The majority of parishes in Northeast and Northwest Louisiana boarding the Mississippi and Red Rivers, as well as St. Landry Parish, now have documented cases of glyphosate resistant Palmer amaranth. Seed from resistant populations in neighboring states is carried along these main waterways and distributed in adjacent fields with back water flooding and seep water in spring.
Three characteristics that make glyphosate resistance in Palmer amaranth such a problem are a very aggressive growth habit, tremendous seed producing capability, and a 73 percent germination rate. In our perfect growing conditions in mid to late summer, it is not uncommon to see Palmer amaranth plants put on 8 to 10 inches of growth in a week’s period and produce viable seed. Studies in Arkansas have shown that female plants are capable of producing up to 1.77 million seed. At that rate, managing the weed becomes a numbers game. Take for instance an area with 50 female plants that each produce 500,000 seed. Let’s say that 90 percent of those seed are lost to predation or rot or other means. Also, let’s say a producer implements a management strategy that provides 99 percent control. He/she is still left with 4.975 million seed. With a 73 percent germination rate, that means there is a potential to have 3.63 million plants, each capable of producing 1.77 million seed!!!
The main recommendation for preventing introduction of or managing an established population of glyphosate resistant Palmer amaranth is to start clean with a planned rotation of herbicides that are effective on the weed species and offer a completely different mode of action, or means of controlling the weed, than glyphosate.
An effective strategy for management of this weed is to overlay residual herbicides to never let the weed off the mat. This includes application of residual herbicides preplant, at planting, in early season over-the-top applications, and at layby. You always want an effective material present at peak strength when another is playing out. This will ensure season long control of the weed.