Japanese foxtail is a predominant tetraploid grass weed in wheat and oilseed rape fields in eastern China. In China, pyroxsulam is mainly used to manage annual grass weeds, especially those resistant to acetyl coenzyme A carboxylase (ACCase)-inhibiting herbicides. Using dose–response studies, a pyroxsulam-resistant population, ACTC-1, was identified with a resistance index value of 58. Additionally, ACTC-1 was cross-resistant to sulfonylureas, imidazolinones, triazolopyrimidines, pyrimidinyl-benzoates, and sulfonylaminocarbonyl-triazolinones and multiresistant to ACCase and photosystem II inhibitors. Sequence analysis revealed four gene fragments encoding acetolactate synthase (ALS) from ACTC-1, and three from JNXW-1, a pyroxsulam-sensitive population. An Asp-376-Glu substitution was found in ALS1;2 and an Ile-2041-Asn in Acc1;1, which may be responsible for its resistance to pyroxsulam and ACCase inhibitors, respectively. In vitro assays of ALS activity revealed that in ACTC-1, the sensitivity of ALS to pyroxsulam was lower, and the basal ALS activity was twofold higher than that of sensitive population JNXW-1. Additionally, the combined application of pyroxsulam with malathion or piperonyl butoxide increased the sensitivity of ACTC-1 to pyroxsulam, although it could not completely overcome the resistance. It was inferred that both target-site-based resistance and nontarget-site-based resistance may be involved in the resistance to pyroxsulam.

Nomenclature: Malathion; piperonyl butoxide (PBO); pyroxsulam; Japanese foxtail, Alopecurus japonicas Steud; birdsrape mustard, Brassica campestris L.; wheat, Triticum aestivum L.