Abstract Horseweed [Conyza canadensis(L.) Cronquist] is a facultative winter annual weed that can emerge from March to November in Michigan. Fall-emergingC. canadensisoverwinters as a rosette, while spring-emergingC. canadensisskips the rosette stage and immediately grows upright upon emergence. In Michigan, primary emergence recently shifted from fall to spring/summer and therefore from a rosette to an upright growth type. Growth chamber experiments were conducted to determine (1) whether bothC. canadensisgrowth types could originate from a single parent and (2) whether common environmental cues can influence growth type. Variations in temperature, photoperiod, competition, shading, and soil moisture only resulted in the rosette growth type in fourC. canadensispopulations originating from seed collected from a single parent of the upright growth type. However, a vernalization period of 4 wk following water imbibition, but before germination, resulted in the upright growth type. Dose–response experiments were conducted to determine whether glyphosate sensitivity differed betweenC. canadensisgrowth types generated from a single parent of the upright growth type. Upright-typeC. canadensisfrom known glyphosate-resistant populations ISB-18 and MSU-18 were 4- and 3-fold less sensitive to glyphosate than their rosette siblings, respectively. Interestingly, differences in glyphosate sensitivity were not observed between growth types from the susceptible population. These results suggest that whileC. canadensispopulations shift from winter to summer annual life cycles, concurrent increases in glyphosate resistance could occur.

Abstract Winter wild oat [Avena sterilis ssp. ludoviciana (Durieu) Gillet & Magne; referred to as A. sterilis here] is one of the major weed species of the Avena genus, given its high competitive ability to infest cereal crops worldwide, with special concern in Spain. A nine-location field experiment was established across Spain where a total of 400 A. sterilis seeds per location were sowed in four replicates in autumn 2016 to monitor the emergence during two growing seasons in dryland conditions. The data were used to test the prediction ability of previously published thermal (TT) and hydrothermal time (HTT) models and to develop new models, if required. Overall, the average percentage of emergence was 30% during the first season and 21% during the second season. In both seasons, the main emergence flush occurred between November and February. According to the phenological stage, A. sterilis achieved the tillering earlier in southern sites, between November 25 and the end of December, compared with northern sites, where this stage was reached at the end of January. The newly developed model described the emergence with precision, using three cardinal temperatures to estimate the TT. The three cardinal points were established at −1.0, 5.8, and 18.0 C for base (Tb), optimum (To), and ceiling temperature (Tc), while the base water potential (Ψb) was established at −0.2 MPa for the HTT estimation. This study contributes to improving prediction of the emergence of A. sterilis and provides knowledge for decision support systems (DSS) for the control of this weed.