A grower provided us a sample of the seed mix used on one of the conservation plantings where Palmer amaranth developed following planting. We were able to extract Amaranthus seed from the native seed mix. Since it is difficult, if not impossible, to visually differentiate the Amaranthus species by seed, we planted them and allowed them to grow until they reached a size I was confident in confirming the seed truly was Palmer amaranth. Based on this grow out I am 95% confident that the seeds were Palmer amaranthus rather than waterhemp or some other pigweed.
Since we had the Palmer amaranth I decided to do a highly non-scientific screening to see if the plants were resistant to glyphosate. I sprayed the flat (18 Palmer amaranth seedlings) with 32 oz of Roundup Powermax (1.4 lbs ae/A). I covered four of the plants with a paper cup to act as an untreated control. Based on the results of this 'experiment' (did I mention it was highly unscientific) I have concluded that this population is not likely to be resistant to glyphosate.
The field where this seed mix was planted was treated with Plateau (imazapic), a Group 2 herbicide. Because there was a near monoculture of Palmer amaranth in the field, I'm assuming the herbicide was used in an effective manner. This indicates the Palmer amaranth is resistant to Group 2 herbicides, which shouldn't be a surprise.