Previous studies have demonstrated benefi ts of individual cover crop species, but the value of diverse cover crop mixtures has received less attention. Th e objectives of this research were to determine the eff ects of spring-sown cover crop mixture diversity and mechanical cover crop termination method on cover crop and/or cash crop productivity, soil moisture and N, and profi tability in an organic cropping system. An experiment was conducted between 2009 and 2011 near Mead, NE, where mixtures of two (2CC), four (4CC), six (6CC), and eight (8CC) cover crop species, or a summer annual weed mixture were included in a sunfl owersoybean-corn rotation. Cover crops were terminated in late May using a fi eld disk or sweep plow undercutter. Undercutting cover crops increased soil NO 3 -N (0-20 cm) by 1.0 and 1.8 mg NO 3 -N kg -1 relative to disk incorporation in 2010 and 2011, respectively. Cover crop mixtures oft en reduced soil moisture (0-8 cm) before main crop planting, though cover crop termination with the undercutter increased soil moisture content by as much as 0.024 cm 3 cm -3 compared to termination with the disk during early main crop growth. Crop yields were not infl uenced by cover crop mixture, but termination with the undercutter increased corn and soybean yield by as much as 1.40 and 0.88 Mg ha -1 , respectively. Despite diff erences in productivity between spring cover crop mixtures and weed communities, crop yield was not diff erent among these treatments; thus, profi tability of the weed mixture-undercutter treatment combination was greatest due to reduced input costs. ).Abbreviations: CC, cover crop mixture; DAT, days aft er termination; DOY, day of year; NC, weed-free and cover crop-free control; WD, weedy mixture and cover crop-free.

Figueiredo, Marcelo R. A.; Leibhart, Lacy J.; Reicher, Zachary J.; Tranel, Patrick J.; Nissen, Scott J.; Westra, Philip; Bernards, Mark L.; Kruger, Greg R.; Gaines, Todd A.; and Jugulam, Mithila, "Metabolism of 2,4-dichlorophenoxyacetic acid contributes to resistance in a common waterhemp (Amaranthus tuberculatus) population" (2017 This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. Please cite this article as doi: 10.1002/ps.

A waterhemp population from a native-grass seed production field in Nebraska was no longer effectively controlled by 2,4-D. Seed was collected from the site, and dose-response studies were conducted to determine if this population was herbicide resistant. In the greenhouse, plants from the putative resistant and a susceptible waterhemp population were treated with 0, 18, 35, 70, 140, 280, 560, 1,120, or 2,240 g ae ha−12,4-D. Visual injury estimates (I) were made 28 d after treatment (DAT), and plants were harvested and dry weights (GR) measured. The putative resistant population was approximately 10-fold more resistant to 2,4-D (R:S ratio) than the susceptible population based on both I50(50% visual injury) and GR50(50% reduction in dry weight) values. The R:S ratio increased to 19 and 111 as the data were extrapolated to I90and GR90estimates, respectively. GR50doses of 995 g ha−1for the resistant and 109 g ha−1for the susceptible populations were estimated. A field dose-response study was conducted at the suspected resistant site with 2,4-D doses of 0, 140, 280, 560, 1,120, 2,240, 4,480, 8,960, 17,920, and 35,840 g ha−1. At 28 DAT, visual injury estimates were 44% in plots treated with 35,840 g ha−1. Some plants treated with the highest rate recovered and produced seed. Plants from the resistant and susceptible populations were also treated with 0, 9, 18, 35, 70, 140, 280, 560, or 1,120 g ae ha−1dicamba in greenhouse bioassays. The 2,4-D resistant population was threefold less sensitive to dicamba based on I50estimates but less than twofold less sensitive based on GR50estimates. The synthetic auxins are the sixth mechanism-of-action herbicide group to which waterhemp has evolved resistance.

Corn and soybean growers in Illinois, Indiana, Iowa, Mississippi, Nebraska, and North Carolina, as well as cotton growers in Mississippi and North Carolina, were surveyed about their views on changes in problematic weeds and weed pressure in cropping systems based on a glyphosate-resistant (GR) crop. No growers using a GR cropping system for more than 5 yr reported heavy weed pressure. Over all cropping systems investigated (continuous GR soybean, continuous GR cotton, GR corn/GR soybean, GR soybean/non-GR crop, and GR corn/non-GR crop), 0 to 7% of survey respondents reported greater weed pressure after implementing rotations using GR crops, whereas 31 to 57% felt weed pressure was similar and 36 to 70% indicated that weed pressure was less. Pigweed, morningglory, johnsongrass, ragweed, foxtail, and velvetleaf were mentioned as their most problematic weeds, depending on the state and cropping system. Systems using GR crops improved weed management compared with the technologies used before the adoption of GR crops. However, the long-term success of managing problematic weeds in GR cropping systems will require the development of multifaceted integrated weed management programs that include glyphosate as well as other weed management tactics.

Field experiments were conducted in 11 states to evaluate broadleaf weed management programs in dicamba‐resistant soybeans which involved the use of preemergence and postemergence dicamba. Preemergence (PRE) dicamba at 0.25 lb ae/acre provided less than 60% control of smooth pigweed, giant ragweed, velvetleaf, palmer amaranth, waterhemp, and morningglory spp., but 97% control of common lambsquarters and horseweed at 3 weeks after treatment (WAT). Preemergence flumioxazin plus chlorimuron or sulfentrazone plus cloransulam provided 66 to 100% control of these weeds. Use of dicamba postemergence (POST) improved uniformity of control of velvetleaf, smooth pigweed, morningglory, and glyphosate‐susceptible waterhemp. However, combining dicamba at 0.25 lb/acre with glyphosate resulted in 30% to 65% greater control of glyphosate‐resistant palmer amaranth, glyphosate‐resistant common waterhemp, glyphosate‐resistant horseweed, and glyphosate‐resistant giant ragweed compared to sequentially applied glyphosate.