Characterization of herbicide resistance of Spanish populations of Chloris truncata against glyphosate

Abstract

Chloris truncata is a perennial grass with a C4 photosynthetic pathway. It is native to Australia, but it is currently distributed in other subtropical regions of the world. In Spain, it is considered a naturalized allochthonous species, mainly in Extremadura and Catalonia. Currently, there have been reported failures in its control with glyphosate. Previous studies report that herbicide resistance is based on EPSPS gene amplification. In 2022, seeds were collected from field plant survivors of glyphosate treatments in Talayuela – Extremadura (CTT-R); a population from Australia (CTA-S) was used as sensitive reference material. Preliminary tests (greenhouse) were carried out on seedlings with four leaves (BBCH 14), with doses of (1.5 and 3 L ha^-1) 540 and 1080 g a.i. ha^-1 of glyphosate, with 100% survival. In the same year, dose-response trials were carried out with a dose range between 270 and 8,640 g a.i. ha^-1. At 28 days after treatment (DAT), the survival percentage was evaluated and values of: 1,146.7 and 321.34 g a.i. ha^-1 were determined with a Resistance Factor (RF) of 3.7 times according to GR₅₀; for LD₅₀, 1,882.9 and 712.5 g a.i. ha^-1 with RF of 2.6 for the CTT-R and CTA-S populations, respectively. Complementary dose-response experiments are currently being carried out with new populations collected in Extremadura and Catalonia, to confirm or not, the presence of other glyphosate-resistant biotypes and to elucidate the resistance mechanism involved.