<p>Acetolactate synthase (ALS) inhibitors are a widely used class of selective herbicides used to control grass and broadleaf weeds. The repeated use of ALS-inhibiting herbicides has selected for biotypes of weeds resistant to ALS inhibitors, especially in the weeds most problematic to growers in the Midwest. While ALS inhibitor use seems futile, new mechanisms of herbicide action are not predicted to be commercialized in the near future to solve this problem. This leads to the main objective of this research, determining what value ALS inhibitors provide in controlling populations of weeds with resistance to ALS inhibitors. </p> <p>Field experiments with soil-applied (PRE) applications of ALS inhibitors on horseweed (<i>Erigeron canadensis</i>) and tall waterhemp (<i>Amaranthus tuberculatus </i>var. <i>rudis</i>) exhibited higher efficacy than would be expected given the frequency of the ALS resistance trait in the population. Whereas control of these species with POST-applied applications was similar or less than the proportion of the population characterized as susceptible using molecular techniques. Soil-applied applications, therefore, resulted in relatively greater control than POST applications in populations with known ALS-inhibitor-resistance mechanisms.</p> <p>Greenhouse experiments showed that overall resistance ratios were higher for PRE applications of ALS inhibitors in horseweed, tall waterhemp, and Palmer amaranth (<i>Amaranthus palmeri</i>). However, GR₅₀ values decreased for both susceptible and resistant biotypes for the PRE applications compared to POST, suggesting the biologically effective dose of these herbicides is lower in soil residual applications. This research found that PRE applications of ALS inhibitors resulted in some level of control on horseweed and tall waterhemp classified as resistant to ALS inhibitors due to the higher efficacy of PRE herbicide applications.</p> <p>Genetic analysis assessing the amino acid substitutions that confer resistance to ALS inhibitors in tall waterhemp confirmed a difference in selection pressure between PRE and POST applications and between ALS active ingredients in tall waterhemp. Applications of chlorimuron PRE at 11 g ai ha^-1selected for 35% homozygous W574L genotypes and at 44 g ha^-1 selected for 70% homozygous W574L genotypes. An increase of homozygous W574L individuals along with a decrease in heterozygous individuals from 65 (11 g ha^-1) to 29% (44 g ha^-1) suggests that W574L is semi-dominant in tall waterhemp and that high labeled rates of chlorimuron applied PRE can partially overcome the heterozygous W574L-resistance mechanism. In horseweed, no difference in selection pressure was observed between application timing or between chlorimuron or cloransulam. A new mutation conferring ALS-inhibitor resistance in horseweed was discovered, a Pro197Leu amino acid substitution, with resistance ratios of 21X to chlorimuron and 8.6X to cloransulam. These resistance ratios are slightly less than those reported for the Pro197Ala and Pro197Ser amino acid substitutions in conferring ALS-inhibitor resistance in horseweed. </p> <p>Finally, a survey of 42 populations of tall waterhemp in Indiana counties with confirmed ALS-inhibitor resistant populations of tall waterhemp found that all populations contained at least 16% individuals with the W574L amino acid substitution, 35 populations contained at least 1% individuals with the S653N substitution, and 9 populations contained at least 1% individuals with the S653T substitution. Taking into consideration the three mutations tested, 8 of the 42 populations tested contained <50% ALS-inhibitor resistant individuals within the population. Using the same tall waterhemp populations as collected in the survey, Next-Generation Sequencing was used to determine if other amino acid substitutions conferring resistance to ALS inhibitors existed. Results from WideSeq revealed that 10 other amino acid substitutions in the ALS protein may be conferring resistance in tall waterhemp in Indiana: A122T, A122N, A122S, P197T, P197L, P197H, D376E, and G654F. Further research from this survey also suggests that metabolic resistance to ALS inhibitors is likely a contributor to resistance in tall waterhemp in Indiana.