Plant defence traits, such as herbicide resistance mutations, may incur a fitness cost to plants that become evident when the trait is not needed. However, individuals with multiple herbicide resistance traits may decrease fitness beyond that of plants with a single herbicide resistance mutation. Multiple herbicide-resistant (MHR) Amaranthus tuberculatus populations are becoming more prevalent in Midwest United States agroecosystems. The objective was to determine whether selected MHR A. tuberculatus populations express differential development when grown in a herbicide-free environment. The hypothesis was that MHR A. tuberculatus populations become increasingly less fit when additional herbicide resistances evolve. Multiple herbicide-resistant and herbicide-susceptible A. tuberculatus populations were grown in a herbicidefree field for 20 weeks for two seasons. Differences (P < 0.001) in apical growth were detected 5 and 7 weeks after transplanting for all populations in 2016 and 2017 respectively. Gender and population influenced (P < 0.001) flowering date, with males flowering up to 1.5 weeks earlier than females, but did not cause pollination asynchrony. Shoot biomass was not different (P = 0.84) across A. tuberculatus populations, but there were differences (P < 0.001) for gender and year. Seed production was different amongst A. tuberculatus populations (P = 0.001), but was not influenced by the number of MHR traits. Conversely, a negative quadratic relationship between seed mass and the number of MHR traits was observed (r 2 = 0.32; P < 0.001). The experiment results demonstrate that MHR in A. tuberculatus populations is not incurring a fitness penalty that will remove the populations in the immediate future.

Complaints of control failures with acetolactate synthase (ALS)-inhibiting and protoporphyrinogen oxidase (PPO)-inhibiting herbicides on redroot pigweed (Amaranthus retroflexus L.) were reported in conventional soybean [Glycine max (L.) Merr.] fields in North Carolina. Greenhouse dose-response assays confirmed that the Camden and Pasquotank County populations were less sensitive to ALS and PPO-inhibiting herbicides compared to susceptible A. retroflexus populations, suggesting the evolution of resistance to these herbicides. Sanger sequencing of target genes determined the Camden County population carried a Trp574Leu mutation in the ALS gene and an Arg98Gly mutation in the PPX2 gene, while the Pasquotank County population carried a His197Pro mutation in the ALS gene (first documentation of the mutation in the Amaranthus genus) but no mutation was detected in the PPX2 gene. Single nucleotide polymorphism (SNP) genotyping assays were developed to enable efficient screening of future control failures in order to limit the spread of these herbicide-resistant populations. In addition, preliminary testing of these assays revealed the three mutations were ubiquitous in the respective populations. These two populations represent the first confirmed cases of PPO-inhibiting herbicide-resistant A. retroflexus in the United States; as well as the first confirmed cases of this particular herbicide resistance profile in A. retroflexus inhabiting North America. While no mutation was found in the PPX2 gene of the Pasquotank County population, we suggest that this population has evolved resistance to PPO-inhibiting herbicides but the mechanism of resistance is to be determined.

