A 2-yr field study was conducted to compare the growth of Amaranthus palmeri, A. rudis, A. retroflexus, and A. albus planted in June and July. Rates of height increase (centimeters per growing degree day) were 0.21 and 0.18 for A. palmeri, 0.16 and 0.11 for A. rudis, 0.12 and 0.09 for A. retroflexus, and 0.08 and 0.09 for A. albus in 1994 and 1995, respectively, when planted in June. A. palmeri had among the highest values for plant volume, dry weight, and leaf area, while A. albus had the lowest. Specific leaf area values (cm2 g−1) ranged from 149 to 261 for A. palmeri, 160 to 205 for A. rudis, 150 to 208 for A. retroflexus, and 127 to 190 for A. albus. Maximum relative growth rates (g g−1 day−1) for any measured period were 0.32 for A. palmeri, 0.31 for A. rudis, 0.30 for A. retroflexus, and 0.26 for A. albus. Recent increases in species range and observed changes in weed community structure may be partially explained by the growth characteristics of A. palmeri and A. rudis. Herbicide rate and timing recommendations for mixed populations of these weeds should be based on A. palmeri because of its high growth rates.

Field studies were conducted in 1997 and 1998 at Manhattan and Topeka, KS, to examine the competitive effects of redroot pigweed, Palmer amaranth, and common waterhemp on soybean yield. The experiments were established as a randomized complete block design in a factorial arrangement of three pigweed species, two pigweed planting dates (soybean planting and cotyledon stage), and seven weed densities (0.25, 0.5, 1, 2, 4, and 8 plants m−1 of row, plus a weed-free control). The effect of weed density on soybean yield loss, pigweed biomass, and pigweed seed production were described using a rectangular hyperbola model. Soybean yield loss varied between locations depending on the weed species, density, and time of emergence. Yield loss increased with weed density for each species and location with the first pigweed emergence time. The maximum soybean yield loss occurred at the first planting and 8 plants m−1 of row density, and was 78.7, 56.2, and 38.0% as determined by the model for Palmer amaranth, common waterhemp, and redroot pigweed, respectively. The second planting of pigweed did not significantly reduce soybean yield. The relative ranking of the pigweed species biomass was Palmer amaranth > common waterhemp > redroot pigweed. Maximum seed production for Palmer amaranth, common waterhemp, and redroot pigweed was 32,300, 51,800, and 9,500 seeds m−2. Palmer amaranth produced a larger quantity of seed than did common waterhemp or redroot pigweed at low weed densities (0.25 to 4 plants m−1 of row). But common waterhemp seed production equaled or surpassed Palmer amaranth at high weed densities.

Palmer amaranth (Amaranthus palmeri) is a major weed in corn (Zea mays) fields in the southern Great Plains of the United States. Field studies were conducted in 1996, 1997, and 1998 near Garden City, KS, to evaluate the effects of Palmer amaranth density and time of emergence on grain yield of irrigated corn and on seed production of Palmer amaranth. Palmer amaranth was established at densities of 0.5, 1, 2, 4, and 8 plants m−1 of corn row both concurrently at corn planting and when corn was at the three- to six-leaf stage. The control plots were weed free. The Palmer amaranth planted with corn emerged with corn, whereas that planted later emerged at the four-, six-, and seven-leaf stages of corn. The Palmer amaranth emerging with corn reduced yield from 11 to 91% as density increased from 0.5 to 8 plants m−1 of row. In contrast, yield loss from Palmer amaranth emerging later than corn was observed only when the emergence occurred at the four- and six-leaf stages. The corn leaf area index (LAI) decreased as Palmer amaranth density increased. Reduction in corn LAI from Palmer amaranth interference was smaller for the second emergence date than for the first emergence date. Seed production per Palmer amaranth plant decreased with greater density, but seed per unit area increased from 140,000 to 514,000 seeds m−2 at densities of 0.5 and 8 plants m−1 of row, respectively, when Palmer amaranth emerged with corn and from 1,800 to 91,000 seeds m−2 at the same densities for later emergence dates. Although Palmer amaranth is highly competitive in corn, this study shows that yield loss is affected more by time of emergence than by density.

Seeds of suspected herbicide-resistant Palmer amaranth and common waterhemp were collected in Clay County and Douglas County, KS, respectively. An experiment was established in a greenhouse to determine if these species had developed resistance to imazethapyr and thifensulfuron. Imazethapyr was applied pre- (PRE) and postemergence (POST) at 1×, 2×, 4×, and 8× the suggested use rate (70 g/ha), and thifensulfuron was applied POST at 1×, 2×, 4×, and 8× the suggested use rate (4.5 g/ha). Both species had developed resistance to all rates of these herbicides. The occurrence of resistance at the Clay County site (Palmer amaranth) fit the typical pattern for the development of herbicide resistance, i.e., multiple applications of the same class of herbicide for several years. However, the Douglas County (common waterhemp) site had a limited history of use of ALS-inhibiting herbicides and did not follow typical models of resistance development.

Herbicides classified as synthetic auxins have been most commonly used to control broadleaf weeds in a variety of crops and in non‐cropland areas since the first synthetic auxin herbicide (SAH), 2,4‐D, was introduced to the market in the mid‐1940s. The incidence of weed species resistant to SAHs is relatively low considering their long‐term global application with 30 broadleaf, 5 grass, and 1 grass‐like weed species confirmed resistant to date. An understanding of the context and mechanisms of SAH resistance evolution can inform management practices to sustain the longevity and utility of this important class of herbicides. A symposium was convened during the 2nd Global Herbicide Resistance Challenge (May 2017; Denver, CO, USA) to provide an overview of the current state of knowledge of SAH resistance mechanisms including case studies of weed species resistant to SAHs and perspectives on mitigating resistance development in SAH‐tolerant crops. © 2017 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.