Multiple resistance to glyphosate and haloxyfop‐p‐methyl in <i>Sorghum halepense</i> from Argentina

Abstract The objective of this study was to characterise multiple resistance to glyphosate and acetyl‐CoA carboxylase (ACCase)‐inhibiting herbicides from an Argentine population of Sorghum halepense (Johnsongrass) (GC‐R). Whole plant dose response trials with haloxyfop‐p‐methyl revealed that the GC‐R population was highly resistant, with LD 50 and GR 50 values exceeding eight times the recommended field dose (64.8 g ai ha −1 ). Sequence analysis identified three unique ACCase mutations in GC‐R: Ile2041Asn and Ala2096Gly, and a novel mutation, Ile1699Thr. Dose–response experiments were performed with three different clones selected by PCR‐based markers. The clone carrying Ile2041Asn + Ile1699Thr (GC‐4) showed the highest LD 50 (&gt;2074 g ai ha −1 ), while the clone carrying Gly2096Ala (GC‐6) and the clone carrying Gly2096Ala + Ile1699Thr (GC‐7) showed similar LD 50 (1241 and 1021 g ai ha −1 , respectively). Glyphosate dose–response showed LD 50 slightly above 1X (1494 g ai ha −1 ), but biomass was significantly reduced (GR50 = 715 g ai ha −1 ). Molecular analysis ruled out mutations in the 5‐enolpyruvylshikimate‐3‐phosphate synthase ( EPSPS ) sequence and gene amplification. The two known ACCase substitutions conferred high levels of resistance to haloxyfop‐p‐methyl, consistent with previous reported inhibition patterns. The newly found mutation could be involved in the resistance phenotype, although its contribution is unclear. Glyphosate resistance likely involves a non‐target site resistance mechanism (NTSR).