Tolerance of Amaranthus palmeri populations from California to postemergence herbicides at various growth stages

• Suspected glyphosate-resistant Amaranthus palmeri plants were collected from 23 locations of the San Joaquin Valley.
• They did not survive the label rate of glyphosate at the 4 to 6 leaf growth stage.
• However, when they were sprayed at later growth stages several plants survived the label rate of glyphosate.
• Therefore, escapes of A. palmeri in these locations may be due to glyphosate applications at later growth stages.
• Several alternative herbicides and herbicide combinations were identified for the control of A. palmeri populations.

In recent years, poor control of Amarathus palmeri S. Wats. plants with glyphosate in many agricultural and non-crop has been observed in the San Joaquin Valley (SJV), California, USA. Studies were conducted to assess if these were glyphosate-resistant (GR) populations. Populations from 23 different locations of the SJV were exposed to glyphosate application of 840 g ae ha−1 at the 5 to 8 leaf stage of the plant and compared against a known GR and glyphosate-susceptible (GS) population from New Mexico, USA. None of the plants from the SJV survived the glyphosate application suggesting that they were GS. Plant mortality following application of glyphosate (840 g ae ha−1), glufosinate (490 g ai ha−1), paraquat dichloride (660 g ai ha−1), saflufenacil (50 g ai ha−1), rimsulfuron (70 g ai ha−1), and a tank-mix of glyphosate (840 g ae ha−1) + saflufenacil (50 g ai ha−1) applied at the 4 to 6, 8 to 10, and 12 to 16 leaf stages of A. palmeri was determined on potted plants grown outdoors. Complete control was obtained with all the treatments applied at the 4 to 6 leaf stage but control was reduced to less than 70% and 20% with glyphosate and glufosinate, respectively at the later stages. The other treatments provided 100% control at all growth stages. Combinations of saflufenacil + glyphosate, saflufenacil + glufosinate, saflufenacil + dicamba, rimsulfuron + glyphosate, tembotrione + glyphosate, flumioxazin + pyroxasulfone + glyphosate, flumioxazin + pyroxasulfone + glyphosate, dicamba + paraquat dichloride, and glyphosate + glufosinate were also tested on 8 to 10 leaf stage A. palmeri plants and all the combinations provided 100% control.