Redroot pigweed (Amaranthus retroflexus L.) is a common broadleaf weed species in Manitoba, Canada. Producers in this region have been reporting reduced response of this weed to common Acetolactate synthase (ALS) inhibiting herbicides.

Three suspected resistant populations of redroot pigweed were compared to known susceptible populations in a greenhouse study to determine the level and mechanism of resistance of these populations to the ALS-inhibiting herbicides imazethapyr and thifensulfuron which are commonly applied in the region. Dose response curves to these herbicides indicated that all three pigweed populations were resistant to imazethapyr (30 to >170-times), but only one of these populations showed a low level of resistance to thifensulfuron.

ALS sequence analysis detected a Ser653Asn amino acid substitution that was consistent in the two populations resistant to imazethapyr only while a Ser653Ile substitution was more common in the population resistant to both imazethapyr and thifensulfuron. No appreciable level of non-target site resistance (NTSR) was observed excluding this as a major resistance mechanism in these three pigweed populations. These results show that target site resistance is the principal mechanism conferring resistance to these two ALS inhibitors in these populations. This enables the development of rapid molecular tests, albeit different primer pairs are required for the two different mutations found in redroot pigweed. One of these mutations has not been reported in redroot pigweed before.