evolutionary consequences of crop rotation structure and the use of more competitive crops, which might promote the selection of more aggressive biotypes in weed species with high genetic variability. Nomenclature: glyphosate; Palmer amaranth, Amaranthus palmeri S. Wats.; corn, Zea mays L.; cotton, Gossypium hirsutum L.; peanut, Arachis hypogaea L.

Novel approaches are needed for overcoming barriers to successful association of herbaceous legumes with grasses in warm‐climate pastures and to identify low‐cost, long‐term solutions to the problem of N limitation in low‐input systems. The objective of this experiment was to evaluate defoliation management options during the year of establishment when rhizoma peanut (RP) (Arachis glabrata Benth.) was strip planted into existing bahiagrass (Paspalum notatum Flüggé). Treatments were four defoliation strategies: (i) Control (no defoliation of the planted RP strip and adjacent bahiagrass harvested for hay), (ii) Hay Production (RP strip and adjacent bahiagrass harvested for hay every 28 d), (iii) Simulated Continuous Stocking (pastures grazed weekly), and (iv) Rotational Stocking (pastures grazed every 28 d). Simulated Continuous and Rotational Stocking reduced RP canopy cover and frequency of occurrence. Greatest RP cover during the establishment year was achieved in August with 32 and 29% for the Control and Hay Production treatments compared to 5 and 4% for Simulated Continuous and Rotational Stocking, respectively. Spread of RP was least in Simulated Continuous Stocking. Light penetration to the level of RP in the canopy was not a primary driver of RP response because it was greatest for grazed plots where RP performed poorest. Results show that defoliation management during the establishment year is critical and if pastures are defoliated, hay production is the recommended option.

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.

No abstract

Glyphosate was compared with other commonly used corn herbicides at three locations in Kentucky in 1998 and 1999. Sequential glyphosate treatments provided greater than 87% control of common cocklebur, ivyleaf morningglory, common lambsquarters, and giant ragweed. Control of these species with glyphosate treatments was similar to the control with atrazine plus other postemergence (POST) herbicides. Generally, treatments containing s-triazines and chloracetamides applied to the soil surface were not as effective as sequential glyphosate or atrazine plus POST herbicides. Corn yield was not significantly different at any location or for any year, whereas differences in net return occurred at one location in 1998.