A survey questionnaire was sent to cotton consultants of Arkansas and Mississippi through direct mail and Louisiana and Tennessee consultants through on-farm visits in fall of 2011. The survey was returned by a total of 22 Arkansas, 17 Louisiana, 10 Mississippi, and 11 Tennessee cotton consultants, representing 26, 53, 13, and 38% of total cotton planted in these states in 2011, respectively. Collectively, the area planted to glyphosate-resistant (Roundup Ready®, RR) cotton was 97%, glyphosate plus glufosinate-resistant (Widestrike® Flex, WRF) cotton was 30%, and glufosinate-resistant (Liberty Link, LL) cotton was 2.6% of the total cotton surveyed in 2011. Seventy percent of area in all states is still under continuous RR/WRF cotton. Average cost of herbicides in RR systems was $114 ha−1 and in LL systems was $137 ha−1. Across the states, cotton planted under no-tillage, conservation tillage, and conventional tillage was 31, 36, and 33%, respectively, of total scouted cotton. Area under conventional tillage increased and conservation tillage decreased in Arkansas compared with a previous survey conducted in 2006. Palmer amaranth, morningglories, and horseweed in the order of listing were the most problematic weeds of cotton across Arkansas, Mississippi, and Tennessee. In Louisiana, however, morningglories were the most problematic weed followed by Palmer amaranth and common waterhemp. Glyphosate-resistant (GR) Palmer amaranth infested only 13% of scouted cotton area in Louisiana compared with 75% in the remaining three states, and consequently, hand-weeding to control GR Palmer amaranth is practiced on only 2.5% of total scouted area of Louisiana and 49% of the scouted area of the remaining three states. Hand-weeding added an additional $12 to 371 ha−1 to weed-management costs. One-half (50%) of the cotton consultants emphasized the need for more research on residual herbicides that can control GR Palmer amaranth effectively.

A survey of soybean fields containing waterhemp infestations was conducted just prior to harvest in 2008 and 2009 to determine the frequency and distribution of glyphosate-resistant waterhemp in Missouri, and to determine if there are any in-field parameters that may serve as indicators of glyphosate resistance in this species in future crop production systems. Glyphosate resistance was confirmed in 99 out of 144, or 69%, of the total waterhemp populations sampled, which occurred in 41 counties of Missouri. Populations of glyphosate-resistant waterhemp were more likely to occur in fields with no other weed species present at the end of the season, continuous cropping of soybean, exclusive use of glyphosate for several consecutive seasons, and waterhemp plants showing obvious signs of surviving herbicide treatment compared to fields characterized with glyphosate-susceptible waterhemp. Therefore, it is suggested that these four site parameters, and certain combinations of these parameters, serve as predictors of glyphosate resistance in future waterhemp populations.

Field studies were conducted in 1998 and 1999 to evaluate the sensitivity of a common sunflower population to acetolactate synthase (ALS)-inhibiting herbicides and evaluate alternative control methods for ALS-resistant common sunflower in soybean. Imazaquin, imazethapyr, imazamox, flumetsulam, chlorimuron, cloransulam, and CGA277476 provided 35 to 73% control of common sunflower with labeled rates. Postemergence glyphosate and sequential applications of bentazon effectively controlled ALS-resistant common sunflower. Although pendimethalin + imazaquin + imazethapyr preemergence followed by glyphosate postemergence with and without cultivation provided 100% control of ALS-resistant common sunflower, sequential applications of glyphosate provided net incomes approximately $50 to $110/ha higher than the alternative control methods. Pendimethalin preemergence followed by acifluorfen + bentazon postemergence followed by bentazon 10 d later provided the lowest overall weed control and net income. In treatments with a soil-applied herbicide followed by glyphosate postemergence, cultivation generally did not improve weed control. Soybean yields were the same comparing glyphosate with a soil-applied herbicide followed by glyphosate.