</p> <p>This research suggests that ALS-inhibiting herbicides, more specifically chlorimuron, would provide the greatest contribution to management of tall waterhemp. Chlorimuron would perform best when used in soil residual applications and in populations of tall waterhemp containing either individuals susceptible to chlorimuron or individuals heterozygous for ALS inhibitor resistance conferred by the W574L mutation. This research also demonstrates the specificity of the amino acid substitutions in the ALS protein and by weed species to realize the benefit of these herbicides for management of weeds resistant to ALS inhibitors. Molecular characterization of target site resistance to ALS inhibitors has traditionally been considered relatively simple. However, we found 11 new amino acid substitutions that confer resistance to ALS inhibitors in horseweed and tall waterhemp. The complexity of ALS inhibitor resistance calls for the use of methods such as NGS to detect all potential resistance mutations in a timely manner and for the use of tests detecting metabolic resistance. Overall, this research demonstrates that ALS inhibitors still provide some utility for management of weed populations classified as resistant to ALS inhibitors and that the resistance mechanisms in horseweed and tall waterhemp are more numerous than previously reported. </p>

<p dir="ltr">Palmer amaranth (Amaranthus palmeri S. Watts) and waterhemp [Amaranthus tuberculatus (Moq.) J. D. Sauer] are two of the most troublesome weeds in U.S. soybean production and the auxin herbicides dicamba and 2,4-D, are currently used extensively for postemergence control of these species. In some cases, complete control of weeds at the time of auxin application is not achieved due to adverse environmental conditions, plant factors or misapplications. In these instances, a subsequent postemergence herbicide may be required to control any plant that survived the initial auxin herbicide application. This research was conducted to determine the efficacy and optimal successive time interval between applications of viable postemergence herbicides in soybean on Palmer amaranth and waterhemp plants surviving a previous application of 2,4-D or dicamba. Results from this research indicated that respraying a failed auxin herbicide application with a subsequent auxin herbicide, especially dicamba, was less effective than respraying with glufosinate or fomesafen to control waterhemp in addition to being a less desirable approach for resistance management. Additionally, respray herbicide applications should target 7 to 14 d after the initial failed herbicide application on waterhemp for optimal overall efficacy. When dicamba was the initial herbicide sprayed on Palmer amaranth, 94% or greater control was achieved with glufosinate in 2019 on a 7- and 14-d respray interval, which was greater than the efficacy observed with either dicamba or 2,4-D (< 82%). In 2020, these differences in herbicide efficacy were not evident within these time intervals. These general trends in treatment differences were also manifested in the data for plant height, biomass and viable apical meristems. When 2,4-D was the initial herbicide sprayed on Palmer amaranth, fomesafen and glufosinate applications on a 7-d respray interval and glufosinate on a 14-d respray interval resulted in greater control than 2,4-D in 2019. All other trends in herbicide treatments and time intervals were consistent with dicamba applied as the initial herbicide. Another part of his research was conducted with the objective of quantifying herbicide spray solution deposition and herbicide efficacy on waterhemp growing in different densities plant densities: low, high-thinned and high densities. In both field and greenhouse experiments, spray deposition (µl cm-2) on waterhemp leaves was up to 53% less on plants grown in high density compared to the other density treatments. Even though no differences in herbicide deposition between low and the high-thinned densities were observed, there were differences in herbicide efficacy. When applied to plants growing in low density, applications of glufosinate, fomesafen and topramezone reduced weed biomass to a greater extent than plants growing in the high-thinned density. Following herbicide damage to the apical meristem, plants growing in high-density produced new branches from axillary buds that were previously dormant, whereas plants growing in the low density already had axillary branches initiated from these buds and did not provide an opportunity for new shoot growth. In general, no differences in herbicide efficacy were observed across weed densities following dicamba applications. Source-to-sink translocation of dicamba to previously active meristems, or axillary buds that broke dormancy after the herbicide application, may have compensated for less spray solution interception on plants growing in high density. Lower levels of postemergence herbicide efficacy in high density weed populations are not only influenced by spray deposition differences, but also by changes in plant growth and apical dominance. This research provides further evidence that justifies the need for weed managers to reduce weed densities as much as possible, via non-chemical or soil residual herbicides (preemergence), as much as possible to optimize the efficacy of foliar herbicide applications.</p>