Greenhouse studies were conducted to evaluate the effects of soil organic matter content and soil pH on initial and residual weed control with flumioxazin by planting selected weed species in various lab-made and field soils. Initial control was determined by planting weed seeds into various lab-made and field soils treated with flumioxazin (71 g ha−1). Seeds of Echinochloa crus-galli (barnyard grass), Setaria faberi (giant foxtail), Amaranthus retroflexus (redroot pigweed), and Abutilon theophrasti (velvetleaf) were incorporated into the top 1.3 cm of each soil at a density of 100 seeds per pot, respectively. Emerged plants were counted and removed in both treated and non-treated pots two weeks after planting and each following week for six weeks. Flumioxazin control was evaluated by calculating percent emergence of weeds in treated soils compared to the emergence of weeds in non-treated soils. Clay content was not found to affect initial flumioxazin control of any tested weed species. Control of A. theophrasti, E. crus-galli, and S. faberi was reduced as soil organic matter content increased. The control of A. retroflexus was not affected by organic matter. Soil pH below 6 reduced flumioxazin control of A. theophrasti, and S. faberi but did not affect the control of A. retroflexus and E. crus-galli. Flumioxazin residual control was determined by planting selected weed species in various lab-made and field soils 0, 2, 4, 6, and 8 weeks after treatment. Eight weeks after treatment, flumioxazin gave 0% control of A. theophrasti and S. faberi in all soils tested. Control of A. retroflexus and Chenopodium album (common lambsquarters) was 100% for the duration of the experiment, except when soil organic matter content was greater than 3% or the soil pH 7. Eight weeks after treatment, 0% control was only observed for common A. retroflexus and C. album in organic soil (soil organic matter > 80%) or when soil pH was above 7. Control of A. theophrasti and S. faberi decreased as soil organic matter content and soil pH increased. Similar results were observed when comparing lab-made soils to field soils; however, differences in control were observed between lab-made organic matter soils and field organic matter soils. Results indicate that flumioxazin can provide control ranging from 75–100% for two to six weeks on common weed species.

Glufosinate is among the few remaining effective herbicides for postemergence weed control in North Carolina crops. The evolution of glufosinate resistance in key weeds is currently not widespread in North Carolina but to better assess the current status of glufosinate effectiveness, surveys were distributed at extension meetings in 2019 and 2020. The surveys were designed to provide information about North Carolina farmers’ perception of and use of glufosinate. Many North Carolina farmers (≥26%) apply glufosinate at the correct timing (5-10 cm weeds). In addition to applying glufosinate at the correct timing, North Carolina farmers (≥22%) are applying glufosinate as a complementary herbicide to other efficacious herbicides and to control herbicide-resistant weeds, suggesting glufosinate is part of a diverse chemical weed management plan. Conversely, survey findings indicated some North Carolina farmers (13 to 17%) rely exclusively on glufosinate for weed control. Additionally, North Carolina farmers (28 to 30%) reported glufosinate control failures and control failures were observed on several weed species in corn, cotton, and soybean. The results of the survey suggest that most North Carolina farmers are currently stewarding glufosinate but also support the need for extension personnel to keep educating farmers on how to correctly use glufosinate to delay the evolution of glufosinate-resistant weeds. Yearly surveys should be distributed to monitor where glufosinate control failures occur and the weed species not being controlled.

Italian ryegrass [Lolium perenne L. ssp. multiflorum (Lam.) Husnot] is one of the most challenging weeds for winter wheat (Triticum aestivum L.) growers to manage. Italian ryegrass has evolved resistance to the majority of the herbicides labeled for use in wheat and the competitive ability of the species makes it a significant factor driving winter wheat production practices around the world. Previous research has utilized remotely sensed spectral imagery to detect Italian ryegrass in winter wheat to aid weed control decisions. Two studies from 2016 to 2017 were initiated with the intent of identifying the spectral reflectance properties of Italian ryegrass and winter wheat using an unmanned aerial vehicle (UAV) equipped with a 5-band multispectral sensor. Image analysis was conducted to determine the potential for species discrimination throughout the growing season. Supervised classification of the imagery was used to evaluate the ability of the UAV platform for further discrimination between Italian ryegrass and winter wheat. Species differentiation proved to be possible, however the data was not able to be referenced across dates. Due to light variability, the reflectance values changed to such a degree that unsupervised classifications were not possible using a database of values from previous flights. Supervised classification of the multispectral image resulted in &gt;70% classification accuracy between the species. However, near infrared light consistently differed enough for accurate classification between Italian ryegrass and winter wheat across different weed densities, flight altitudes, and imaging dates. On a single field basis, species differentiation was successful and resulted in classified maps of Italian ryegrass and winter wheat. This study also analyzed the exact accuracy of the species differentiation based on the quality and uniformity of light conditions and growth stage of plants.