VanGessel: Glyphosate-resistant horseweed • 703 Weed Science, 49:703–705. 2001 Rapid Publication Glyphosate-resistant horseweed from Delaware Mark J. VanGessel Department of Plant and Soil Sciences, University of Delaware, Research and Education Center, 16684 County Seat Highway, Georgetown, DE 19947; mjv@udel.edu No-tillage corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] production has been widely accepted in the mid-Atlantic region, favoring establishment of horseweed [Conyza canadensis (L.) Cronq.]. Within 3 yr of using only glyphosate for weed control in continuous glyphosate-resistant soybeans, glyphosate failed to control horseweed in some fields. Seedlings originating from seed of one population collected in Delaware were grown in the greenhouse and exhibited 8- to 13-fold glyphosate resistance compared with a susceptible population. There were no differences be- tween the isopropylamine or diammonium salts of glyphosate. Nomenclature: Corn; Zea mays L.; horseweed; Conyza canadensis (L.) Cronq. ER- ICA; soybean; Glycine max (L.) Merr.; glyphosate. Key words: Herbicide resistance, glyphosate resistance. Glyphosate is a broad-spectrum, nonselective herbicide with limited or no soil activity. In recent years, glyphosate has been used annually in many no-till fields for controlling vegetation prior to planting. Prior to herbicide-resistant crops, glyphosate was applied in combination, or in se- quence, with other herbicides that reduced pressure for se- lection of resistant biotypes. However, with widespread use of glyphosate-resistant soybean [Glycine max (L.) Merr.], many no-tillage fields have been treated only with glyphos- ate. Some of the soils in Delaware are very drought prone and growers have continuously planted soybeans in these fields because they are more drought tolerant than corn (Zea mays L.). In 2000, a field treated with 1.6 kg ai ha -1 of glyphosate had unaffected horseweed [Conyza canadensis (L.) Cronq.] plants next to plants that were completely dead 3 wk after application. The surviving horseweed plants were not controlled with an additional 1.6 kg ha -1 of glyphosate. Previous research has reported naturally occurring gly- phosate tolerance in field bindweed (Convolvulus arvensis L.) (DeGennaro and Weller 1984; Duncan and Weller 1987). Two Australian populations of rigid ryegrass (Lolium rigi- dum Gaud.) exhibited a 7- to 11-fold resistance to glyphos- ate, whereas goosegrass [Eleusine indica (L.) Gaertn.] in Ma- laysia had similar levels of glyphosate resistance (Lee and Ngim 2000; Powles et al. 1998; Pratley et al. 1999). In the case of rigid ryegrass, uptake, translocation, or metabolism could not explain the mechanism of resistance (Feng et al. 1999). Glyphosate resistance has not been reported for an annual broadleaf species. These studies were conducted to confirm glyphosate re- sistance in a population of horseweed from Delaware. In 1999 and 2000, glyphosate was used exclusively for preplant and in-crop weed control in glyphosate-resistant soybeans. Prior to 1999, glyphosate was used infrequently for preplant weed control. Materials and Methods Horseweed seed was collected at maturity from a soybean field located near Houston, DE, from plants surviving two 1.6 kg ha -1 applications of glyphosate. Seeds were also col- lected from horseweed plants growing at the University of Delaware’s Research and Education Center (UD-REC), near Georgetown, DE. The field at UD-REC had not been treat- ed with glyphosate during the past 5 yr and previous gly- phosate applications effectively controlled horseweed. Seed from UD-REC served as a susceptible check. Seeds were planted 0.5-cm deep in commercial potting mix 1 in 9 by 9 by 9 cm pots. Plants were placed in a green- house maintained at 28 C and daylength was extended to 15 h using high-pressure sodium lights. 2 Upon emergence, seedlings were thinned to five per pot. Later-emerging plants were removed. Plants were irrigated twice a day and fertil- ized once, 5 wk after planting, with a commercial fertilizer. 3 Seedlings were treated 3 wk after planting with either the isoproplyamine 4 or diammonium 5 salt of glyphosate at rates of 0, 0.28, 0.56, 0.84, 1.1, 2.2, 4.4, 8.8, or 13.4 kg ai ha -1 . Glyphosate was applied with a compressed air cabinet spray- er calibrated to deliver 234 L ha -1 at 173 kPa. The experimental design was a randomized complete block with a three-factor factorial arrangement of treat- ments. The first factor was biotype, suspected resistant (R) from Houston or susceptible (S) from UD-REC. The sec- ond factor was the glyphosate salt and the third was the glyphosate rate. Each treatment was replicated four times and the experiment was repeated (referred to as Run 1 and Run 2). Replicates were determined by plant size at the time of application. At application, the Houston biotype rosette ranged from 1.5 to 4 cm in diam and the UD-REC biotype ranged from 1 to 3.5 cm for the first run. For the second run, rosette diam ranged from 1 to 3.5 cm and 1 to 3 cm for the Houston and UD-REC biotypes, respectively. Plants were visually evaluated for percent control at week- ly intervals after treatment (0 = no apparent reduction in plant biomass and 100 = dead horseweed). Four weeks after treatment, plants were clipped at the soil surface, dried at 43 C for 4 d, and weighed. Average biomass per plant was expressed as the percentage of the untreated control plants and square root transfor-