The effect of application technology on efficacy has been thoroughly investigated for sprays of chemical pesticide but not biological pesticides. This study investigates the effect of applying conidia of Microsphaeropsis amaranthi, a candidate bioherbicide for Amaranthus spp., with a range of different hydraulic nozzle tips. The nozzle tips were selected to deliver sprays with different spectra of droplet sizes deployed at different angles. We found that sprays of large droplets gave poor coverage of the target and resulted in low levels of disease severity on common waterhemp. The most effective nozzle tip tested was a hollow cone nozzle tip, such as is commonly used for the application of fungicides. This nozzle tip deployed large numbers of fine droplets that swirled within the plant canopy, provided good coverage of all plant parts, and resulted in the highest levels of disease severity, particularly on stems.

Common waterhemp is one of the most commonly encountered and troublesome weeds in the midwestern United States. It is well known that water stress adversely affects crop growth and yield; however, the effects of water stress on weed growth and seed production are poorly understood. The objective of this study was to determine the effects of degree and duration of water stress on growth, development, and fecundity of two common waterhemp biotypes in greenhouse experiments conducted at the University of Nebraska–Lincoln. No difference was observed in growth, development, and seed production between two biotypes in response to degree and duration of water stress; therefore, data were combined. The degree of water stress study included five treatments, where the amount of water applied to each pot at 2-d interval was equivalent to 100, 75, 50, 25, and 12.5% of pot (soil) water content. The highest plant height (163 cm), number of leaves (231 plant−1), and growth index (4.4 × 105 cm3) were recorded at 100% of pot water content (no water stress). Similarly, aboveground biomass, total leaf area, and seed production reached their maximum at 100% of pot water content treatment, whereas they were reduced as degree of water stress increased. The study of water stress duration included five treatments, where amount of water applied to each pot at 2-, 4-, 6-, 8-, and 10-d intervals was equivalent to 100% of pot water content. The highest plant height (150 cm), number of leaves (210 plant−1), and growth index (3.8 × 105 cm3) were observed at 2-d interval of water stress, whereas seed production was similar at 2-d (36,549 seeds plant−1) and 4-d (34,176 seeds plant−1) intervals. This study shows that common waterhemp has capacity to survive and reproduce even under a higher degree and duration of water stress.

A total of four field experiments were conducted over a 2-yr period (2011 and 2012) near Mokane and Moberly, Missouri, to determine the control of glyphosate-resistant (GR) waterhemp with dicamba and glyphosate applied alone or as a tank-mix combination. In one experiment, dicamba was applied at 0.14, 0.28, 0.42, and 0.56 kg ae ha−1 with or without 0.86 kg ae ha−1 glyphosate to GR waterhemp plants 7.5, 15, and 30 cm in height. In a second experiment, sequential treatments of dicamba or dicamba plus glyphosate were applied 4, 7, and 14 d after the initial herbicide treatment to plants measuring either 7.5 or 23 cm in height. Control of GR waterhemp ranged from 7 to 62%, 11 to 40%, and 8 to 30% when applied to 7.5-, 15-, and 30-cm plants, respectively. Control of 7.5-cm GR waterhemp increased by 16 to 36%, and biomass reduction increased by 29 to 52% in response to 0.14, 0.28, 0.42, and 0.56 kg ha−1 dicamba plus glyphosate compared to these same rates of dicamba alone. When sequential dicamba-containing treatments were averaged across all treatments and application timings, GR waterhemp control ranged from 46 to 47%, and biomass reduction ranged from 55 to 66%. No differences in control were observed based on the timing of the sequential herbicide treatment. However, in terms of GR waterhemp biomass reduction, sequential treatments applied 4 d after the initial treatment reduced GR waterhemp biomass more than sequential treatments applied 14 d after the initial treatment. Results from these experiments indicate that, in the absence of crop competition, a single treatment of dicamba up to 0.56 kg ha−1 provides less than 62% control of GR waterhemp, and sequential dicamba plus glyphosate treatments targeting 7.5 cm plants are required to achieve at least 72% control.