In the southeastern United States, Amaranthus, or pigweed species, have become troublesome weeds in agricultural systems. To implement management strategies for the control of these species, agriculturalists need information on areas affected by pigweeds. Geographic information systems (GIS) afford users the ability to evaluate agricultural issues at local, county, state, national, and global levels. Also, they allow users to combine different layers of geographic information to help them develop strategic plans to solve problems. Furthermore, there is a growing interest in testing free and open-source GIS software for weed surveys. In this study, the free and open-source software QGIS was used to develop a geographic information database showing the distribution of pigweeds at the county level in the southeastern United States. The maps focused on the following pigweeds: Palmer amaranth, redroot pigweed, and tall waterhemp. Cultivated areas and glyphosate-resistant (GR) pigweed data were added to the GIS database. Database queries were used to demonstrate applications of the GIS for precision agriculture applications at the county level, such as tallying the number of counties affected by the pigweeds, identifying counties reporting GR pigweed, and identifying cultivated areas located in counties with GR pigweeds. This research demonstrated that free and open-source software such as QGIS has strong potential as a decision support tool, with implications for precision weed management at the county scale.

In a survey of herbicide responses among Illinois waterhemp half-sib populations, several were observed with differential responses to imazethapyr and thifensulfuron, two acetolactate synthase (ALS)–inhibiting herbicides. Plants from two waterhemp populations were verified resistant to imazethapyr, but susceptible to chlorimuron, using a nondestructive leaf-disc assay. Sequencing of the ALS gene revealed that imazethapyr-resistant waterhemp plants from both populations had inferred amino acid substitutions at position 653 of ALS. Depending on the population, the serine at position 653 of ALS was substituted with either asparagine (S653N) or threonine (S653T). Waterhemp lines were derived from each population to create uniformly imidazolinone-resistant (IR) waterhemp biotypes, designated IR-62 and IR-101. ALS-inhibitor responses of each IR biotype were compared with a previously identified ALS inhibitor–resistant biotype with a tryptophan to leucine substitution at position 574 (W574L) and an herbicide-susceptible control. Whole-plant dose–response experiments with waterhemp biotypes containing W574L, S653N, or S653T mutations indicated that each biotype was resistant to imazethapyr, but only the biotype with a W574L mutation was resistant to thifensulfuron. In vitro ALS-activity assays revealed unique patterns of cross-resistance among protein extracts derived from each biotype in response to imazethapyr, thifensulfuron, cloransulam, and pyrithiobac. In conclusion, three different forms of target-site–based resistance to ALS inhibitors have been identified in waterhemp.

Prior research indicated that analysis of nuclear DNA content by flow cytometry could be used to distinguish smooth pigweed × tall waterhemp hybrids when the parent lines are known. Flow cytometry was performed on nuclei isolated from several Illinois populations of smooth pigweed and tall waterhemp. The smooth pigweed and tall waterhemp analyzed had nonoverlapping 2C nuclear DNA content values, with mean values of 1.04 and 1.34 pg, respectively. The consistent difference in DNA content observed between the two species indicates that DNA content analysis can be used to distinguish their hybrid progeny in natural populations.

A population of waterhemp was identified in Adams County, Illinois, that survived treatment of several acetolactate synthase (ALS) inhibitors and a postemergence (POST) application of lactofen, a protoporphyrinogen oxidase (PPO)–inhibiting herbicide. Greenhouse studies were conducted to quantify the responses of this waterhemp population, designated ACR, to multiple PPO inhibitors and various other herbicides with different sites of action. Resistance ratios were obtained by comparing herbicide dose–response curves between the ACR population and a herbicide-susceptible waterhemp population. The ACR population was resistant to lactofen (23-fold) and to five other PPO-inhibiting herbicides (ranging from 2.2- to 6.2-fold). Furthermore, the ACR waterhemp population was 17,000-fold and 18,000-fold resistant to imazamox and thifensulfuron, respectively, two ALS-inhibiting herbicides. In response to atrazine, a Photosystem II inhibitor, the ACR population was 38-fold resistant. Plants within the ACR waterhemp population survived treatment of a herbicide mixture containing lactofen at 175 g ai ha−1, imazamox at 44 g ae ha−1, and atrazine at 1,000 g ai ha−1. Thus, individual plants—not just the population as a whole—displayed multiple herbicide resistance. The ACR population was not resistant to glyphosate or paraquat. This is the first reported weed population from the United States with resistances to herbicides inhibiting three unique sites of action. Furthermore, this research identifies a significant reduction in the number of POST herbicide options available for waterhemp control in soybean production.

Common ragweed and common cocklebur plants were collected at two sites each in Illinois, Minnesota, and Ohio to analyze intraspecific variability of the gene encoding acetolactate synthase (ALS). A 385-nucleotide fragment within the coding sequence of ALS was compared among 24 plants of each of these two species from the six locations. Common ragweed ALS was highly variable, with polymorphisms observed at 48 (12.5%) of the 385 nucleotides among the 24 plants. Despite the numerous nucleotide polymorphisms, only two inferred amino acid polymorphisms were identified. No apparent population structure was suggested by the ALS sequence data, indicating widespread gene flow consistent with the wind-pollinated nature of common ragweed. In contrast to common ragweed, no ALS polymorphisms were identified among the common cocklebur plants used in this study. As a basis for comparing the extremes observed between common ragweed and common cocklebur, ALS intraspecific variability also was investigated in 10 plants each of tall waterhemp and smooth pigweed. Normalized to the number of plants analyzed, the number of nucleotide polymorphisms for both tall waterhemp and smooth pigweed was greater than that in common cocklebur but less than that observed in common ragweed. Information on variability of herbicide target-site genes may be useful in predicting the likelihood for herbicide-resistance development. However, all four of the species investigated in this study have evolved resistance to ALS-inhibiting herbicides, despite the different levels of ALS variability observed.

Several populations of different Amaranthus species have been reported resistant to single or multiple herbicides. Interspecific hybridization among amaranths is hypothesized to contribute to the evolution of herbicide resistance. Although other studies have shown the occurrence of interspecific Amaranthus hybrids, little has been done to establish the likelihood of hybridization under field conditions. The main objective of this study was to determine potential field frequencies of hybridization between tall waterhemp females and smooth pigweed. Field hybridization plots were established during each of two growing seasons. Individuals of the two species were transplanted to field plots and arranged at varying distances from each other. Hybrid progeny were detected using the acetolactate synthase (ALS) gene as a marker. Smooth pigweed parents were homozygous for a herbicide-resistance ALS allele, whereas maternal tall waterhemps were homozygous for a herbicide-sensitive ALS form. Heterozygous interspecific progeny were thus detected by means of herbicide selection. Molecular and cytogenetic tools were used to verify the validity of the data obtained. Averaged among female waterhemp plants and across the two field seasons, hybridization occurred at a frequency of 33%. A single tall waterhemp plant was capable of producing more than 200,000 hybrids, suggesting little if any gametic incompatibility. All flowering hybrids obtained from tall waterhemp females were of dioecious condition and female sex. Observed sexual segregation was consistent with previously postulated chromosomal XY-type system in tall waterhemp sex determination, where males are the heterogametic sex.